Metathesis by Partner Interchange in σ‐Bond Ligands: Expanding Applications of the σ‐CAM Mechanism

Abstract In 2007 two of us defined the σ‐Complex Assisted Metathesis mechanism (Perutz and Sabo‐Etienne, Angew. Chem. Int. Ed. 2007, 46, 2578–2592), that is, the σ‐CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal–element bonds through partner‐interchange of σ‐bond complexes. The key concept that defines a σ‐CAM process is a single transition state for metathesis that is connected by two intermediates that are σ‐bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ‐bond complexes have been isolated or computed as well‐defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ‐bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ‐CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ‐CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional σ‐bond ligands, agostic complexes, sp2‐carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal–element bonds.


Introduction
In 2007 two of us published areview entitled "The s-CAM mechanism: s-complexes as the basis of s-bond metathesis at late-transition-metal centers". [1] Thep rinciple behind the proposed s-CAM (s-Complex Assisted Metathesis) mechanism is that s-bond complexes can interchange the partners that form the s-bond(s) donating to the metal. This interchange could lead to metathesis at constant oxidation state (Scheme 1a). We proposed that such am echanism would compete with oxidative addition/reductive elimination mechanisms (Scheme 1b)i ns ituations where the s-bond complexes acted as intermediates both preceding and following asingle transition state that interchanged partners.Evidence for the existence of such s-complex intermediates may come from their direct spectroscopic observation or even crystallographic characterization. They are also often identified using computational methods,w hen their existence is fleeting or equilibrium concentrations are low in an overall reaction manifold. There was also acontrast with the standard s-bond metathesis mechanism of d 0 complexes (Scheme 1c), because that did not require s-bond complexes as (potentially) observable intermediates. [2] Similarly,1 ,2-addition (Scheme 1d)i sa nother transformation that breaks an E À H bond but does not require s-bond complexes as intermediates.
Theunderlying concept of a s-CAM process is an overall metathesis reaction that facilitates the replacement of one covalently,2 c-2e,m etal-bonded ligand by another [Eq. (1)].
Fore xample,a na lkyl ligand is replaced by as ilyl ligand or vice versa. Thee ntering reagent in this case is as ilane and the co-product is an alkane alongside the required metal silyl complex. This reaction could be used in synthesis where the new MÀE' complex is the main output, or as part of al arger catalytic manifold yielding the co-product, E À H.
In the first step of the s-CAM mechanism, E'ÀH coordinates to the metal center to form a3 c-2e s-bond complex. This precursor s-bond complex undergoes ad ynamic rearrangement to an ew 3c-2e s-bond complex with coordinated E À H, and finally the co-product E À Hleaves In2007 two of us defined the s-Complex Assisted Metathesis mechanism (Perutz and Sabo-Etienne, Angew.C hem. Int. Ed. 2007Ed. , 46, 2578Ed. -2592, that is,t he s-CAM concept. This new approachtor eaction mechanisms brought together metathesis reactions involving the formation of avariety of metal-element bonds through partner-interchange of s-bond complexes.The key concept that defines a s-CAM process is a single transition state for metathesis that is connected by two intermediates that are s-bond complexes while the oxidation state of the metal remains constant in precursor,intermediates and product. This mechanism is appropriate in situations where s-bond complexes have been isolated or computed as well-defined minima. Unlike several other mechanisms,itdoes not define the nature of the transition state.Inthis review,wehighlight advances in the characterization and dynamic rearrangements of s-bond complexes,m ost notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out aselection of catalytic and stoichiometric examples of the s-CAM mechanism that are supported by strong experimental and/or computational evidence.W ethen draw on these examples to demonstrate that the scope of the s-CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional s-bond ligands,a gostic complexes,s p 2 -carbon, surfaces). Finally,w eprovide acritical comparison to alternative mechanisms for metathesis of metal-element bonds.
generating the MÀE' product (Scheme 1a). In by far the commonest version, hydrogen is exchanged between the sbond partners (Scheme 1a), but al ess frequent alternative with E' occupying the central position is discussed in section 3.4.
Theo verall reaction can be productive (E ¼ 6 E')o r degenerate (E = E'). Common to both situations are three key features of the mechanism:( 1) that av acant site is required for the initial coordination of E' À H, (2) that two successive s-bond complexes are formed as reaction intermediates or isolable species,( 3) the oxidation state of the metal center in the precursor,i ntermediates and product remains constant throughout the process.U nlike some other mechanisms,itdoes not specify the nature or oxidation state of the transition state (TS) between the two s-bond complexes,which can involve varying degrees of bonding between E, Hand E'.However,importantly,asingle TS should link the s-complexes which interchange partners.
The s-bond complexes may be detected by experiment or by computational methods.T he formation of s-bond complexes depends on synergic bonding in which back-bonding from the metal to the ligand is significant, although not dominant. Consequently, s-bond complexes require the presence of d-electrons and are observed most commonly in d 6 and d 8 electron configurations.
Thec oncept requires metathesis (i.e.c onversion of MÀE to MÀE')a nd not just dynamic interchange between two sbond complexes.T he s-bond metathesis mechanism com-Robin Perutz is an emeritus professor at the University of York. During his PhD under J. J. Turner in Cambridge and Newcastle, Robin revealed the formation of metal methane complexes. Subsequently, he worked in Mülheim, Edinburgh and Oxford, before moving to York in 1983. Robin aims to understand the activation of small molecules and the mechanisms of catalytic reactions, often through photochemical reactions. He has received awards from the Royal Society of Chemistry,the Italian and the French Chemical Societies. He was elected aF ellow of the Royal Society,the UK'snational academy of sciences, in 2010. Robin has been active in the women in science agenda. He is also concernedw ith protecting human rights of scientists fleeing conflict.
Sylviane Sabo-Etienne has just retired as CNRS Director of Research Exceptional Class. She spent most of her career at the Laboratoire de Chimie de Coordination in Toulouse, France. Her broad research interests encompassc oordination chemistry and organometallic chemistry for applications in the fields of energy and catalysis with aspecific focus on polyhydrides and dihydrogen transition metal complexes. She is now active in promoting projects combining arts and sciences with astrong commitment to audiences unfamiliar with these cultures and to those with disabilities. monly (but not exclusively) observed for d 0 configurations shares the feature of constant oxidation state but no s-bond complexes have been observed experimentally as intermediates.I nstead, ak ite-shaped 4-center TS is formed directly. These comparisons will be developed in sections 4a nd 5.
In this review,w er eturn to the s-CAM mechanism and examine arange of examples from many authors which have offered strong evidence in favor of the mechanism (Section 3). We also examine several different extensions of the principle and compare the s-CAM mechanism to other competing mechanisms (Sections 4,5). Before this,weoutline advances in s-bond complex synthesis and characterization since the 2007 review,toprovide context for the discussion of mechanism.

