Synthetic, Structural, and Theoretical Studies of η2-Acyl Complexes of Molybdenum

Article abstract of DOI:10.1021/ja00267a026

Hydridotris(pyrazolyl)borate (Tp) complexes of the type TpMo(CO)2(η²-COR) (R = Me (1), Ph (2) are formed from the reaction of TpMo(CO)3^- with MeI, Me3O⁺, MeCOBr, or PhCOBr.The use of 13C-labeled PhC*OBr in the reaction established that a carbonyl initially on the metal is lost in the decarbonylation and that the η²-acyl is not in equilibrium with a low concentration of the isomeric ?-alkyltricarbonyl complex.The derivatives Tp(CO)LMo(η²-COMe) (L = P(OMe)3 (3) and PEt3 (4) are made from 1 and the appropiate phosphine.The structures of 1-4 were determined by X-ray crystallography: (1) a, b, and c = 8.995(3), 12.803(4), and 14.999(6) Angstroem; β = 103.98(3) deg; V = 1677(1) Angstroem³; z = 4; space group P2¹/n (no. 14); (2) a, b, and c = 9.405(2), 12.505(3), and 9.025(2) Angstroem; α,β, and γ = 114.59(2), 92.85(2), and 95.45(2) deg; V = 956.0(4) Angstroem³; Z = 2; space group = P1/ (no. 2); (3) a, b, and c = 8.953(2), 15.684(6), and 8.032(4) Angstroem; α,β, and γ = 99.47(4), 102.79(3), and 97.78(3) deg; V = 1067.5(7) Angstroem³; Z = 2; space group = P1/.Values of R₁ and R₂ for 1-4 are 0.045, 0.059 (1); 0.038, 0.063 (2); 0.038, 0.056 (3); and 0.44, 0.061 (4).The bonding of an L₅M(d⁴) fragment to the η²-acyl group, the rotational conformations of the latter, and the realative stabilities of Cp(CO)2Mo(η²-HCO),Cp(CO)3Mo-H, (N-donor)3(CO)2Mo(η²-HCO), and (N-donor)3(CO)3Mo-H are comprehensively treated within the framework of the Extended Hueckel MO formalism.It is shown that there is substantial double-bond character in the Mo=C(acyl) bond but a very weak Mo-O bond and that the compounds should be regarded as stabilized 16-electron complexes.There is an inherent tendency toward a bending distortion in the L₅M fragment, and η²-acyl group tends to be aligned with the axis of distortion and/or with the M-CO bond(s).