INFLUENCE OF THE C5R5 (R=H OR ME) RING SUBSTITUENTS ON THE ELECTROCHEMISTRY OF (MO-2(C5R5)(2)(MU-SR')(2)) COMPLEXES (R'=ME, PH) AND ON THE REACTIVITY OF ELECTROGENERATED INTERMEDIATES WITH UNSATURATED SUBSTRATES

The effect of the C5R5 ring substituents on the electrochemical behavi or of various complexes possessing the {Mo2Cp'(2)(mu-SR)(2)} core (Cp' = C5H5 or C5Me5) and on the reactivity of electrogenerated intermedia tes with unsaturated substrates has been investigated. The increase of the electron density at the metal centers and the increase of the bul kiness of the rings resulting from the C5H5-->C5Me5 substitution produ ce various effects (shift of the redox potentials, ring-dependent geom etrical change, stabilization of Mo-NCX bonds (X=Me or O), stabilizati on of intermediates generated from the reduction of the compounds) dep ending on the nature of the complex considered. Comparison of the elec trochemistry of analogs in the C5H5 and C5Me5 series allows a better u nderstanding of the reduction mechanisms ofthe complexes with C5H5 rin gs: the C5H5-->C5Me5 substitution leads to the stabilization of interm ediates whose formation was not detected by CV in the C5H5 series, but could only be inferred from trapping experiments. The C5H5-->C5Me5 su bstitution also results in reactive sites showing a lower selectivity, since the sites with C5Me5 rings still bind electron-withdrawing liga nds (CO, CNR), but also ligands with mediocre pi-accepting capabilitie s (MeCN), which are not strongly retained at the metal centers for Cp' = C5H5.