ELECTROCHEMISTRY OF [MO2CP2(CO)(4)(MU-N(2) - ETA(3)-HC-C-C(R1)(R2))](- R1 = ME, R2 = ME, PH) - CONTROL OF THE REDUCTION PROCESS (2-ELECTRONVS ONE-ELECTRON) BY THE SUBSTITUENTS R1 AND R2 - EHMO RATIONALIZATION() COMPLEXES (R1 = H, R2 = H, ME, ET, FC )

The electrochemical reduction of Mo2Cp2(CO)(4){mu-eta(2):eta(3)-HC=C-C (R1)(R2)}](+) complexes has been investigated by cyclic voltammetry, c ontrolled-potential electrolysis, and coulometry. On the cyclic voltam metry 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 q uasi-reversible, two-electron process. Two different chemical reaction s are involved in the overall reduction mechanism. The first chemical step is assigned as a structural rearrangement, responsible for slowin g down the heterogeneous electron transfer. Extended Huckel MO calcula tions 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 o ne metal center and the carbon of the C(R1)(R2) group could trigger th e two-electron transfer process. The second chemical reaction leading to the final product(s) of the reduction involves radical species, eve n when a two-electron transfer is observed by cyclic voltammetry. The final products formed in these processes have been identified either b y H-1 NMR spectroscopy of the compounds extracted from the catholyte a fter controlled-potential electrolyses or from a comparison of their c haracteristic redox potentials with those of authentic samples. The na ture 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.