teraction between the two ruthenium centers in the imidazolate case. This is valid if the electron mediation is accomplished with the -bond system of the ligand. Imidazolate ion, however, can possibly make use of its -bond system in a manner similar to halides and hydroxide ligands. The relatively fast net rate of electron transfer for -Ru^-Im-Co111-(k = 6 ± 1 s_1) may be a result of such a mechanism. Note that the rate is faster than the rate of electron transfer in similar binuclear complexes with other bridging N-heterocyclic ligands that have been studied.17 The -bond system of imidazolate anion can interact more effectively with a dacceptor orbital as in cobalt(III) than can many pyridine-type heterocycles. The possible use of the t-as well as the -bond system of imidazolate anion renders it a versatile ligand which can interact withand -donor and -acceptor metal ion orbitals.Acknowledgment. We thank Professor D. Shombert and Jersey Technical Electronics for help in setting up the electrochemical equipment. Financial support from the Research Corporation, the Rutgers University Charles and Johanna Busch Memorial Fund, the Biological Sciences Research Grant, and the Research Council is gratefully acknowledged. References and Notes(1) Eichorn, G. L, Ed. "Inorganic Biochemistry", Elsevier: New York, 1973; Vol. II.(2) The pKa for the ionization of the pyrrole hydrogen of free imidazole, ImH -Im" + H+, is 14.2-14.5 (see ref 5). In metal complexes of imidazole, the acidity of the pyrrole nitrogen increases resulting in a decrease in the pKa by as much as five units.

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