When Is an Odd-Electron Dinuclear Complex a Mixed-Valent Species? Tuning of Ligand-to-Metal Spin Shifts in Diruthenium(III,II) Complexes of Noninnocent Bridging Ligands OC(R)NNC(R)O

The complexes { (adc-R)[Ru(bpy)z]z}" with bpy = 2,2'-bipyridine and adc-R = azodicarbonyl ligands O=C(R)-N=N-C(R)=O, R = NR'2 (piperidyl), OC~HS, OCH2C&, CH3, C~5,4-C6C however, the EPR spectra observable only below 50 K reveal a strongly substituent(R-) dependent degree of metal contribution to the singly occupied MO. X P S measurements of two 3+ ions which show Ru(I1) and Ru(1II) signals also exhibit a marked substituent effect on the electronic structure. Whereas the 4 f ions formed at rather positive potentials also seem to possess strongly mixed frontier orbitals, the stable 2+ ions clearly contain two Ru(I1) centers and fully reduced, i.e. 1,2-dicarbonylhydrazid0(2-) bridging ligands, [O=C(R)-N-N-C(R)=O]2-. The results are interpreted within a hole vs electron transfer scheme, based on a three-site MO model for the mewmetal communication in ligand-bridged mixed-valent dimers according to which the intermediates { (ad~-R)[Ru(bpy)2]2}~+ are best described as delocalized systems with varying contributions from Ru"/(adc-R)'-/Ru" and Run/(adc-R)2-/Run1 resonance forms.