EPR characteristics of radical complexes with coordinated ammineruthenium(II) fragments. Evidence for the metal-to-ligand charge transfer (MLCT) nature of the low-lying excited states in precursor complexes

The first EPR study of one-electron reduced ammineruthenium(II) complexes is reported, based on the reversible reduction of the following precursors: [(mpz)Ru(NH3)5]3+(mpz =N-methylpyrazinium), [(bpym)Ru(NH3)4]2+(bpym = 2,2′-bipyrimidine), [(bptz)Ru(NH3)4]2+ and {(µ-bptz)[Ru(NH3)4]2}4+[bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine]. The partially resolved EPR spectra of [(mpz·)-Ru(NH3)5]2+ and [(bptz)Ru(NH3)4]+ confirm unambiguously that the unpaired electron resides mainly in the π system of the unsaturated ligand, however, the spin distribution and the 99,101Ru isotope coupling reveal non-negligible contributions from the metal 4d orbitals to the singly occupied MO. The two other one-electron reduced compounds exhibit less well-resolved EPR spectra but are also RuII complexes of radical ligands as judged by their g anisotropy in comparison to the RuIIIcontaining oxidized forms. Distinctly different EPR characteristics of the oxidized and reduced forms support the MLCT formulation of low-lying excited states in the precursor compounds. The effect of the RuII ammine complex fragments on the EPR characteristics of the organic radical ligands is compared to that of other low spin d6 systems.