Energetics of Metal−Ligand Multiple Bonds. A Combined Solution Thermochemical and ab Initio Quantum Chemical Study of MO Bonding in Group 6 Metallocene Oxo Complexes

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Abstract

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 WO distance of 2.04(1) Å and an average WC(Cp) distance of 2.371(8) Å. Thus, WC(Cp) exhibits a ∼0.07 Å elongation over the corresponding distance in typical Cp2WX2 complexes and a WO distance which appears to be elongated versus what might be expected for a formal triple bond. D(MO) values obtained from (MeCp)2MO silanolytic (Me3SiCl, Me3SiI) batch titration calorimetry are very large: 110(11) kcal/mol (M = Mo) and 132(10) kcal/mol (M = W). The corresponding D(WOTMS) value is determined to be 65(18) kcal/mol. Ab initio relativistic core potential calculations reveal significantly weakened MCp bonding versus that in the corresponding Cp2MCl2 compounds, weakened MO bonding due to population of MO 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(MO) values at the MP2 level are in favorable agreement with experiment. The D(MO) and D(MO) 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.

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