Electrochemistry of [Mo2Cp2(CO)(4){mu-n(2): eta(3)-HC C-C(R1)(R2)}](+) complexes (R1 = H, R2 = H, Me, Et, Fc; R1 = Me, R2 = Me, Ph). Control of the reduction process (two-electron vs one-electron) by the substituents R1 and R2: EHMO rationalization

The electrochemical reduction of [Mo2Cp2(CO)(4){mu-eta(2):eta(3)-HC=C-C(R1)(R2)}](+) complexes has been investigated by cyclic voltammetry, controlled-potential electrolysis, and coulometry. On the cyclic voltammetry time scale, the complexes with R1 = H, R2 = H (1(+)): Me (2(+)), Et (3(+)) undergo an irreversible or a quasi-reversible one-electron reduction whereas the analogues with R1 = H, R2 = Fc (4(+)) and R1 Me, R2 = Me (5(+)) and Ph (6(+)) reduce in a single-step, reversible or quasi-reversible, two-electron process. Two different chemical reactions are involved in the overall reduction mechanism. The first chemical step is assigned as a structural rearrangement, responsible for slowing down the heterogeneous electron transfer. Extended Huckel MO calculations indicate that in the case of the complexes with R1 = H, R2 = Pc and R1 = Me, R2 = Me or Ph, a small increase in the distance between one metal center and the carbon of the C(R1)(R2) group could trigger the two-electron transfer process. The second chemical reaction leading to the final product(s) of the reduction involves radical species, even when a two-electron transfer is observed by cyclic voltammetry. The final products formed in these processes have been identified either by H-1 NMR spectroscopy of the compounds extracted from the catholyte after controlled-potential electrolyses or from a comparison of their characteristic redox potentials with those of authentic samples. The nature of the final product(s), either a dimer or mu-alkyne and mu-enyne complexes, is also dependent on the nature of R1 and R2.