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) comple xes is reported, based on the reversible reduction of the following pr ecursors: [(mpz)Ru(NH3)5]3+ (mpz = N-methylpyrazinium), [(bpym)Ru(NH3) 4]2+(bpym = 2,2'-bipyrimidine), [(bptz)Ru(NH3)4]2+ and {(mu-bptz)[Ru(N H3)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]+ con firm unambiguously that the unpaired electron resides mainly in the pi system of the unsaturated ligand, however, the spin distribution and the Ru-99, Ru-101 isotope coupling reveal non-negligible contributions from the metal 4d orbitals to the singly occupied MO. The two other o ne-electron reduced compounds exhibit less well-resolved EPR spectra b ut are also Ru(II) complexes of radical ligands as judged by their g a nisotropy in comparison to the Ru(II) containing oxidized forms. Disti nctly different EPR characteristics of the oxidized and reduced forms support the MLCT formulation of low-lying excited states in the precur sor compounds. The effect of the Ru(II) ammine complex fragments on th e EPR characteristics of the organic radical ligands is compared to th at of other low spin d6 systems.