ELECTROCHEMICAL AND STRUCTURAL CHARACTERIZATION OF CIS-[CP(CO)RU(MU-AS(C6H5)(2))](2) AND TRANS-[CP(CO)RU(MU-AS(C6H5)(2))](2), ISOMERS THAT UNDERGO 2-ELECTRON-TRANSFER OXIDATIONS

The structures and oxidations of the cis and trans isomers of the doub ly bridged dinuclear species [Cp(CO)Ru(mu-AsPh2)](2) (Ph C6H5) have be en studied by X-ray crystallography, electrochemistry, and IR and NMR spectroscopies. Each complex oxidizes in a single two-electron voltamm etric process, the E-1/2 values being -0.36 V for the cis isomer (1) a nd -0.30 V for the trans isomer (2) in CH2Cl2/0.1M [NBu4][PF6] (refere nced to ferrocene). These are apparently the first comparative redox p otentials published for cis and trans isomers of bridged dinuclear org anometallic complexes. Oxidation of 1 to 1(2+) was cleanly accomplishe d either by electrolysis or by oxidation of 1 by 2 equiv of ferroceniu m, allowing isolation of the dication. The split carbonyl absorptions in the IR spectra of 1(2+) (nu(co) = 2032, 2050 cm(-1)) are consistent with formation of a Ru-Ru bond in the oxidation reaction. The electro de reaction 1/1(+) + e(-) is much slower than the reaction 1(+)/1(2+) + e(-), implying that the metal-metal bond is formed in the former pro cess. This conclusion is supported by the observation that the inner-s phere activation barrier, Delta G double dagger, is about 8.5 kcal/mol , close to that (similar to 10 kcal/ mel) estimated for a one-electron oxidation involving formation and cleavage of a Ru-Ru bond. The elect ron-transfer (ET) activation barrier is higher in these Ru complexes t han in analogous Fe complexes, which are known to undergo large ET-ind uced changes in metal-metal bond lengths, most likely because M-M bond strengths are larger when M = Ru than when M = Fe.