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)(2)](2)}(n) with bpy = 2,2'-bipyridine and adc-R = azodicarbonyl ligands O=C(R)-N=N-C(R)=O, R = NR'(2) (piperidyl), OC2H5, OCH2C6H5, CH3, C6H5, 4-C6H4COOH, and 4-C6H4COOCH3, can exist in several oxidation states (n = 2-, 0, 2+, 3+, 4+) of which the stable (comproportionation constant K-C > 5 x 10(7)) and isolable paramagnetic intermediates (n = 3+) exhibit a considerable and remarkably tunable degree of metal/Ligand/metal orbital mixing. The occurrence of intense (epsilon > 7000 M(-1) cm(-1)) and hardly solvatochromic absorption bands at about 1500 nm, the temperature-dependent magnetic moment (mu(eff) = 1.6-2.1 mu(B)) determined for one derivative, and the IR vibrational spectra did not allow us to make a conclusive determination of metal oxidation states; however, the EPR spectra observable only below 50 K reveal a strongly substituent-(R-) dependent degree of metal contribution to the singly occupied MO. XPS measurements of two 3+ ions which show Ru(II) and Ru(III) signals also exhibit a marked substituent effect on the electronic structure. Whereas-the 4+ ions formed at rather positive potentials also seem to possess strongly mixed frontier orbitals, the stable 2+ ions clearly contain two Ru(II) centers and fully reduced, i.e. 1,2-dicarbonylhydrazido(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 metal/metal communication in ligand-bridged mixed-valent dimers according to which the intermediates {(adc-R)[Ru(bpy)(2)](2)}3+ are best described as delocalized systems with varying contributions from Ru-II(adc-R)(.-)/Ru-II and Ru-II/(adc-R)(2-)/Ru-III resonance forms.