THE STRUCTURE OF HEXAMETHYLTUNGSTEN, W(CH3)(6) - DISTORTED TRIGONAL PRISMATIC WITH C-3 SYMMETRY

Ab initio and density functional calculations show that the equilibriu m structure of hexamethyltungsten is a distorted trigonal prism of C-3 symmetry (with local C-3v symmetry for the WC6 skeleton). A regular p rismatic D-3 structure (with D-3h skeleton) is found to be ca. 20 kT m ol(-1) higher in energy at correlated levels of theory. It is a transi tion state connecting two C-3 minima. These results extend a recent ga s-phase electron diffraction study which favored a regular prismatic s tructure but could not rule out a distortion to C-3v The failure of a previous theoretical study to locate the distorted minimum is due to t he neglect of electron correlation and to some other restrictions duri ng the structure optimizations. Correlation is important, e.g. for the description of hyperconjugative ''agostic'' C-H-->W interactions whic h are found to be pronounced in W(CH3)6 Structures optimized with grad ient-corrected or hybrid density functionals, or at the MP2 level, des cribe these interactions well. The observed single-line C-13 and H-1 N MR spectra are explained by dynamic motions due to the low D-3 inversi on and methyl rotation barriers. C-13 chemical shifts calculated using density functional theory differ by ca. 18 ppm between the two nonequ ivalent sets of methyl groups in the distorted trigonal prismatic stru cture. Low-temperature NMR experiments could be useful to confirm this value and thus the distortion. Harmonic vibrational frequency analyse s are consistent with experimental results and have been used to chara cterize stationary points on the potential energy surface. Differences between the structural preferences of W(CH3)(6) and WH6 are investiga ted via detailed bonding analyses.