Vibronic coupling in dicyano-complex-bridged mixed-valence complexes. Relaxation of vibronic constraints in systems with degenerate bridging-ligand and electron-transfer excited states

Intense near-infrared (NIR) absorption bands have been found in mixed-valence Ru(NH3)(5)(2+,3+) complexes bridged by trans-Ru(py)(4)(CN)(2) and cis-Os(bpy)(2)(CN)(2), epsilon(max) similar to 1.5 X 10(3) cm(-1) and Delta v(1/2) similar to 5 x 10(3) cm(-1) for bands at 1000 and 1300 nm, respectively. The NIR transitions implicate substantial comproportionation constants (64 and 175, respectively) characteristic of moderately strong electronic coupling in the mixed-valence complexes. This stands in contrast to the weakly forbidden electronic coupling of Ru(NH3)(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, Q(N), of the bridging ligand: H-DA = HDA(o) + bQ(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 bQ(N) is larger for D(NC)M but very small (bQ(N) similar to 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 = HDAHAM/E-ave independent of M and consistent with the earlier report. When E-DM congruent 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.