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)₂MO, M = Cr, Mo, W. Efficient, high-yield syntheses of the pairs Cp₂MH₂/(MeCp)₂MH₂, Cp₂MCl₂/(MeCp)₂MCl₂, and Cp₂MO/(MeCp)₂MO where M = Mo or W are reported. The molecular structure of (MeCp)₂WO features a 'bent sandwich' geometry with a W=O distance of 2.04(1) Å and an average W- C(Cp) distance of 2.371(8) Å. Thus, W-C(Cp) exhibits a ~0.07 Å 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)₂MO silanolytic (Me₃SiCl, Me₃-SiI) 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 π antibonding levels, and a pronounced accumulation of negative charge on the oxo ligand (consistent with observed nucleophilicity of these complexes). Calculated Cp₂MCl₂ 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.