High-frequency EPR and structural data as complementary information on stable radical complexes containing the semi-reduced azo function

Dinuclear metal compounds [(LnM)(μ-BL−)(MLn)]k involving radical anions of bridging ligands BL such as 2,2′-azobispyridine or 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine and coordinated inorganic or organometallic complex fragments MLn containing copper(I), rhenium(I), ruthenium(II), osmium(II), rhodium(III) or iridium(III) have been investigated in frozen solutions using high field EPR spectroscopy at W band (95 GHz) or higher frequencies. The generally small but still rather variable anisotropy of the g factor shows typical patterns reflecting the specific metal/ligand interactions, the competition between bridging and ancillary ligands for spin density, and the spin-orbit coupling constants of the metals. The EPR data are thus shown to specify the structural data as obtained from X-ray crystallography for a standard series of diphosphinocopper(I) complexes involving the radical ligands indicated. The (RNNR)− group can be placed next to the related superoxide (OO)− as a paramagnetic ligand with specific geometrical and electronic structure.