Major advances in structural variety of s-bond ligands and s-bond complexes
Tr aditional ligands for transition metals are bonded via donation of an electron pair either as 2c-2e (dative) covalent bond or by a p-bond. It is also possible for as imple s-bond between ap air of atoms to act as donor to am etal center in a3 c-2e interaction. Ther esulting complexes are termed sbond complexes,o rs imply s-complexes. [3,4] Thep rototypical s-bond ligand is H 2 ,f irst recognized by Kubas, [5] but established examples can be found with alkanes,s ilanes, boranes and germanes.T hey typically exhibit h 1 -o rh 2 -E-H geometries (E = H, C, B, Si, Ge) and are unsupported by other bonds to the metal, that is,t hey are intermolecular complexes.O ther possible geometries are given in Scheme 2 in the context of alkanes together with definitions of our nomenclature. [6][7][8] Them ost appropriate nomenclature in systems with the potential for 4-center interactions (middle of top row of Scheme 2) depends on detailed analysis of bonding [9] which is beyond the scope of this review.T his Scheme also illustrates the close relation to agostic complexes in which the M···HÀC s-bond interaction is supported by another bond in an intramolecular chelate. [10] Many authors have extended the agostic concept to other elements,m ost frequently silicon and boron;w es pecify the elements concerned if not C À H. [11][12][13][14] When ac omplex contains both s-bond ligand(s) and either hydride or dihydrogen ligands, additional secondary interactions often occur revealing themselves by shorter internuclear distances than would otherwise be expected. In the specific case of Si···H, they are known as SISHA interactions (Secondary Interactions between Silicon and Hydrogen Atoms).
Dynamic NMR experiments often reveal the fluxional processes that occur as the component nuclei of the s-ligand undergo interchange with their neighbors.S ince the s-CAM mechanism demands the lengthening of the coordinated sbond and the shortening of the distance to an eighboring ligand (Scheme 1a), it is closely associated with the dynamic interchange of s-partners at constant oxidation state (Scheme 3a). Such interchange may be assisted by the secondary interactions mentioned above.I nternal rotation may also be required in some s-CAM mechanisms (Scheme 3b). Geminal exchange and chain-walking (Scheme 3c,d) are related, dynamic processes that can occur in s-bond complexes but are not required in the s-CAM mechanism. This s-partner interchange also contrasts with the oxidative cleavagereductive coupling mechanism that requires an intermediate of higher oxidation state (Scheme 3e).

HÀH s-bond complexes
Molecular dihydrogen complexes are now recognized to exist with aw ide range of H À Hd istances (0.8-1.3 ). [5,15] Although H À Hd istances can be measured by single-crystal X-ray diffraction and estimated by NMR methods,t he gold standard remains single-crystal neutron diffraction structures. [16] An example of aw ell-characterized dihydrogen complex whose structure has been determined by singlecrystal neutron diffraction is Ru(H) 2 (H 2 ) 2 (PCyp 3 ) 2 (Cyp = cyclopentyl, C 5 H 9 )i nw hich the dihydrogen and hydride ligands occupy the equatorial belt around Ru, with the phosphine ligands axial. [17] Access via H/D exchange to the deuterium isotopologue,Ru(D) 2 (D 2 ) 2 (PCyp 3 ) 2 ,allows for the exploitation of the very different scattering cross-sections for hydrogen and deuterium to trace the isotopic exchange.T he structure also provides Ru À Da nd D À Db ond lengths with estimated standard deviations of 0.002 or less (see section 3.1 for adiscussion of the exchange mechanism). [16] Even with neutron diffraction, however,t he distinctions between hydride and dihydrogen can sometimes be blurred due to disorder [18] or nuclear motion on af lat potential energy surface. [19] Dihydrogen complexes have also been identified at metal nodes in metal-organic framework materials using powder neutron diffraction and IR spectroscopy, [20] and on metal nanoparticle surfaces using 2 Hs olid-state NMR techniques. [21]

C À H s-bond complexes
Major advances in the understanding of alkane s-complexes have been made since our review in 2007. [8,[22][23][24] Most notably,s everal rhodium complexes and one cobalt complex have been characterized by single crystal X-ray diffraction, using single-crystal to single-crystal reactivity of molecular alkene precursors by simple addition of H 2 ,providing the long sought geometric proof of structure ( Figure 1a). [25][26][27][28][29][30][31] The majority of these complexes contain an alkane ligand coordinated by two CÀHb onds on different carbon atoms to the metal, each in aM(h 2 -CÀH) mode ( Figure 1). M(h 1 -CÀ H) coordination modes are also reported depending on the identity of the metal/ligand/alkane.Many of these complexes can be observed at room temperature,aconsequence of the stabilizing non-covalent interactions provided by the secondary anion microenvironment in the solid state. [31,32] Isotope H/D exchange at the bound alkane ligand using D 2 allows for remarkable selectivity in such processes as determined using single crystal neutron diffraction techniques. [33] Alkane complexes have also been synthesized in solution by low-temperature photolysis of metal carbonyl, or metal dinitrogen, precursors and by protonation of metal methyl complexes ( Figure 1b). Low temperature 1 HNMR spectroscopy in solution has revealed the isotopic perturbation of resonance for the h 2 -C À Hbond of the partially deuterated isomers.This effect demonstrates that rapid and reversible exchange processes are occurring between C À H( C À D) bonds that can interact with the metal center. [34] Additionally,t he corresponding 13 Cr esonance of the alkane ligand can lie at an exceptionally high field and exhibits ar educed CÀH coupling constant when compared to the free alkane (Figure 1b,c). [34][35][36][37][38][39][40][41][42] Metal centers that have been shown to engage in s-alkane complex formation now include W, Mn, Re,a nd Rh and contain av ariety of supporting ligands.T his improved characterization has been accompanied by quantitative measurements of reactivity including dynamic exchange processes in solution [34][35][36][37][38][39][40]43] revealed by time-resolved infrared spectroscopy, [34,40,44,45] or by time-resolved EXAFS. [46] Dynamic exchange in the solid-state has been observed using low temperature solid-state NMR spectroscopy. [26,28,33] Thel evel of theory in computational studies has also improved considerably so that reliable comparisons may be made of the interactions of different alkanes with metal centers using isolated molecule (gas-phase) calculations. [47][48][49] Fore xample,t he methane complex CpRe(CO) 2 (h 2 -CH 4 )h as been analyzed using coupled-cluster methods by two groups (Figure 1d). [48,49] These two papers agree broadly on binding energies (62.0 kJ mol À1 and 70.0 kJ mol À1 )a nd on the importance of dispersion, but disagree on the magnitude of the dispersion contribution. Neither of them account for the solvent contribution to dispersion. In the solid-state,periodic DFT methodologies can be used to interrogate binding, stability and reactivity in s-alkane complexes. [50] Theability to stabilize s-alkane complexes at room temperature using single-crystal methodologies means that onward reactivity of the M···HÀCi nteraction becomes kinetically accessible. Reactions have been studied that connect s-alkane complexes with the products of CÀHa ctivation:f or example,s elective at CCSD(T)/def2-QZVPPlevel. [48] H/D exchange and acceptorless alkane dehydrogenation. [26,31,33] s-Alkane complexes have also been directly characterized on metal oxide surfaces,s uch as RuO 2 or PdO,a tl ow temperatures (e.g.90K)using acombination of temperatureprogrammed desorption, surface IR spectroscopy and DFT techniques. [51] Reassuringly,t hese M···H-C interactions are broadly similar to those observed and calculated for molecular species,albeit now with the possibility of interaction with multiple surface metal sites for alkanes larger than methane ( Figure 2). Theinteraction of cyclic alkanes with small (Ru 13 ) nanoparticles has been studied using DFT computational methods to understand empirically observed H/D exchange processes.T hese calculations indicate the formation of salkane complexes on the nanoparticle surface prior to C À H bond cleavage. [52] We return to these systems in our discussion of the s-CAM mechanism later (Section 3.5).

