SYNTHESES, ELECTROCHEMISTRY, AND BONDING OF BIS(CYCLOPENTADIENYL)MOLYBDENUM ALKYL COMPLEXES - MOLECULAR-STRUCTURE OF MO(ETA-5-C5H5)2(C4H9)2 - THERMOCHEMISTRY OF MO(ETA-5-C5H5)2R2 AND MO(ETA-5-C5H5)2L (R = CH3, C2H5, C4H9, L = ETHYLENE, DIPHENYLACETYLENE)

Mo(Cp)2I2 (Cp = eta-5-C5H5) reacts with lithium alkyls to afford the new complexes Mo(Cp)2R2 (R = Et, Bu) in reproducible yields. Reaction of Mo(Cp)2Me2 with 1 equiv of HX gives the complexes Mo(Cp)2XMe (X = Cl, O2CPh). The mixed dialkyl Mo(Cp)2MeBu is prepared from Mo(Cp)2ClMe and LiBu, whereas EtMgBr reacts with Mo(Cp)2Me(O2CPh) to give Mo(Cp)2BrMe. Cyclic voltammetry studies on the dialkyls and halo alkyls show that they undergo one-electron oxidation. The ease of oxidation of these complexes decreases in the order Me2 > MeBu > Bu2 > Et2 > ClMe > BrMe. The corresponding 17-electron cations are prepared by oxidation of Mo(Cp)2R2 and Mo(Cp)2XMe with the ferrocenium cation. The possible existence of agostic interactions in alkyl derivatives of M(Cp)2 fragments (M = Ti, Zr, Mo) was investigated by extended Huckel molecular orbital calculations. These interactions do not appear to be favored in the 17-electron Mo(Cp)2Me2+. The ease of oxidation of the dimethyl complex is attributed to a relatively high energy HOMO caused by the methyl groups acting as pi-donors. A similar pi-donor interaction also explains the relative order of oxidation potentials found for Mo(Cp)2XMe (X = Cl, Br). The structure of the complex Mo(Cp)2Bu2 was determined by X-ray diffraction. The crystals are monoclinic, space group C2/m, with a = 2160.8 (7) pm, b = 786.3 (3) pm, c = 1955.2 (8) pm, beta = 100.65 (3)-degrees, V = 3264.7 x 10(6) pm3, and Z = 8. The butyl ligands lie almost in the plane bisecting the angle defined by the ring normals, the C-Mo-C angle being 76.6 (2)-degrees. Metal-carbon bond enthalpies in the complexes Mo(Cp)2R2 (R = Me, Et, Bu) and Mo(Cp)2L (L = C2H4, C2Ph2) were derived from reaction-solution calorimetry studies. The values for the dialkyl compounds, together with literature data, indicate that the differences DBAR(M-Me) - DBAR(M-higher alkyl) are nearly constant along the periodic table, in contrast with DBAR(M-H) - DBAR(M-alkyl) values, which are lower for more electropositive metals. The influence of these trends on the energetics of beta-elimination and olefin insertion reactions is discussed. The Mo-C2H4 bond dissociation enthalpy is about 60 kJ mol-1 lower than D(Mo-C2Ph2).