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).

Antiferromagnetic exchange constants were calculated by using the Mulliken-Hush treatment for metal-ligand coupling elements (J. Photochem. Photobiol. A: Chem. 1994, 82, 47) and the valence bond model of antiferromagnetic exchange (Inorg. Chem. 1993, 32, 2850, from the spectral data of the solvent-dependent ligandto-metal charge-transfer bands of [{(NH 3 ) 5 Ru} 2 (µ-L)] 4+ complexes, where L is a substituted 1,4-dicyanamidobenzene dianion derivative. These calculated values were compared to the corresponding experimental exchange constants that were estimated from the complexes' solvent-dependent room-temperature magnetic moments. The correlation between these values is quite good, and this in turn implies that a relatively unsophisticated level of theory in conjunction with spectroscopy may be all that is necessary to predict trends in molecular properties derived from frontier orbitals.

The complexes, [(NH3)5Ru(L)][C104]2, where L = cyanamide, phenylcyanamide, 4-cyanamidobiphenyl, 1-cyanamidonaphthalene, 2-cyanamidonaphthalene, 2-cyanamidophenanthrene, and 1-cyanamidopyrene anions, were synthesized and characterized by cyclic voltammetry and electronic absorption spectroscopy. The Ru(HI/ II) couple was shown to shift positively with increasing conjugation of the group attached to the cyanamide moiety and indicated withdrawal of cyanamide electron density onto the conjugated group. Extended Hueckel calculations of the free anion ligands permitted estimates of transition dipole moment lengths, R, for the bi*bi transition of the cyanamide complexes. Only an approximate positive correlation was shown between oscillator strength and R* 12.

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