ORBITAL ANALYSIS OF METAL-TO-LIGAND CHARGE-TRANSFER AND OXIDATION IN (NH3)(5)RUL(2- EFFECTIVE T(2G) ORBITAL ORDERING AND THE ROLE OF LIGAND-PI AND LIGAND-PI-ASTERISK ORBITALS() COMPLEXES )

Both metal-to-ligand charge transfer (MLCT) and oxidation (ionization) processes have been investigated for (NH3)(5)RuL(2+) complexes with L = pyridine, pyrazine, and protonated pyrazine. Calculations were carr ied out using ZINDO-95. Self-consistent-field molecular orbitals (MO's ) were obtained at the restricted Hartree-Fock level for the closed-sh ell ground state (t(2g))(6) (NH3)(5)RuL(2+) species. The MLCT wave fun ctions and excitation energies were obtained by configuration interact ion among all singlet configurations generated by single excitations f rom the highest 11 occupied MO's to the lowest 11 unoccupied MO's. We find that the Ru(II) d(pi) orbital is the HOMO for all three complexes . The results provide clear indication that the relative energies for removing an electron from the three types of t(2g) orbital depend on t he effective distance that the electron is transferred. For relatively long-distance transfers (e.g., greater than or equal to 5 Angstrom as for intra- or intermolecular thermal or optical electron transfer to a weakly coupled Ru(III) site), the lowest energy process involves the d,orbital. In contrast, for the shorter range MLCT process, for which the electron interacts with the newly created hole to an extent deter mined by the relevant Coulomb and exchange integrals, the d(pi) orbita l participates in the highest energy of the three possible t(2g) --> L (pi) excitations. Accordingly, to the extent that the electronic coup ling element for thermal or optical metal-to-metal electron transfer i s enhanced by superexchange of the ''electron'' type involving the d(p i) orbital, the relevant MLCT intermediate state is not the lowest ene rgy but rather the highest energy one. It also follows that an electro n from the same symmetry orbital (d(pi)) is transferred in both optica l and thermal electron transfer processes involving the Ru(II)-Ru(III) couples considered.