Synthesis, reactions, and molecular and electronic structure of the radical cation [Mo-2(mu-C(8)Me(8)) (eta-C5H5)(2)](+): An intermediate in the redox activation of an alkyl C-H bond

The reaction of [Mo-2(mu-C(8)Me(8))(eta-C5H5)(2)] (1) with 1 equiv of [Fe(eta-C5H5)(2)][PF6] in CH2Cl2 gives [Mo-2(mu-C(8)Me(8))(eta-C5H5)(2)][PF6] (1(+)[PF6](-)), which reacts with a second 1 equiv of [Fe(eta-C5H5)(2)][PF6] or 1 equiv of the trityl radical, .CPh(3), to give [Mo-2(mu-C(8)Me(7)CH(2))(eta-C5H5)(2)](+) (2(+)); depending on the oxidant the activation of one C-H bond of 1, with the formation of 2(+), can occur by an EC or EEC mechanism. Complexes 1 and 1(+) constitute the first isolable redox pair to show the structural effects of one-electron oxidation on a metal-alkene bond. An X-ray crystal structure analysis shows that the geometry of 1(+) is similar to that of 1 and has approximate C-s, symmetry. The Mo-Mo distance in 1(+) is consistent with the presence of a Mo=Mo double bond. The C-8 chain acts as a double mu-allylidene ligand while binding to Mo(2) as an eta-alkene through C(4) and C(5). The Mo-C distances for atoms C(1), C(2), C(3), C(6), C(7), and C(8) of the C-8 chain are remarkably similar in 1 and 1(+) while the Mo(2)-C(4) and Mo(2)-C(5) distances are significantly increased and the C(4)-C(5) distance is decreased in 1(+) compared with that of 1. These observed structural changes are consistent with a model for the bonding in the paramagnetic cation in which the unpaired electron occupies an orbital of a'' symmetry which is largely localized on Mo(2) and involved in pi-back-bonding with the alkene function of the C(8)Me(8) ligand in the classic Dewar-Chatt-Duncanson manner. This conclusion is supported by detailed NMR and ESR spectroscopic studies on 1 and 1(+) and by EHMO calculations.