SiÀH s-bond complexes
Awide variety of silane and disilane complexes as well as SiH-agostic complexes have been prepared. [11,54] It has now been demonstrated that as imple hydrosilane (Et 3 SiH) can bond in an h 1 -geometry (1)a sw ell as an h 2 -geometry, paralleling the behavior of alkanes mentioned above.D FT calculations (B3LYP/LANL2DZ/6-311G**) suggest less Ir dp to SiH s*b ackbonding in the h 1 -SiH complex than with aconventional h 2 -silane. [55] An example of an h 1 -SiH complex undergoing onward reactivity comes from ac yclometalated platinum complex with asupporting s-bond silane ligand that reacts to form ac onventional PtÀSi bond and open the cyclometalated ring. [56] Theruthenium silazane complex 2 which contains ametalhydrogen and three metal-silicon bonds represents an example of structural characterization of an h 2 -SiH complex. It has been studied by neutron diffraction, solution and solid state NMR and by DFT calculations (B3PW91). [57] Thes tructural evidence ( Figure 3a)shows that the distances from the three Si atoms to the single hydride are all different. One is described as aSi À Hbond (Si a 1.874(3) ), the next as aSi···H or SISHA interaction (Si B 2.099(3) ), and the third as nonbonding (Si c 3.032(3) ,q uoting neutron diffraction distances). TheR u ÀSi a and RuÀSi b distances are essentially equal while the Ru-Si c distance is slightly shorter.Insolution, the Si nuclei are indistinguishable by NMR at all temperatures accessed, but the solid-state Si À HHETCOR NMR spectrum ( Figure 3b)c learly shows the Si nuclei as inequivalent with two of them coupled to the hydride.I nc ontrast, the related complex 3 shows equal RuÀSi distances and equivalent Si nuclei even in the solid-state NMR spectrum;t his species is described as aRu IV (SiMe 2 R) 3 Hcomplex stabilized by SISHA interactions between the hydride and all three Si atoms. [58] Complexes of iron and ruthenium formed by reaction of precursors with 1,2-bis(dimethylsilyl)benzene formally con-  tain one SiMe 2 (C 6 H 4 )Me 2 Si unit bound to the metal by conventional 2c-2e s-bonds,and one H-SiMe 2 (C 6 H 4 )Me 2 Si-H unit bound by h 2 -Si-H interactions (A Fe ,A Ru ). However, there is an alternative formulation in which each hydrogen is bound as ah ydride to the metal and engaged in secondary SISHA bonding to two silicons (B Fe ,B Ru ). Thes pectroscopic and crystallographic data support the M-H + SISHA formulation (Scheme 4). Thus the metals are coordinated by 2L ligands (carbonyl or isonitrile), 4silicon atoms and 2hydrides.T hese hydrides lie midway between pairs of silicon atoms and undergo secondary interactions with them rendering the sbond complex description inappropriate. [59][60][61] AN i 2 complex with ad inucleating P 2 SiOSiP 2 ligand provides an intriguing example of dynamic exchange between dihydrogen and silane ligands that is the key step in catalytic silane deuteration. Thesquare-planar precursor contains two Ni II units,e ach with ah ydride,asilyl and two phosphine ligands bridged by the SiOSi group.Onreaction with H 2 ,this complex reacts to generate first one,and subsequently two Ni 0 units,e ach with ad ihydrogen, an h 2 -SiH and two phosphine ligands. [62] NMR spectra show that the SiH and H 2 groups undergo dynamic exchange at room temperature but coalesce at À908 8 C. Thep roposed exchange mechanism (BP86, 6-31G(d)) is presented in Figure 4.
Theability of mono-silanes to bridge two metals has been illustrated previously. [63] More recently,anintriguing example of such behavior was reported for aN i 2 complex bridged by H 2 SiR 2 (R = Ph, Et) 4 in which the hydrogen atoms lie midway between Ni and Si and the H-Si-H angle is opened to 156(3)8 8. [64] Calculations suggest that, due to the doubly reduced naphthyridine-diimine ligand, these complexes are best considered as Ni I -Ni I species on the continuum between a s-complex and final double Si-H oxidative addition. Most unusually for as ilane complex, the NMR spectra reveal that there is atriplet excited state slightly above the singlet ground state.

BÀH s-bond complexes
Since the initial review article in 2007 there has been significant interest in the coordination chemistry,and onward reactivity,o fB ÀH s-bond complexes.S uch complexes play ar ole in:( a) the construction of CÀBb onds via CÀH activation strategies [68] using 3-coordinate boranes such as  HBCat or HBpin (Cat = catecholate,p in = pinacolate); (b) the catalytic removal of H 2 from amine-boranes,p rototypically H 3 B·NR 3 (R = alkyl or H), for proposed hydrogen storage applications [69][70][71][72][73] and for the synthesis of new B-N main chain containing polymeric materials. [74] Related to these studies,t he coordination chemistry and reactivity of dihydrido-boranes (H 2 BR), amino-boranes (H 2 B = NR 2 )a nd phosphine-boranes H 3 B-PR 3 has been developed. Aw ide range of s-bonding coordination modes are expressed in such complexes,and selected examples (Scheme 5) of 3-coordinate (A-D)and 4-coordinate (E-F)borane species include:M(h 2 -B-H), A; [75] M(h 2 ,h 2 -BH 2 ), B [76] and C; [77][78] [81] In addition to these mono-boron species, s-bond complexes from ligands that contain more than one boron (including boron clusters) are being actively investigated. [12,82] Computational studies on these complexes demonstrate that the bonding between the metal and the borane is best described as arising from donation from aB À Hb onding orbital. For3 -coordinate boranes back-donation into low lying B À H s*o rbitals,o ra nu noccupied p-orbital, is also significant. [77] In 4-coordinate boranes, s-donation to the metal dominates and there is little evidence for back-bonding since the s*B ÀHorbital lies at high energy.
Boron-hydrogen bonds are rather hydridic due to the electronegativity difference between boron and hydrogen, and this,i nt urn, is ac ontributor to the greater stability in solution of s-borane complexes compared to their s-alkane counterparts.C onsequently,d etailed NMR characterization is possible at room temperature and single crystals may be produced using traditional solution techniques.T his difference is illustrated by ac omparison of the stabilities of two closely related complexes:M n(h 5 -C 5 H 5 )(CO)(propane) [34] and Mn(h 5 -C 5 H 5 )(CO)(H 3 B·NMe 3 ). [83] Thef ormer is only observed at 134 Kinliquid propane,while the latter is stable at room temperature and can be recrystallized to allow for as tructural characterization. Like their isoelectronic alkane counterparts,a mine borane ligands (H 3 B·NR 3 )a re often highly fluxional, undergoing exchange of geminal M···H À B interactions,aswell as H/D exchange at the BÀHgroups with D 2 .F igure 6demonstrates that both of these process occur in , where partial substitution of B À Hf or B À Du sing D 2 leads to as eries of isotopologues with B(H 3Àx D x )(x = 3-0). [84] This H/D exchange results in an example of isotopic perturbation of equilibrium of a s-borane complex detected in the resulting 1 Ha nd 2 HNMR spectra. This phenomenon comes from the preference for B À Dt oa dopt terminal rather than bridging positions [85] when in fast exchange with B À Hb onds on the NMR timescale.C onsequently,t here are downfield shifts of the BH 2 Ds ignal (dÀ0.6) and the BHD 2 signal (dÀ1.1) relative to the BD 3 (dÀ1.5) in the 2 HNMR spectrum. The mechanism by which H/D exchange proceeds is discussed in the s-CAM section (Section 3.4).
Finally,t he structure of a s-bound BH 3 ligand, Ir- 2 ]h as been characterized using single-crystal neutron diffraction (Figure 7). [75] Careful consideration of the bonding metrics points to aformulation as a s-bond complex of BH 3 rather than atetrahydridoborate complex with avery  activated BÀHb ond. Thus,B 1 ÀH1c is lengthened compared to the non-interacting terminal BÀHbonds [1.45(5) versus 1.18(2) and 1.22 (5) ], consistent with s-coordination, and the distance to the proximal Ir-hydride is too long to be considered ac ovalent bond [B1 À H2, 1.74 (5) ].

