DONOR-ACCEPTOR ELECTRONIC COUPLING AS A FUNCTION OF BRIDGING GROUP - MIXED-VALENCE DIRUTHENIUM(II,III) COMPLEXES BRIDGED BY ISONICOTINATO AND ISONICOTINAMIDO LIGANDS

Binuclear, mixed-valence mu-isonicotinato and mu-isonicotinamido complexes capped by pentaammineruthenium(II) at the pyridine site and pentaammineruthenium(III) at the carboxylato or amido site have been synthesized and characterized. On the basis of their electronic absorption spectra and redox properties, the amido complexes are assigned as the N-bonded (Ru(III)-NHC(O)R) isomers. The mixed-valence complexes exhibit intervalence charge-transfer bands at 720 nm (epsilon = 2.6 x 10(2) M-1 cm-1) and 761 nm (0.9 x 10(3) M-1 cm-1), respectively, in aqueous acetate buffer at pH 4.8. The redox potentials of the two sites differ by 0.44 V in the isonicotinato and by 0.54 V in the isonicotinamido complexes in the above medium, and the donor-acceptor coupling elements, evaluated from the intensities of the intervalence bands, are 300 and 510 cm-1, respectively. Solvent dependences of the electronic spectra and electrochemical parameters are also reported. Both bridges provide significant coupling between the Ru(II) and Ru(III) metal centers in the mixed-valence complexes. These coupling energies serve as a point of departure for a consideration of coupling mechanisms in related polyproline-bridged systems for which thermal electron-transfer rates have been determined. It is concluded that "hole" transfer pathways predominate when osmium(II) pentaammine is the donor and cobalt(III) or ruthenium(III) pentaammine is the electron acceptor.