Orbital specific charge transfer distances, solvent reorganization energies, and electronic coupling energies: Electronic stark effect studies of parallel and orthogonal intervalence transfer in (NC)₅Os(II)-CN- Ru(III)(NH₃)₅^-

For the mixed-valent chromophore, (NC)₅Os(II)-CN-Ru(III)(NH₃)₅^-, spin-orbit coupling and ligand-field asymmetry effects lead to multiple visible region intervalence (metal-to-metal) charge transfer transitions (Forlando et al. Inorg. Chim. Acta 1994, 223, 37). The higher energy transition is associated with transfer from an Os 5dπ orbital that is nominally orthogonal to the charge transfer axis. The lower energy transition, on the other hand, involves a degenerate pair of Os 5dπ donor orbitals directed along the charge transfer axis. Low-temperature electronic Stark effect measurements of the partially resolved transitions permit donor- orbital-specific one-electron-transfer distances to be directly evaluated. The distances, R, are remarkably dependent upon donor orbital orientation (R(parallel) = 2.8 ± 0.2 Å; R(orthogonal) = 4.0 ± 0.4 Å) and significantly shorter than simple geometric estimates (5.0 Å). From the distance information, donor-orbital-specific coupling energies and solvent reorganization energies can also be estimated. These also differ substantially from those obtained by equating the charge transfer distance with the geometric donor/acceptor separation distance.