Abstract
Total geometry optimizations are reported for Cr(CO)₆ , HMn(CO)₅ , Fe(CO)₅ , Ni(CO)₄ , Cr(C₆H₆)₂, Fe(C₅H₅)₂ , Ni(C₄H₄)₂, Cr(NO)₄, (C₅H₅)Mn(CO )₃, and (C₆H₆)Cr(CO)₃ . A variety of basis sets were examined, and, based on the results, a relatively compact and accurate basis set is proposed. The differences between the calculated and experimental metal-carbonyl, metal-benzene, and metal-nitrosyl bond distances average 0.03, 0.08, and 0.07 A, respectively. Calculated metalcyclopentadienyl bond lengths were found to be an average of 0.15 A longer than experimental bond lengths. Addition of electron correlation at the perfect pairing GVB level reduced the average difference in the metal-cyclopentadienyl bond length to 0.08 A. Optimization of the geometry of TiCl₄ and TiCl₃CH₃ at the self-consistent-field (SCF) level results in Ti-Cl bond lengths longer than the experimental values, even when d- and f-type polarization functions are added to the basis set. The bond lengths remain too long even as the Hartree-Fock limit is approached because the SCF level of theory over-estimates the noble-gas-like Cl...Cl repulsions, which hinder close Ti-Cl approach. The Ti-C-H angle of TiCl₃CH₃ is calculated to be close to tetrahedral geometry with little flattening of the hydrogen atoms, which apparently was observed in the electron diffraction. These same calculations do predict the anomalously low methyl-rocking frequency for the titanium complex in agreement with the experimental IR. This low methyl rocking frequency is due to stabilization of the Ti-C bond during the rocking motion by low lying empty d-orbitals on Ti. The large positive geminal hydrogen coupling constant observed in the NMR experiment is due primarily to the σ-donor and π-acceptor character of the TiCl₃ moiety and not to any flattening of the methyl group..
Williamson, Rodney Lowell (1989). Molecular orbital, generalized valence bond, and complete active space calculations on transition metal compounds with agostic hydrogen and bridging carbonyls : new basis sets, geometry of trichloromethyltitanium, and bonding of Bis(cyclopentadienyldicarbonylchromium). Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1048868.