EÀH s-bond complexes (E = Al, Ga, Zn)
New additions to the range of s-complexes derive from the chemistry of hydrides of main group metals of groups 12 and 13, when coordinated to transition metals;b oth h 2 -E,H and h 1 ,h 1 -H,H forms have been reported for E = Al, Ga, Zn (Scheme 6). [86][87][88] Ac ritical assessment provides parameters for deciding whether the s-alane, s-gallane, s-zincane formulation best represents the bonding situation. [88] Tw o important criteria are (a) the formal shortness ratio,d efined as the ratio of the M À M' distance to the sum of the single bond radii of the transition metal Mand the main group metal M' and (b) the CO-stretching frequency of metal carbonyl derivatives.F or an example of astructurally characterized bis s-zincane complex, we consider Cr(CO) 4 (h 2 -HÀZnR) 2 (R = b-diketiminate) in which the hydrogen atoms have been located using single-crystal X-ray diffraction. Notably,the two CrHZn units are coplanar (Scheme 6). Thef ormal shortness ratio exceeds 1.0 as expected for a s-zincane complex and the low wavenumbers of n(CO) support aformulation as Cr 0 .The alternative formulation as ad ihydrogen complex was excluded in solution by NMR relaxation time measurements (definitely for Mo and W, less decisive for Cr). TheMoand W analogues exhibit two isomers that interconvert by an intramolecular mechanism. [89] In as imilar way to that found with boron, the h 2 -Zn,H geometry of these neutral ligands is different from that of formally anionic dihydrozincate ligands that show a h 1 ,h 1 -H,H geometry as in the ruthenium complex in Scheme 7. This complex exhibits exchange between the dihydrogen ligand and the dihydrozincate hydrogens that proceeds via an h 2 -ZnHEt ligand (Scheme 7). [90] In further examples,t he bis(h 2 -zincane) complexes M(PCy 3 )(h 2 -HÀ ZnR) 2 (M = Pd, Pt) are formed by reaction of the bketiminate zinc hydride RZnH (see Scheme 6f or ligand R) with M(PCy 3 ) 2 . [91] TheP tc omplex reacts with pentafluorobenzene to form Pt(C 6 F 5 )(PCy 3 )(h 1 ,h 1 -H,H-H 2 ZnR) in which both hydrides interact with both Pt and Zn. [91] Thee xtent of GaÀHb ond activation in [Rh(bisphosphine){H 2 Ga(NacNac)}][BAr F 4 ][ NacNac = HC-(MeCN(2,6-i Pr 2 -C 6 H 3 ) 2 ]c an be systematically controlled by the combined effects exerted by bite angle of the chelating ligand and the steric bulk of ancillary Rgroups. [92,93] This leads to structural snapshots of GaÀH s-bond activation at ametal center ( Figure 8): from abis-s-bond complex (dppp), through stretched GaÀHb onds (dcypp) to af ully GaÀHa ctivated Rh III dihydride (PCy 3 )w ith aG a I L-type ligand. Computational studies (BP86-def2TZVP-D3BJ) on the bis s-bond complex [Rh(dppp){H 2 Ga(NacNac)}][BAr F 4 ]s how the expected synergic bonding,w ith donation from the HOMO GaÀHb ond to the LUMO of the cationic Rh-fragment complemented by back donation from the metal into GaÀH s*orbitals.

E À E s-bond complexes
Once considered exceedingly rare, s-bond complexes that involve EÀEbonds (e.g.B ,C,Si) have peppered the literature over the last 15 years. [94,95] While still relatively uncommon, such interactions are now firmly established, and have been characterized by structural (single crystal X-ray diffraction), spectroscopic (NMR) and computational (DFT/QTAIM) techniques.S elected examples are included here to highlight key advances,alongside the various descriptors that are used to identify EÀE s-bond coordination.  In these Rh III complexes, am etallocyclobutane unit is partnered with ac yclopropyl C À Ca gostic interaction with the metal from the Binor-S derived ligand. This agostic CÀC s-interaction results in asignificant lengthening of the CÀCbond compared with free derivatives of Binor-S,t he observation of RhÀCc oupling in the low temperature 13 C{ 1 H} NMR spectra, and bond critical points between the C À Cu nit and the Rh as determined by both DFT/QTAIM (LDA/VWN/BP86-6-31G**) and experimental charge density studies. [96] An Ir-congener is also reported, that undergoes reversible CÀCactivation in singlecrystal to single-crystal processes in the solid-state. [97] CÀC agostic interactions have also been studied extensively in early transition metal systems that contain cyclopropyl ligands. [98] In ar ecent example,S c(L)(c-C 3 H 5 ) 2 [L = N(2,6i Pr 2 -C 6 H 3 )C(Me)CHC(Me)N(2,6-i Pr 2 -C 6 H 3 )], B, [99] C À Cagostic interactions between the cyclopropyl group and the metal center are signaled by lengthening of the CÀCb ond that closely approaches the metal, ar educed 13 CÀ 13 C 1 J coupling constant, and NBO analysis (PBE0-GD3-BJ). A b-agostic CÀ Hbond in the cyclopropyl ring is also involved in donation to the metal center in some cases.S imilar h 3 -C À C À Ha gostic interactions have been mentioned as intermediates calculated in CÀCa nd CÀHo xidative cleavage processes at Rh I centers. [100] Intramolecular SiÀSi s-bond interactions with Cu [101] have also been described in which aSiÀSi single bond is brought in close approach to aC u I center by ap hosphine brace, C.E xtension of the SiÀSi bond compared with free ligand, and as ignificant donor/acceptor interaction with the Cu I center that is identified computationally (B3PW91/SDD/ 6-31G**), signal the formation of a s-interaction.
Intermolecular EÀE s-bond complexes have also been reported. The3 -coordinate Pt complex, Pt[NHC(Dip) 2 ]-(SiMe 2 Ph) 2 (NHC = N-heterocyclic carbene;D ip = 2,6-diisopropylphenyl), D has been characterized as being aP t 0 sdisilane complex (Scheme 8). [102] Computational studies (B3PW91/BS-II/B3PW91/BS-I)s how that there is significant back donation from the Pt 0 into the s*o rbitals of the R 3 Si À SiR 3 ligand that complements donation to the metal from the SiÀSi bonding orbital, together resulting in as ignificant lengthening of the SiÀSi bond. Important supporting spectroscopic evidence comes from the 195 Pt chemical shift that signals aPt 0 center rather than Pt II .
Ap latinum complex, Pt(PEt 3 ) 2 {h 2 -(Ph)CC(2,4,6-Me 3 C 6 H 2 )B(2,4,6-Me 3 C 6 H 2 )}, E, [103] has an unsupported borirene ligand that interacts with the metal center through aB À Csingle bond, which is significantly lengthened by coordination to the metal center.T he bonding is best represented as aP t 0 metal center in which donation from aB À C s bond is supported by Pt to Bd ative bonding.Abase-stabilized diborane(5) complex of Cu, F, [104] features alengthened BÀB single bond that forms a s-interaction with the metal (Scheme 8). As for the other EÀE s-bond complexes,D FT calculations (OLYP/TZ2P) show significant s-donation from the B À Bb ond to the metal center, accompanied by backdonation to vacant B À Bo rbitals.W hile undoubtedly ac omplex where aB À Bs ingle bond interacts with am etal center, this unusual single bond shows significant p-character.C onsequently,itisperhaps not as immediately clear whether this is at rue s-complex rather than an analogue of Chatt-Dewar bonding of an alkene (C = C···M).

