Vibronic Coupling in Dicyano-Complex-Bridged Mixed-Valence Complexes. Relaxation of Vibronic Constraints in Systems with Degenerate Bridging-Ligand and Electron-Transfer Excited States

摘要

Intense near-infrared (NIR) absorption bands have been found in mixed-valence Ru(NH_3)_5~(2+,3+) complexes bridged by trans-Ru(py)_4(CN)_2 and cis-Os(bpy)_2(CN)_2, #epsilon#_(max) approx1.5X10~3 cm~(-1) and #DELTA#_(v1/2) approx 5X10~3 cm~(-1) for bands at 1000 and 1300 nm, respectively. The NIR transitions implecate substantial comproportionation constants (64 and 175, respectvely) characteristic of moderately strong electronic coupling in the mixed-valence complexes. This stands in contrast to the weakly forbidded electronic coupling of Ru(NH_3)_5~(2+,3+) couples bridged by M(MCL)-(CN)_2~+ complexes (MCL=a tetraazamacrocyclic ligand) (Macatangay; et al. J. Phys. Chem.1998, 102, 7537). A straightforward perturbation theory argument is used to account for this contrasting behavior. The electronic coupling between a cyanide-bridged, donor-acceptor pair, D-(CN~-)-A, alters the properties of the bridging ligand. Such systems are described by a “vibronic” model in which the electronic matrix element, H_(DA), is a function of the nuclear coordinates, O_N, of the bridging ligand: H_(DA)=H_(DA)+bO_N. Electronic coupling in the dicyano-complex-bridged, D-[(NC)M(CN)]-A, systems is treated as the consequence of the perturbational mixing of the “local”, D(NC)M and M(CN)A, vibronic interactions. If M is an electron-transfer acceptor, then the nuclear coordinates are assumed to be configured so that bO_N is larger for D(NC)M but very small(bO_N approx 0) for M(CN)A. When the vertical energies of the corresponding charge-transfer transitions, E_(DM) and E_(DA), differ significantly, a perturbation theory treatment results in H_(DA)=H_(DA)H_(AM)/E_(ave) independent of M and consistent with the earlier report. When E_(DM)ident to E_(DA), configurational mixing of the excited states leads to H_(DA) proportional to H_(DM), consistent with the relatively intense intervalence bands reported in this paper. Some implications of the model are discussed.