INTRAMOLECULAR ELECTRON-TRANSFER FROM RU-II(EDTA)(2-) TO CO-III(NH3)(5)(3+) VIA NITROGEN HETEROCYCLE BRIDGING LIGANDS

A series of binuclear complexes of formula (EDTA)Ru(III)LCo(III)(NH3)( 5)(2+) (EDTA = ethylenediaminetetraacetate, L = pyrazine, 4,4'-bipyrid ine, 3,3'-dimethyl-4,4'-bipyridine, trans-1,2-bis(4-pyridyl)ethylene, 1,4-bis(4-pyridyl)butadyne) were prepared in aqueous solution by react ion of Ru-III(EDTA)OH2- with Co(NH3)(5)L(3+). The reduction potentials of the binuclear complexes were measured. Reactions of the binuclear complexes with ascorbic acid or dithionite result in the preferential reduction of Ru(III) to Ru(II) and produce (EDTA)Ru(II)LCo(III)(NH3)(5 )(+). The latter undergo intramolecular electron transfer from Ru(II) to Co(III) with rate constants (25.0 degrees C, I=0.20 M) 22.7+/-0.4, 0.64+/-0.02, 0.067+/-0.002, 0.21+/-0.01 and 0.039+/-0.001 s(-1) (in th e same order as above), The decrease in rate constant with increasing distance between metal centers is ascribed to the increase in solvent reorganization energy with increasing metal to metal distance. The reo rganization energies corrected for the solvent contribution (ellipsoid al cavity model) have values of 10.6, 10.5, 11.8, 10.5 and 10.9 kcal m ol(-1). Except for 3,3'-dimethyl-4,4'-bipyridine, it is suggested that the electron transfer is adiabatic. The results are compared with res ults from previous studies with (NC)(5)Fe(II)LCo(III)L(NH3)(5).