Valence-Shell Charge Concentrations and Electron Delocalization in Alkyllithium Complexes: Negative Hyperconjugation and Agostic Bonding
Scherer, W and Sirsch, P and Shorokhov, D and McGrady, GS and Mason, SA and Gardiner, MG, Valence-Shell Charge Concentrations and Electron Delocalization in Alkyllithium Complexes: Negative Hyperconjugation and Agostic Bonding, Chemistry - A European Journal, 8, (10) pp. 2324-2334. ISSN 0947-6539 (2002) [Refereed Article]
In this paper we present the results of density functional theory (DFT) calculations on the ethyl ligand and some related organic moieties; we then proceed to consider a range of alkyllithium complexes studied by DFT calculations and high-resolution X-ray and neutron diffraction. Topological analysis of the charge density is used to follow changes in the electronic structure of the organic fragment. The charge concentrations (CCs) in the valence shell at the α and β atoms reveal faithfully the delocalization of the lone pair at the Cα atom or of the Li-C bonding electrons. Negative hyperconjugation is thus shown to arise from delocalization of the lone pair or the Li-C bonding electrons over the alkyl fragment, with depletion of the metal-directed charge concentration at Cα, and characteristic ellipticity profiles for the bonds involved in hyperconjugative delocalization. In the case of so-called lithium agostic complexes, we show that close Li ···H contacts are a consequence of this delocalization and further secondary interactions, with Li···H-C agostic interactions, playing only a minor role. The ellipticity profiles and the magnitude of the CCs at Cα provide a quantitative measure of the extent of delocalization, and show excellent agreement between experiment and theory.