ESR OF HOMOLEPTIC AND HETEROLEPTIC MONONUCLEAR AND DINUCLEAR TRIS(ALPHA-DIIMINE)RUTHENIUM RADICAL COMPLEXES

A comprehensive ESR study of 11 mononuclear and 5 dinuclear singly reduced tris(α-diimine)ruthenium(II) complexes of the general formulas [(L)3Ru.+, [(L)(bpy2Ru, and [(bpy)2Ru(μ-L)Ru(bpy2.3(bpy, 2,2’-bipyridine; L, other α-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 π-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. © 1990, American Chemical Society. All rights reserved.