ESR of homo- and heteroleptic mono- and dinuclear tris(.alpha.-diimine)ruthenium radical complexes

A comprehensive ESR study of 11 mononuclear and 5 dinuclear singly reduced tris({alpha}-diimine)ruthenium(II) complexes of the general formulas ((L){sub 3}Ru){sup {sm bullet}+}, ((L)(bpy){sub 2}Ru){sup {sm bullet}+}, and ((bpy){sub 2}Ru({mu}-L)Ru(bpy){sub 2}){sup {sm bullet}3+} (bpy, 2,2{prime},-bipyridine; L, other {alpha}-diimine) shows a variety of g factors and spectral resolution. All paramagnetic species are true anion-radical complexes with little g anisotropy and relatively small but characteristically positive differences g(ligand radical) - g(complex). The variations correlate with the calculated properties of the ligands and with spectroscopic and electrochemical data for the diamagnetic precursor complexes. In particular, the g shifts depend (i) on the extent of metal-ligand interaction and (ii) on the energy differences between the singly occupied and neighboring unoccupied or completely filled orbitals. Virtually complete localization of the unpaired electron on the better {pi}-accepting ligand L has been established for the mono- and dinuclear heteroleptic systems, while fast spin exchange on the ESR time scale is evident from the ESR line width of all singly reduced homoleptic complexes.