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
L. Luo et al., 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, Journal of the American Chemical Society, 120(13), 1998, pp. 3111-3122
In this paper, we report a synthetic; molecular structure, thermochemi
cal, and ab initio Hartree-Fock/Moller-Plesset level study of bonding
and bonding energetics in the group 6 metallocene oxo series Cp2Mo/(Me
Cp)(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 wher
e M = Mo or W are reported. The molecular structure of (MeCp)(2)WO fea
tures a ''bent sandwich'' geometry with a W=O distance of 2.04(1) Angs
trom and an average W-C(Cp) distance of 2.371(8) Angstrom. Thus, W-C(C
p) exhibits a similar to 0.07 Angstrom elongation over the correspondi
ng distance in typical Cp2WX2 complexes and a W=O distance which appea
rs to be elongated versus what might be expected for a formal triple b
ond. D(M=O) values obtained from (MeCp)(2)MO silanolytic (Me3SiCl, Me2
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 i
s determined to be 65(18) kcal/mol. Ab initio relativistic core potent
ial calculations reveal significantly weakened M-Cp bonding versus tha
t 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 nucleo
philicity of these complexes). Calculated Cp(2)MCh and Cp2MO molecular
geometries and D(M=O) values at the MP2 level are in favorable agreem
ent with experiment. The D(M=O) and D(M-O) data provide significant in
sight into the chemistry of Cp2MO complexes, especially in regard to c
onstraints on oxo transfer as well as oxametallacycle formation and sc
ission processes.