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 f irst isolable redox pair to show the structural effects of one-electro n oxidation on a metal-alkene bond. An X-ray crystal structure analysi s shows that the geometry of 1(+) is similar to that of 1 and has appr oximate C-s, symmetry. The Mo-Mo distance in 1(+) is consistent with t he 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 t he 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 occu pies an orbital of a'' symmetry which is largely localized on Mo(2) an d 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.