Hypervalent bond Subject

It is obvious that studying interligand Si-H interactions has reached a great extent of sophistication. At least three classes of nonclassical Si-H bonding can be identified. These are the electron-deficient residual Si H interactions in silane a-complexes and agostic complexes, electron-rich IHI MH SiX, and the more recent multicenter H Si interactions, which are the subject of current debate and have features common to both IHI and a-complexes. This surprising diversity stems from the special role the substituent at silicon can play in tuning the extent of Si H interaction, and from the propensity of silicon to be hypervalent. 

Hypervalent carbocations have received some attention this year.14 The concept of three-centre, two-electron bonding in these entities is supported by a topological bifurcation analysis of the electronic structure of CH5 15 this and the related species CHg+ and CH3+ are also the subject of a review.16 The CHj + H2 reaction has been studied theoretically.17... 

The classical explanation for the increased coordination or valency of sulphur is its use of atomic d orbitals in molecules to form more hybrid orbitals for bonding than can be formed from just s- and p-type orbitals10. The dominant thinking on this subject today11 is that d-type orbitals provide needed spatial flexibility12 for bonding molecular orbitals that are formed even without the d orbitals (in theoretical descriptions or calculations, for example). The d orbitals in hypervalent sulphur are needed for quantitative accuracy and have not been found to be required for the qualitative electronic structure description13. 

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