A quantitative study of hole-transfer superexchange in Class II mixed-valence complexes is presented. The free energy of resonance exchange was calculated from metal−metal coupling elements derived from Hush and CNS models and compared to experimental values that were factored from the free energy of comproportionation. The Hush model gave acceptable results for only the most weakly coupled systems while the CNS model gave reasonable predictions throughout the range of coupling studied (valence trapped to nearly delocalized behavior).

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The dinuclear Ru(III) complexes trans-[{(NH(3))(4)Ru(py)}(2)(&mgr;-L)][PF(6)](4), where py represents pyridine and L represents 1,4-dicyanamidobenzene dianion (dicyd(2)(-)) derivatives dicyd(2)(-) (1), Me(2)dicyd(2)(-) (2), Cl(2)dicyd(2)(-) (3), and Cl(4)dicyd(2)(-) (4), have been prepared and characterized by electronic absorption spectroscopy and cyclic voltammetry. A crystal structure of the complex trans-[{(NH(3))(4)Ru(py)}(2)(&mgr;-dicyd)][PF(6)](4).(1)/(2)H(2)O showed the dicyd(2)(-) ligand to be approximately planar with the cyanamido groups in a syn configuration. Crystal structure data are space group P2(1), with a, b, and c = 7.826(3), 20.455(7), and 14.428(5) Å, respectively, beta = 95.76 (3) degrees, V = 2296.7(14) Å(3), and Z = 2. The structure was refined by using 3292 reflections with I > 2.5sigma(I) to an R factor of 0.069. Solid state magnetic susceptibility measurements of the Ru(III)-Ru(III) dimers showed diamagnetic behavior at room temperature, and this is suggested to be due to strong antiferromagnetic superexchange via the HOMO of the dicyd(2)(-) ligand. The bridging ligand dependence of metal-metal coupling in the Ru(III)-Ru(II) complexes of 1, 2, 3, and 4 in acetonitrile solution was demonstrated by the trend in comproportionation constants, 1.5 x 10(6), 5.7 x 10(6), 1.4 x 10(4), and 1.1 x 10(3), respectively. In addition, comparison to the analogous pentaammineruthenium dimers showed that the magnitude of metal-metal superexchange could be controlled by the nature of the spectator ligand. Spectroelectrochemical methods were used to acquire the absorption spectra of the mixed-valence complexes, and the intervalence band properties were modeled with PKS theory. Metal-metal coupling in the Ru(III)-Ru(II) complexes of 1, 2, 3, and 4 was analyzed by using Hush and CNS theories.