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-valen ce 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 coupl ing in the mixed-valence complexes. This stands in contrast to the weakly f orbidden 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 c oupling between a cyanide-bridged, donor-acceptor pair, D-(CN-)-A, alters t he properties of the bridging ligand. Such systems are described by a "vibr onic" 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, sys tems is treated as the consequence of the perturbational mixing of the "loc al", 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 tha t 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 transitio ns, E-DM and E-DA, differ significantly, a perturbation theory treatment. r esults in H-DA = HDAHAM/E-ave independent of M and consistent with the earl ier report. When E-DM congruent to E-DA, configurational mixing of the exci ted states leads to H-DA proportional to H-DM, consistent with the relative ly intense intervalence bands reported in this paper. Some implications of the model are discussed.