The electronic structure of pentaammine(pyrazine) ruthenium(II) and (III): the metal-ligand π conjugation and its implications in electron delocalization

Abstract

Results of electronic structure calculations on the title cations are reported. Our transition operator calculations have confirmed the original assignments of optical absorption transitions at 250 and 470 nm for the 2+ ion and have suggested assignments for transitions observed at 238 and 270 nm for the 3+ ion. Because the pyrazine ligand resides in an asymmetric environment, the identities and symmetry designations of the six π states of the free ligand are heavily obscured in these cations. The 2+ species exhibits substantial mixing of the Ru 4d xz orbital with a π * orbital of pyrazine, while the 3+ species does not. Conversely the 3+ ion shows some mixing of the 4d xz orbital with a π orbital of pyrazine, while the 2+ ion has only weak mixing of these orbitals. We find that the polarization of the pyrazine ligand is toward the metal ion, which is consistent with simple electrostatic predictions. The dependence of the π-bonding and backbonding strengths upon the electron occupation (or charge) of the metal species is an important feature of the ruthenium-pyrazine system. The electron density distribution within the pyrazine ligand also depends strongly on the charge. Implications for extended and complexed mixedvalence species are discussed.