A Simple Electrostatic Model for Trisilylamine: Theoretical Examinations of the nfσ* Negative Hyperconjugation, p π fd π Bonding, and Stereoelectronic Interaction Yirong Mo,* ,²,‡ Yongqing Zhang, ‡ and Jiali Gao* ,‡ Contribution from the Department of Chemistry and State Key Laboratory for Physical Chemistry on Solid Surface, Xiamen UniVersity, Xiamen 361005, People’s Republic of China, and Department of Chemistry and Center for Computational Research, State UniVersity of New York at Buffalo, Buffalo, New York 14260 ReceiVed February 16, 1999. ReVised Manuscript ReceiVed April 22, 1999 Abstract: A block-localized wave function method was used to examine the stereoelectronic effects on the origin of the structural difference between trisilylamine and trimethylamine. The pyramidal geometry of trimethylamine along with its high basicity is consistent with the traditional VSEPR (valence shell electron- pair repulsion) model for σ bonding. On the other hand, in trisilylamine, the silicon d orbitals make modest contribution to the electronic delocalization, although the key factor in charge delocalization is still n N fσ SiH * negative hyperconjugation. Interestingly, the gain in p π fd π bonding stabilization is offset by a weaker negative hyperconjugation effect in trisilylamine, resulting in an overall smaller delocalization energy (-18.5 kcal/ mol) than that in trimethylamine (-23.9 kcal/mol), which contains little p π fd π bonding character. Significantly, because of the relatively low electronegativity of silicon, the N-Si bond is much more polar than the N-C bond. Weinhold’s natural population analyses of the BLW and HF wave functions for these compounds reveal that the origin of the planar geometry of trisilylamine is due to the polar σ-effect that yields significant long- range electrostatic repulsion between the silyl groups. In addition, it was found that only the most electronegative substituents such as F and OH can result in a pyramidal geometry at the nitrogen center for silylamines. This is in good accord with the recent X-ray structure of a pyramidal silylamine, N(CH 3 )(OCH 3 )(SiH 3 ). Introduction The notion that p π fd π donation is responsible for the unusually short bond distances of second-row elements of main groups is ubiquitous in the literature and textbooks. 1-10 For example, the short sulfur-oxygen bond distance in SO 4 2- is attributed to the donation of the lone-pair electrons of oxygen to the vacant 3d atomic orbitals of the sulfur atom. Similar explanations can be found to illuminate the structures of PO 4 3- , SiO 4 4- , and ClO 4 - . However, recent theoretical analyses cast serious doubt on the d-orbital hypothesis of main group elements. 5-9 Kutzelnigg 5 showed that the X-O bond in H 3 PO, H 2 P(O)F, H 2 SO, etc. can be described with the traditional valence s- and p-type orbitals by allowing these atomic orbitals to deform, whereas Reed and Weinhold’s natural population analyses revealed that the occupancy of the sulfur d function is very small. 6 Reed and Schleyer 7 further demonstrated that nfσ* negative hyperconjugation 11 is important in controlling hyper- valency, and that the d orbitals on the central atom play only a secondary role in π-bonding. By optimizing the d-function exponents in various molecules of first- and second-row elements systematically, Magnusson 8a was able to show that d functions of second-row atoms are polarization functions in nature. These functions do not take a valence role, a conclusion ² Xiamen University. ‡ State University of New York at Buffalo. (1) See, for example, (a) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and ReactiVity, 4th ed.; Harper Collins College Publishers: New York, 1993; p 871. (b) Sharpe, A. G. Inorganic Chemistry, 3rd ed.; Longman Scientific & Technical: London, 1992; p 298. (c) Lee, J. D. Concise Inorganic Chemistry, 4th ed.; Chapman & Hall Ltd: London, 1991; p 456. (d) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 269. (e) Jolly, W. L. Modern Inorganic Chemistry; McGraw-Hill: New York, 1984; p 128. (f) Ebsworth, E. A. V. Volatile Silicon Compounds; Macmillan: New York, 1963; p 113. (2) (a) Fessenden, R.; Fessenden, J. S. Chem. ReV. 1961, 61, 1. (b) Nagy, J.; Hencsei, P. J. Organomet. Chem. 1972, 38, 261. (c) Shea, K. J.; Gobeille, R.; Bramblett, J.; Thompson, E. J. Am. Chem. Soc. 1978, 100, 1611. (d) Livant, P.; McKee, M. L.; Worley, S. D. Inorg. Chem. 1983, 22, 895. (e) Hallet, J.-F.; Hoffman, R.; Saillard, J.-Y. Inorg. Chem. 1985, 24, 1695. (3) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (4) For example: (a) Janes, N.; Oldfield, E. J. Am. Chem. Soc. 1986, 108, 5743. (b) Dixon, D. A.; Smart, B. E. J. Am. Chem. Soc. 1986, 108, 7172. (c) Nakamura, S.; Takahashi, M.; Okazaki, R.; Morokuma, K. J. Am. Chem. Soc. 1987, 109, 4142. (d) Schmidt, M. W.; Truong, P. N.; Dordon, M. S. J. Am. Chem. Soc. 1987, 109, 5217. (e) Streitwieser, A., Jr.; McDowell, R. S.; Glaser, R. J. Comput. Chem. 1987, 8, 788. (f) Dibbs, K. D.; Boggs, J. E.; Barron, A. R.; Cowley, A. H. J. Phys. Chem. 1988, 92, 4886. (g) Angyan, J. G.; Poirier, R. A.; Kucsman, A.; Csizmadia, I. G. J. Am. Chem. Soc. 1987, 109, 2237. (5) (a) Kutzelnigg, W. Angew. Chem., Int. Ed. Engl. 1984, 23, 272. (b) Wallmeier, H.; Kutzelnigg, W. J. Am. Chem. Soc. 1986, 108, 2804. (6) Reed, A. E.; Weinhold, F. J. Am. Chem. Soc. 1986, 108, 3586. (7) (a) Reed, A. E.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 1434. (b) Reed, A. E.; Schleyer, P. v. R. Inorg. Chem. 1988, 27, 3969. (c) Reed, A. E.; Schade, C.; Schleyer, P. v. R.; Kamath, P. V.; Chandradekhar, J. J. Chem. Soc., Chem. Commun. 1988, 67. (8) (a) Magnusson, E. J. Am. Chem. Soc. 1990, 112, 7940. (b) Magnusson, E. J. Am. Chem. Soc. 1993, 115, 1051 (9) (a) Cruickshank, D. W. J.; Eisenstein, M. J. Mol. Struct. 1985, 130, 143. (b) Cruickshank, D. W. J. J. Mol. Struct. 1985, 130, 177. (c) Cruickshank, D. W. J.; Eisenstein, M. J. Comput. Chem. 1987, 8, 6. (10) Weinhold, F.; Carpenter, J. The Structure of Small Molecules and Ions; Plenum Press: New York, 1988; p 227. (11) Schleyer, P. v. R.; Kos, A. J. Tetrahedron 1983, 39, 1141. 5737 J. Am. Chem. Soc. 1999, 121, 5737-5742 10.1021/ja9904742 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/03/1999