Dynamics and s-CAM involving agostic interactions
We begin our survey of examples of the s-CAM mechanism with agostic interactions because they were not included in the 2007 review and provide aw idespread and productive extension to the concept. (Weu se the half-arrow symbol for agostic interactions,S cheme 2.) [10] Thec entral portion of the s-CAM mechanism is the dynamic interchange with three bisphosphines (a) dppp (Ph 2 PCH 2 CH 2 CH 2 PPh 2 ); (b) dcypp (Cy 2 PCH 2 CH 2 CH 2 PCy 2 ); (c) (PCy 3 ) 2 .[ BAr F 4 ] À anions not shown. [92] of partners between two s-bond ligands.Dynamic rearrangements are typical of agostic complexes,but we need to select examples involving the partner interchange characteristic of the s-CAM mechanism.
Thep rotonation of the ruthenium complex 5 with [H-(OEt 2 ) 2 ][BAr F 4 ]inTHF (Scheme 9) yields the cyclometalated dihydrogen cation 5-THF.Ifthe reaction is performed under dihydrogen, the agostic intermediate 5-H may be isolated. This complex loses H 2 reversibly to form the product. DFT calculations (B3PW91/RECP/6-31G(d,p)) indicated that this reaction is triggered by transfer of the agostic aromatic hydrogen to form the bis-dihydrogen complex via asingle TS. Thus the overall reaction conforms perfectly to a s-CAM mechanism. [105] Reaction with HBAr F 4 under D 2 results in exchange of Hfor Datthe ortho positions of the phenyl ring, consistent with this mechanism with the added step of phenyl rotation. This reaction may be considered as am odel for the mechanism of the Murai reaction which is catalyzed by Ru(H) 2 (H 2 ) 2 (PR 3 ) 2 . [106][107][108] Thea gostic platinum complex 6 is cyclometalated at the aromatic ring but transforms into ac omplex cyclometalated at the alkyl group 8 on reaction with L = SOMe 2 (Scheme 10). This reaction is postulated to proceed via the isomer 7 in which the alkyl group is cyclometalated and the aromatic ligand forms the agostic interaction, which is then displaced by SOMe 2 .T his isomer 7 lies only 25 kJ mol À1 above 6 according to DFT calculations (OPBE, triple z,CoSMO). [109] Thec onversion of 6 to 7 may be an example of an intramolecular s-CAM reaction if this is connected by asingle TS. A s-CAM mechanism was also postulated for the "rollover" [110] reaction of 6 on prolonged reaction with Me 2 SO in which the phenyl pyridine ligand transforms from C,N to C,C coordination, but no details were provided.
Then ature of this rollover process can be understood from the gas-phase study of the rollover and loss of methane from [Pt(bpy)(SMe 2 )(CH 3 )] + when subject to collisioninduced dissociation conditions. [111a] Scheme 11 shows the s-CAMp athway proposed from DFT calculations (B3LYP, TZVP) involving formation of an agostic pyridyl group prior to a s-methane complex, but an oxidative addition-reductive elimination pathway lies close in energy.
Asimilar s-CAM rollover mechanism has been proposed involving an agostic complex and ad ihydrogen complex at ruthenium. [112] This mechanism is supported by EXSY NMR experiments showing exchange between the hydride and appropriate CH protons.
Thecomplex [Ru(dppe) 2 Me][OTf] also exhibits an agostic interaction with aphenyl from dppe that positions the agostic interaction trans to the methyl group.T his complex undergoes cyclometalation with loss of methane.T he reaction proceeds via a s-CAM mechanism involving isomerization to place the agostic interaction cis to the methyl group followed by conversion to am ethane complex and loss of methane (Scheme 12). [113] This mechanism is supported by the presence of NOE interactions between the methyl group and ortho phenyl protons and by DFT calculations (BSLYP/LANL2DZ or MO6-L/QZVPPD or PBE/QZVPPD).
Ar elated mechanism has been postulated for the reversible double C À Ha ctivation of ethers by (k 4 -N,N',N'',C-Tp tol' )Ir(Ph)(N 2 )r equiring formation of a s-complex with the ether followed by an agostic complex involving coordination of one of the ligand tolyl arms. [114] When Ru(D) 2 (D 2 ) 2 (PCyp 3 ) 2 (see section 2.1) is left for af ew days in C 6 D 6 or is pressurized with 3atm D 2 ,H /D exchange occurs,r esulting in endo-selective incorporation of deuterium in the 3-and 4-positions of the cyclopentyl rings,as shown by both NMR spectroscopy and single-crystal neutron diffraction. DFT calculations (B3PW91/RECP/6-31G(d,p)) show that H/D exchange is initiated by isomerization from trans-to cis-phosphines,followed by dihydrogen loss.The C À Ha ctivation step at the C3 or C4 position of the ring leads first to an agostic complex, then to acyclometalated complex with the hydrogen transferred to form an ew dihydrogen ligand, thus retaining the Ru II state throughout. Exchange with D 2 completes the process,w hich overall is fully consistent with a s-CAM mechanism. [16] Ther ole of agostic interactions in s-CAM mechanisms involved in catalytic H/D exchange process has been inves-tigated at Ir III centers. [115] Using the precatalyst Ir(COD)Cl-(NHC) (NHC = N-heterocyclic carbene) and primary sulfonamides as substrates,s elective ortho aryl H/D exchange occurs on addition of D 2 .Acomputational study (M06/6-31G(d)) shows that this operates through asequence of steps, at constant Ir III oxidation state,a fter initial addition of D 2 to the Ir I center and reduction of the cyclo-octadiene ligand (not shown). Ther esulting dideuteride (Figure 9) has an agostic interaction between an ortho-aryl CÀHa nd the Ir III center. This agostic complex connects to an h 2 -D 2 /aryl hydride intermediate via TS1 which moves one s-interaction (agostic C À H) to another (D 2 )w ithout exchange of partners.Alow energy H/D exchange via TS2 (structure shown in inset) results in a s-CAM partner interchange and the formation of h 2 -HÀDand IrÀD. Afinal exchange of s-bonded interactions (TS3)leads to selective installation of an ortho C À Dbond on the substrate.

