Structure of W(CH3)6

Abstract

Letters from: [ Clark R. Landis, et al. ][1] [ K. Seppelt ][1] The exquisite solid-state structural characterizations of W(CH3)6 and Re(CH3)6 reported recently by Valerie Pfennig and Konrad Seppelt (Reports, [2 Feb., p. 626][2]) challenge our understanding of the forces controlling molecular structures. For metal complexes containing only alkyl or hydride ligands, the existence of structures that appear to violate valence shell electron pair repulsion (VSEPR) conventions is particularly intriguing. Application of the simple concept of orbital hybridization, first espoused by Linus Pauling 65 years ago ([1][3]) and recently revised by us ([2][4]), results in (i) the prediction of the observed C 3v -distorted trigonal prismatic geometry for W(CH3)6 and (ii) a robust model for rationalizing the shapes of homoleptic methyl compounds. Transition metals form covalent, two-electron bonds with hydride and alkyl ligands. For complexes with a valence orbital electron count of 12 or fewer electrons [such as the 12-electron W(CH3)6], we have shown that the hybridization of the metal center can be described as sd n −1, where n is the number of bonds plus lone pairs. Thus, W(CH3)6 exhibits sd5 hybridization at the metal center. The shapes of sd5 hybrids are such that orthogonality of hybrid orbital pairs occurs at angles of 63° and 117°. Four arrangements of the six ligands, two that have C 3v and two that have C 5v point group symmetry, are consistent with these angular preferences. For WH6, ab initio computations suggest that all four of these structures are distinct minima of roughly equivalent energy. One might expect the intermethyl steric effects of W(CH3)6 to favor the most open ( C 3v ) geometry and to distort the bond angles to larger values. Using a valence bond theory-based molecular mechanics method (VALBOND), we computed ([2][4]) a C 3v equilibrium geometry for W(CH3)6 (Fig. [1][1]). This structure exemplifies the essential attributes of the crystallographic structure, despite using only generic, rule-based parameters. Molecular dynamics simulations revealed a low energy motion corresponding to movement of the metal atom along the C 3 axis with a maximum of approximately 3.0 kilocalories per mole at the trigonal prismatic D 3h geometry. Our computational results are consistent with the gas-phase electron-diffraction data of Haaland and co-workers ([3][5]), the x-ray crystallographic results of Pfennig and Seppelt, and ab initio results recently communicated to us by M. Kaupp ([4][6]). ![Figure][7] Fig. 1. Schematic representation of the C 3v equilibrium structure for W(CH3)6. Average bond lengths (in picometers) and angles are shown for the VALBOND-computed structure (upper numbers) and for the crystallographic structure (lower numbers, in parentheses). 1. 1.[↵][8] 1. L. Pauling , J. Am. Chem. Soc. 53, 1367 (1931). [OpenUrl][9][CrossRef][10] 2. 2.[↵][11] 1. C. R. Landis, 2. T. Cleveland, 3. T. K. Firman , ibid. 117, 1859 (1995). [OpenUrl][12] 3. 3.[↵][13] 1. A. Haaland, 2. A. Hammel, 3. K. Rypdal, 4. H. V. Volden , ibid. 112, 4547 (1990). [OpenUrl][14] 4. 4.[↵][15] 1. M. Kaupp , ibid. in press. # {#article-title-2} Response : It is a pleasant surprise to learn that, with a simple model, the shape of the W(CH3)6 molecule [and also qualitatively that of Re(CH3)6] can be precisely predicted and that a similar model can predict an ab initio result ([1][3]). For skeptics, it may be stressed that these predictions were published ([2][4]) or submitted ([1][3]) before our Science paper appeared and that neither of the two scientists had previous knowledge about our crystallographic work. I apologize to Landis for overlooking his recent theoretical publication on this subject ([2][4]). 1. 1. 1. M. Kaupp , J. Am. Chem. Soc. in press. 2. 2. 1. C. R. Landis, 2. T. Cleveland, 3. T. K. Firman , ibid. 117, 1859 (1995). 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