Characterizing the intrinsic stability of gas-phase clusters of transition metal complex dianions with alkali metal counterions: Counterion perturbation of multiply charged anions

The authors report the gas-phase generation and characterization of a series of cation-dianion clusters, e.g., M+·PtCl₆^2-, M+·PtCl₄^2-, M+·Pt(CN)₆^2-, and M+·Pd(CN)₄^2-, where M+=Na+,K+,Rb+, as model systems for investigating gas-phase contact ionpairs. Low-energy collisional excitation of these systems isolated within a quadrupole ion trap reveals that the fragmentation products are determined by the dianion and are independent of the counterion. This indicates that cation-dianion clusters represent gaseous ion-pair complexes, in line with recent findings for K+·Pt(CN)_n^2-, n=4,6 [Burke et al., J. Chem. Phys. 125, 021105 (2006)]. The relative fragmentation energies of several cation-dianion systems are obtained as a function of the counterion to explore the nature of ion-pair binding. For most of the systems studied, e.g., M+·PtCl₆^2-, the fragmentation energy increases as the cation size decreases, in line with a simple electrostatic description of the cation-dianion binding. However, the M+·Pt(CN)₄^2- clusters displayed the reverse trend with the fragmentation energy increasing as the cation size increases. Density functional theory calculations of the cation-dianion fragmentation potential energy surfaces reveal the existence of a novel double-minima surface, separated by a repulsive Coulomb barrierlike feature at short range. The experimentally observed trends in the fragmentation energies can be fully understood with reference to the computed surfaces, hence providing strong support for the existence of the double-minima surface.