Alkane CÀH s-bond complexes in combination with other sbond complexes
Thed egenerate interchange reaction of [M(CH 3 )] + with methane is the simplest possible reaction to test the s-CAM mechanism and has been investigated in the gas phase by mass spectrometry and by computation (DFT with B3LYP/ TZVP) for metals of groups 8, 9a nd 10. An exceptionally clear-cut example with both experimental and computational evidence for a s-CAM process is provided by the hydrogenolysis of an Scheme 12. Cyclometalation of dppe at Ru via s-CAM (only phenyl groups involved in the transformation are shown), adapted from ref. [113]. iridium methyl pincer complex. [39] Dihydrogen adds to [Ir-(PONOP)(CH 3 )H] + 9 (PONOP = 2,6-bis(di-t-butylphosphinito)pyridine) at its vacant site at À1008 8 Ct og enerate ad ihydrogen complex 10 which is in equilibrium with the precursor.O nw arming, these species are replaced by the dihydride complex 11 with evolution of methane (Scheme 13). However,i ft he reaction is performed with D 2 , exchange is observed at À908 8 Ci nto the coordinated methyl group and the terminal hydride at equal rates,p roviding decisive evidence for an equilibrium between 10 and the isomeric methane complex 12.I tw as possible to estimate by experiment both the barrier to exchange between 10 and 12 and the barrier to loss of methane and formation of 11.T his process was modelled by DFT (PBE0/6-311G**) successfully as direct conversion of 10 to 12 with no intermediate (i.e. as ingle TS) with ab arrier within 1.3 kJ mol À1 of the observed barrier, entirely consistent with a s-CAM mechanism. [39] TheC À Ha ctivation of benzene by [Pt(CH 3 )(2,2'-bpy)] + (13 a)h as been studied mass spectrometrically in the gas phase using collisioninduced dissociation (CID) methods including use of deuterated isotopologues.I tu ndergoes dissociation of methane at low collision energies without cyclometalation. DFT calculations with the mPW1k functional favor the s-CAM pathway and explain the H/D exchange behavior (Scheme 14). This example illustrates the extension of the s-CAM concept to include the scoordination of aC À Hbond of an arene,here benzene itself. Thebenzene is initially coordinated in a p fashion (13 b)and then moves to s-coordination (13 c/13 d,b arrier 71 kJ mol À1 ) before isomerizing to the h 2 -C,H-methane complex (13 e)and finally losing methane (barrier 60 kJ mol À1 ). [116] However,use of another functional (M05-2X) makes the s-CAM barriers essentially the same as the oxidative-addition/reductive elimination barrier.
Scheme 13. Hydrogenolysis of an iridium methyl hydride complex with experimental energetics and activation barriers. [39] Scheme 14. Dissociation of methane from Pt(bpy)(CH 3 )] + by CID in gas phase. Note formation of s-complex with benzene. [116] 3.3. Silane and germane s-bond complexes in combination with other s-bond complexes One of the simplest reactions of silanes involving the s-CAMm echanism is shown in Scheme 15. [118] Here,t he 16electron RuH(H 2 )species reacts with hydrosilane (HSiMe 2 Cl, HSiMeCl 2 or HSiCl 3 )toform aRu(H 2 )(SiMe 3Àn Cl n )product. Thecrystal structure and DFT calculations (B3PW91) for the Ru(H 2 )(SiMeCl 2 )product provide strong evidence for aclose approach of one hydrogen to silicon (a SISHA interaction). Thec alculations show that the h 2 -silane isomer RuH(h 2 -HSiMe 3Àn Cl n )l ies very close in energy to the dihydrogen complex and may even be preferred in one rotamer of the SiMe 2 Cl complex. When the initial reaction is performed with the RuD(D 2 )c omplex, the product is Ru(HD)(SiMe 3Àn Cl n ). All these observations are consistent with the s-CAM sequence shown in Scheme 15.
In aw ell characterized sequence,t he T-shaped cyclometalated platinum cation 14 reacts with primary silanes to form aC ÀSiÀPt linkage 16 via two intermediates,c haracterized at low temperature (Scheme 16). [119] Thefirst intermediate, 15 a contains an h 1 -SiH 3 Rgroup;itexhibits an SiH proton with large couplings to 29 Si and 195 Pt and has been characterized crystallographically for R = Ph and by NMR spectroscopy for R = n Bu. In the second intermediate, 15 b,the CÀ SiH 2 Rbond has formed and there is an h 2 -SiH link to Pt (also observed for R = n Bu). In the next intermediate, 15 c charac-terized by DFT (M06/SDD/6-31G(d,p)/SMD), at rans-cis isomerization has occurred before a s-CAM step yields ad ihydrogen complex,15 d.F inally,H 2 is lost to form the product. According to the calculations,the conversion of 15 a to 15 b involves an oxidative addition to form aP t IV intermediate and should not be termed a s-CAM mechanism, although this intermediate lies in av ery shallow minimum. Theconversion 15 c to 15 d is a s-CAM reaction according to the calculations.
Thec obalt agostic ethyl cations,[ Cp*Co(CH 2 CH 2 -m-H)-(L)] + (L = PMe 3 ,P(OMe) 3 )are isolable as [BAr F 4 ] À salts and undergo reaction with dihydrogen to form well-characterized h 2 -H 2 complexes and with silanes to form h 2 -silane complexes. Thea gostic salts also catalyze alkene hydrogenation and hydrosilation. Thec ombination of these three groups of scomplexes led the authors to propose that the catalytic hydrogenation is enabled by a s-CAM mechanism in which the agostic cation is converted first to adihydrogen cation and then an h 2 -ethane cation, before ethane is lost to create ac oordinatively unsaturated hydride cation (Scheme 17 a). Similarly,h ydrosilation is initiated by formation of an h 2silane complex followed by an ethane complex generating the unsaturated silyl cation and ethane (Scheme 17 b). In catalytic polymerization of alkenes,r elated mechanisms may also feature in chain transfer or termination by reaction with dihydrogen or silane. [117] Ag ermane s-complex was first characterized crystallographically in 2003, [120] and further examples are reviewed in ref. [121].I nc omparison to silicon, there is as hift from scoordination toward oxidative addition. A s-CAM mechanism has been postulated in the reaction of Ru(H) 2 (H 2 ) 2 -(PCy 3 ) 2 with GeH 2 Ph 2 to form ag ermylene complex. [122]

