Energetics of metal-ligand multiple bonds. A combined solution thermochemical and ab initio quantum chemical study of M=O bonding in group 6 metallocene oxo complexes

In this paper, we report a synthetic; molecular structure, thermochemical, and ab initio Hartree-Fock/Moller-Plesset level study of bonding and bonding energetics in the group 6 metallocene oxo series Cp2Mo/(MeCp)(2)MO, M = Cr, Mo, W. Efficient, high-yield syntheses of the pairs Cp2MH2/(MeCp)(2)MH2, Cp2MCl2/(MeCp)(2)MCl2, and Cp2MO/(MeCp)(2)MO where M = Mo or W are reported. The molecular structure of (MeCp)(2)WO features a "bent sandwich" geometry with a W=O distance of 2.04(1) Angstrom and an average W-C(Cp) distance of 2.371(8) Angstrom. Thus, W-C(Cp) exhibits a similar to 0.07 Angstrom elongation over the corresponding distance in typical Cp2WX2 complexes and a W=O distance which appears to be elongated versus what might be expected for a formal triple bond. D(M=O) values obtained from (MeCp)(2)MO silanolytic (Me3SiCl, Me2SiI) batch titration calorimetry are very large: 110(11) kcal/mol (M = Mo) and 132(10) kcal/mol (M = W). The corresponding D(W-OTMS) value is determined to be 65(18) kcal/mol. Ab initio relativistic core potential calculations reveal significantly weakened M-Cp bonding versus that in the corresponding Cp2MCl2 compounds, weakened M=O bonding due to population of M-O st antibonding levels, and a pronounced accumulation of negative charge on the oxo ligand (consistent with observed nucleophilicity of these complexes). Calculated Cp(2)MCh and Cp2MO molecular geometries and D(M=O) values at the MP2 level are in favorable agreement with experiment. The D(M=O) and D(M-O) data provide significant insight into the chemistry of Cp2MO complexes, especially in regard to constraints on oxo transfer as well as oxametallacycle formation and scission processes.