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

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 Cp 2 Mo/(MeCp) 2 MO, M = Cr, Mo, W. Efficient, high-yield syntheses of the pairs Cp 2 MH 2 /(MeCp) 2 MH 2, Cp 2 MCl 2 /(MeCp) 2 MCl 2, and Cp 2 MO/(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) 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 Cp 2 WX 2 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 (Me 3 SiCl, Me 3 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 Cp 2 MCl 2 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 Cp 2 MCl 2 and Cp 2 MO 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 Cp 2 MO complexes, especially in regard to constraints on oxo transfer as well as oxametallacycle formation and scission processes.