B À H s-bond complexes in combination with other s-bond complexes
We have already shown that 3-and 4-coordinate boranes have arich coordination chemistry associated with s-complex formation. They also undergo aw ide variety of bond activation processes in which s-CAM mechanisms operate, and these are discussed in this section. We highlight three examples where s-CAM mechanisms clearly operate,t hat cover BÀHa ctivation. Diboranes,s uch as R 2 BÀBR 2 also undergo bond activation processes where s-CAM mechanisms may operate. [123] Ac oordinatively unsaturated cationic Pt II complex related to that in Scheme 16, provides av ery well-defined system where s-CAM mechanisms operate in B À Cand C À H bond formation processes. [124] An initial complex Similar B À Ha ctivation/B À Cb ond-forming reactivity has been reported at Pt II centers,this time by sequential s-CAM steps in ad ehydrogenative benzylic borylation reaction (Scheme 20). [125] The1 6-electron Pt II complex A,P t(k 2 -P, N)(h 3 -benzyl) (PN = N-phosphinoamidinate) reacts with HBpin to cleanly afford Pt(k 2 -P,N)(h 3 -PhCH(Bpin)), B with loss of H 2 .C omputational studies on the reaction pathway (M06/def2-TZVP//M06/LANL2DZ[6-31G**]) indicate the initial formation of a s-bond HBpin complex, INT1,t hat undergoes a s-CAM process (TS1)with an h 1 -benzyl group to form the Pt II s-boratoalkane complex INT2.Afurther s-CAMp rocess (TS2)r esults in the formation of aw eakly Scheme 17. Cobalt s-complexes and their s-CAM interconversion in initiation of (a) hydrogenation, (b) hydrosilation. [117] Scheme 18. Kinetic and thermodynamic products in the reaction of [Pt(I t Bu i Pr')(I t Bu i Pr)][BAr F 4 ]with HBpin leading to BÀCb ond formation. [124] Scheme 19. Variant of s-CAM mechanism with E' occupying the central position. Experimentally,e xchange is shown to proceed much faster for M = Ir. Computational studies (BP86/6-31G**) on the allhydrogen system show that interchange operates via a s-CAMm echanism ( Figure 11). [84] TS1 connects the starting amine-borane s-bond complex with ad ihydrogen base-stabilized boryl intermediate.A ne quivalent transition state then connects to the product, in which M À H and M À H À B have exchanged. While the metal center remains at aconstant oxidation state in the intermediates,t he TS has some M V character (a relatively short MÀHcontact for the transferring hydrogen). Consequently,t he barrier to exchange is calculated to be lower for 5d iridium than 4d rhodium, as observed experimentally.

s-bound ligands at nanoparticles
Thedefinitive characterization of chemisorbed molecules and atoms on surfaces is significantly more challenging than for molecular complexes.Despite this,there is growing direct evidence for s-bond complex formation at metal and nanoparticle surfaces [126] as we have discussed in section 2.2. Here we briefly discuss whether such species can also be implicated in s-CAM processes.
Ad etailed kinetic and spectroscopic analysis has led to the postulate that H/D exchange between H 2 and D 2 to form HD occurs on the surface of Ru metal nanoparticles via a, socalled, associative exchange (Figure 12 A). [127] This invokes a s-bond interaction of D 2 with am etal surface already covered in metal hydride groups.D ihydrogen/hydride transfer-presumably via 3-center D···D···H transition state-then leads to surface-bound HD,f ollowed by desorption into the gas phase.Analternative mechanism, based upon initial D À D bond scission to form surface-bound deuterides coupled with exchange mediated by fast surface diffusion, was discounted on the basis of careful kinetics measurements of the headspace of the HD that is formed, using gas-phase NMR spectroscopy. [127] While this "associative" mechanism captures the essential elements of a s-CAM process (s-bond intermediates bookending am etathesis process) details of the transition state that connects them are still to be determined. Thes ame groups have reported ac losely related H/ De xchange of the C À H bonds of cyclopentane on the surface of Ru nanoparticles with D 2 .U sing DFT calculations (PBE) on am odel Ru 13 H 17 system (Figure 12 B), this exchange is proposed to occur via s-alkane complex formation, followed by C À H activation that forms aR u ÀC bond reversibly. [52] While no details of the bonding mode of the released hydrogen were disclosed, it is tempting to speculate that as urfacebound h 2 -H 2 species is possible,w hich would then characterize the exchange process as a s-CAM process.For both Scheme 20. Dehydrogenative benzylic borylationv ia two sequential s-CAM steps. [125]

Assessment of extensions of the original s-CAM concept
In this section, we summarize and assess the extensions to the original s-CAM concept that we have highlighted in the preceding sections providing cross-references to the appropriate schemes and figures.

Additional element-hydrogen and element-element bonds
In our original review,w ep rovided evidence for the operation of s-CAM for H À H, C À H, Si À Ha nd B À Hb onds. Thec urrent review provides some evidence for h 2 -Ge À H bonds in s-CAM processes (sections 3.3). Theinvolvement of BÀHb onds is now seen to include the h 2 ,h 2 -B,H-BH 3 L( L= amine) coordination as well as the h 2 -B,H-BHR 2 ( Figure 11).
Given the clear evidence of s-coordination of ZnÀH, AlÀH and Ga À H( section 2.5), we can anticipate further examples of s-CAM involving groups 12 and 13. Similarly,the isolation of s-E À E' complexes (E À E' = B À C, C À C, Si À C, B À B; section 2.6) provides abasis for the potential involvement of EÀ Eb onds in s-CAM processes.

Agostic complexes
In section 3.1, we provided several examples illustrating s-CAMreactions resulting in conversion of an agostic complex to acomplex with an h 2 -EÀHbond (Schemes 9-12, Figure 9)

Extension to coordination of sp 2 -carbon
Thee xamples of s-CAM mechanisms involving C À H bonds were originally confined to sp 3 -carbon with alkyl groups or alkanes.T he demonstration of comparable mechanisms involving agostic phenyl groups demonstrates that the principle can be extended to sp 2 -carbon in the form of (h 1 -H,C-aryl) (Schemes 9-13 in section 3.1). There are numerous examples of isolable M(h 2 -C,C-arenes) [128][129] and some of early metal M(k 1 -F, C-arene) coordination. [130]

4.4.-Extension to surfaces
In section 3.5, we summarized evidence for reactions at nanoparticle surfaces involving s-complexation ( Figure 12). Given the encouraging clues above,w esuggest that s-CAM processes are possible in EÀHbond activation that occurs at surfaces.

