Comparison of hole-transfer superexchange in dinuclear mixed-valence ruthenium complexes

Three dinuclear complexes [{mer-Ru(NH3)(3)(bpy)}(2)(mu-L)][ClO4](4), where L = 2,5-dimethyl-(Me(2)dicyd(2-)), 2,5-dichloro- (Cl(2)dicyd(2-)), and unsubstituted 1,4-dicyanamidobenzene dianion (dicyd(2-)), have been synthesized and characterized by magnetic resonance, electrochemical, and electronic absorption spectroscopic techniques. A crystal structure of [{mer-Ru(NH3)(3)(bpy)}(2)(mu-dicyd)][ClO4](4). 3H(2)O showed dicyd(2-) to be approximately planar with the cyanamido groups in an anti configuration. Crystal structure data are space group = P (1) over bar, with a, b, and c equal to 12.5613(1), 12.8738(1), and 16.3267(2) Angstrom, respectively, alpha, beta, and gamma equal to 76.756(1), 83.893(1), and 69.053(2)degrees, respectively, V = 2399.28(4) Angstrom(3), and Z = 2. The structure was refined using 6112 independent reflections with I > 2.5 sigma(I) to a final R factor of 0.0568. The strongly coupled mixed-valence complexes [{mer-Ru(NH3)(3)(bpy)}(2)(mu-L)](3+) where L = Me(2)dicyd(2-), dicyd(2-), and Cl(2)dicyd(2-) had decreasing comproportionation constants of 1.3 x 10(7), 9.3 x 10(6), and 3.5 x 10(5), respectively, which are consistent with a hole transfer superexchange mechanism for metal-metal coupling. The mixed-valence properties of these complexes together with analogous systems were compared in the context of a transformation from a localized to a delocalized mixed-valence state.