Reactions of H2 and R3SiH with electrophilic cobalt(III) alkyl complexes:: Spectroscopic characterization, dynamics, and chemistry of [Cp*Co(L)(H)(η2-H2)][B(ArF)4] and [Cp*Co(L)(H)(η2-HSiR3)][B(ArF)4]

Co(III) agostic alkyl complexes [Cp*Co(L)(CH2CH2-mu-H)][B(Ar-F)(4)] (CP* = C-5(CH3)(5), L = P(OCH3)(3), 1a, or P(CH3)(3), 1b; Ar-F = 3,5-(CF3)(2)C6H3) react with H-2 to yield ethane and trihydride complexes characterized as eta(2)-dihydrogen hydride species [Cp*Co(L)(H)(eta(2)-H-2)1[B(Ar-F)], 2a and 2b, in which there is rapid scrambling between the eta(2)-H-2 ligand and the terminal Co-H. Complexes 2a and 2b react with a variety of neutral donor ligands (L' = RCN, PMe3, P(OMe)(3), H2O, CH3OH) to yield [Cp*Co(L)(L')(H)][B(Ar-F)(4)] complexes. Reaction of silanes with either 1a,b in the presence of traces of water or 2a,b yields eta(2)-silane hydrides, [Cp*Co(L)(H)(eta(2)-HSiR3)][B(Ar-F)(4)]. Analysis of the dynamics of these species by NMR spectroscopy provides evidence for an extremely rapid process involving silyl migration between hydrogens and a slower process in which a cobalt-silyl eta(2)-H-2 complex is formed as an intermediate and results in hydrogen scrambling between the two diastereomers of [Cp*Co(L)(H)(eta(2)-H-SiHMePh)][B(Ar-F)(4)]. The structures and dynamics of 2a,b and the eta(2)-silane complexes imply that cleavage of the cobalt-alkyl bonds of la,b in hydrogenation and hydrosilation catalytic cycles occurs by the sigma-CAM (sigma-complex-assisted metathesis) process.