Critical overview
Them ost compelling examples of s-CAM highlighted in this review include ac ombination of extensive experimental Figure 12. s-complexes on Ru nanoparticle surfaces in exchange processes. (A) "Dissociative" and the favored "Associative" mechanisms for H/D exchange between H 2 and D 2 .Reproducedw ith permission from ref. [127].( B) CalculatedC ÀHa ctivation pathway for cyclopentane activation at Ru 13 H 17 .A dapted, with permission,f rom ref. [52]. and computational evidence.Good experimental evidence for the s-complexes and their interconversion by s-CAM processes constrains the computational studies to realistic energy landscapes.O nce suitably calibrated, computation identifies transition states and associated barriers that link the component intermediates of the reaction pathway.S uch examples that include the characteristics of interchange of s-partners at constant oxidation state through asingle TS are illustrated in Schemes 11,13,[16][17][18] Figures 11, 12.
Delineation of the s-CAM pathway benefits from the ability to isolate (or just detect, either directly through experiment or indirectly through computation) the s-complex intermediates and their interconversion for late transition metals,e specially with d 6 or d 8 configurations.T he s-CAM mechanism shares with oxidative cleavage/reductive coupling and 1,2-addition (Scheme 1) the focus on reaction intermediates,r ather than transition states.I nc ontrast, the s-bond metathesis mechanism, so characteristic of d 0 configurations, does not require reaction intermediates.A lthough sigma complexes may be spotted as shallow minima in DFT calculations at d 0 (summarized by ref. [131]), they are rarely, if ever, detected experimentally.
s-Bond metathesis and 1,2-addition are not confined to d 0 systems.T he role of polar metal-heteroatom bonds with alone pair in 1,2-addition has been analyzed for d 0 , d 6 and d 8 configurations and compared to s-bond metathesis or oxidative addition to metal-centers without asuitable heteroatom. Thec omparisons lead to the headline conclusion that facile metal-mediated 1,2-C À Haddition requires astrong s-accepting orbital on the metal and ap olar M À Xb ond that has substantial electron density.T hus 1,2-additon should be considered as an alternative to s-CAM when these features are present. [132] Another question arises when the possibility of a s-CAM mechanism is discussed:w hen will oxidative cleavage/reductive coupling be preferred to s-CAM?O ne approach to answering this question is to compare the energies of scomplexes to their oxidative cleavage analogues.T wo examples demonstrate the subtlety of this issue.T he s-methane complex of [Rh(PONOP)] + lies below the corresponding rhodium methyl hydride complex. Fori ridium, the reverse is true:the iridium s-methane complex lies at higher energy as revealed by EXSY NMR experiments. [38,133] Alkane (ethane and methane) and alkyl hydride complexes of CpRe(CO) 2 and Cp*Re(CO) 2 have been detected directly by TRIR spectroscopy in supercritical methane and ethane. [134] In this example,t he s-complex and the oxidative-cleavage product are at equilibrium. Thep osition of equilibrium shifts toward the alkane complex with the Cp analogue and with use of ethane instead of methane.T hus small changes may induce ac hange in the ground state structure and/or barriers to kinetically accessible bond activation/formation partners.I n turn, these changes will affect the mechanistic path taken.
Thec apability of investigating the structure of transition states by computational methods has led to alternative definitions of mechanism based on transition-state geometries that are unobservable using experimental methods. However,t hese definitions do not restrict the identity of the intermediates on either side of the TS.S uch mechanistic nuances that are relevant to the current review include metalassisted s-bond metathesis (MAsBM), [135,136] oxidativelyadded transition state (OATS), [137] oxidative hydrogen migrations (OHM) [138,139] and ligand-to-ligand hydrogen transfer (LLHT). [140] There is av ery close relationship of several of these mechanisms to one another that may be differentiated by BadersAtoms in Molecules (AIM) methods. [141] Afurther study covering awide range of electron configurations shows that there is ac ontinuum of charge-transfer stabilization during C À Hactivation of methane from electrophilic through to nucleophilic.Moreover,t his applies regardless of whether the mechanism is oxidative cleavage or s-bond metathesis. [142] Most importantly,avariety of transition states are compatible with the s-CAM mechanism, since s-CAM defines the intermediates and not the TS.T he corollary of this principle is that aparticular bond activation process can correspond to both s-CAM (defined by s-complex intermediates) and one defined by the TS.
To illustrate this concept, the MAsBM mechanism is aversion of the s-CAM mechanism in which the TS maintains some bonding between the hydrogen undergoing transfer and the metal, as well as the donating and accepting atoms. [135,136] TheO ATSmechanism can also be considered as aversion of the s-CAM mechanism in which the TS corresponds to oxidative cleavage. [137] Unlike the preceding mechanisms,OHM and LLHT were recognized in systems in which a p-bonded ligand with sp 2 or sp C À Hbonds undergoes metal-mediated hydrogen transfer. OHM is aC À Ha ctivation mechanism identified in catalytic hydroarylation that links an sp 2 C À Hb ond of ac oordinating benzene or alkene to an alkyl ligand through aTSinwhich the Hisbonded to the metal, but not to either donor or acceptor carbon. [138,139] Afurther related mechanism is ligand-to-ligand hydrogen transfer (LLHT). Thec haracteristics of the LLHT transition state are very weak M···H interaction and aC donor ···H···C acceptor angle close to 1808 8.T he transfer can therefore be viewed as ap roton transfer between the two ligands at the transition state. [140] In combination with an entering and leaving ligand, LLHT becomes am etathesis process.T his mechanism was first classified for the hydrofluoroarylation of an alkyne.A fluoroarene coordinates to an ickel(II) alkyne complex to form a s-C À Hc omplex which transfers hydrogen directly to the alkyne without an intervening hydride,g enerating an agostic vinyl group ( Figure 13). In other examples,h ydroarylation of nickel(0) alkene generates an agostic alkyl nickel(II) product by LLHT. [143,144] Although the central process of LLHT can be viewed as ap roton exchange,t here may be ac hange in oxidation state.T his contradiction is apparent rather than real since the proton-accepting alkyne or alkene ligand lies close to the metallacycle limit in its bonding,and so it can be considered as aNi II to Ni II process. In those cases where a s-complex lies either side of the LLHT transition state,w em ay also describe the overall metathesis mechanism as a s-CAM process.
An earlier example of LLHT may be the protonation of Pt(NN)(C 6 H 5 ) 2 generating [Pt(NN)(C 6 H 5 )(h 2 -C 6 H 6 )] + (NN = ArNÀCMeÀCMe=NAr, Ar = 2,6-Me 2 C 6 H 3 )w hich undergoes hydrogen exchange between C 6 H 5 and C 6 H 6 groups according to EXSY experiments,f ollowed by associative displacement of benzene. [145,146] Comparisons between these different mechanisms have also been explored by following the displacements of the centroids of localized molecular orbitals (CLMO) that can help to visualize the mechanisms in the traditional terms of electron pair movements,m ore commonly known as curly arrows. [147] Examples include the s-CAM mechanism of Scheme 16 and acontrasting example of OHM.
Thec entral interchange process in s-CAM involves stretching of the coordinated EÀHb ond and compression of the distance between E' and the transferring hydrogen atom. This can often be observed as adynamic interchange process by NMR spectroscopy.Important developments in computation now allow calculation of reactions dynamics,not just for the interchange but for the complete metathesis.R eaction trajectories for several metathesis mechanisms including s-CAM, oxidative addition/reductive elimination and s-bond metathesis of the types shown in Scheme 1h ave been calculated using structures calculated by DFT combined with quasi-classical dynamics. [131,148,149] In these calculations, the trajectory is calculated starting from the TS moving either toward product or in the reverse direction toward the precursor.T he authors recognize the limitation that their calculations do not include the effect of solvent. Their conclusion is that the minima defining s-complex intermediates may be skipped if the minima are shallow (as for instance in d 0 or d 6 s-complexes) and there is sufficient energy in the system. This principle is analogous to aperson skiing downhill who passes through as hallow dip and continues downhill without stopping. Alternative methods for molecular dynamics including explicit solvent have been explored for transfer hydrogenation. [150] New methodologies involving dynamics are set to enlighten us in the future about the relationship between the different mechanisms of Scheme 1. At present, we infer that mechanistic deductions are more reliable when made from ac ombination of experimental detection of reaction intermediates and calculation of potential energy surfaces.

Conclusions
In conclusion, the s-CAM concept represents an instructive approach to reaction mechanism that brings together metathesis reactions involving the formation of av ariety of metal-element bonds through partner interchange of s-bond complexes.I ti ss upported through experimental measurements and computational studies of stoichiometric and catalytic reactions that are becoming increasingly sophisticated. [147,150,151] Thek ey concept that defines am etathesis reaction as a s-CAM process is the presence of two s-bond complexes as intermediates.They must retain the metal in the same oxidation state and must be connected by a single transition state.T he nature of this transition state,h owever, does not define whether it is a s-CAM process or not. This definition allows for experimental and computational investigation of the intermediates,w hile allowing flexibility and nuance in the nature of the transition states.