CHEMICAL BONDING IN HYPERVALENT MOLECULES - THE DOMINANCE OF IONIC BONDING AND NEGATIVE HYPERCONJUGATION OVER D-ORBITAL PARTICIPATION

Does sulfur form six covalent bonds in CH3SO2Cl, F3S≡N or carbon or phosphorus five bonds in F3C=O-, F3P=O? After a brief history of the viewpoints on hypervalent bonding and a comparison of analysis methods (with CH3SO2Cl as example), natural population and natural bond orbital analysis is applied to a series of 32-valence-electron species of X3AY type (CF4, F3NO, O3C1F, O3PS3-, F3SN, etc.). The σ-bonding in these systems is found to be significantly ionic, and the strongly polar σ*AX orbitals are found to be more effective electron acceptors than the extra-valence dπ(A) orbitals. Negative πY → σ* typehyperconjugation, which results in πAY bonding, is thus the primary contributor to xAY bonding in X3AY species, with πY → dπ(A) overlap secondary. However, the d orbitals serve to polarize the orbitals. This enhances irAY bonding and diminishes σ*AX antibonding interactions and must beincluded to obtain a qualitatively correct description. The strength of πAY bonding increases along the series F3SiF, F3PO, F3SN, F3ClC, but the latter species, as well as F3IC and F3TeC-, are found computationally to be incapable of existence. By generalizing our discussion to n-coordinate 8n-valence-electron species (HF2-, BF3, ClO4-, F4SO, F5TeO-, IF6+, OXeF6, etc.) and their “reduced” analogues that have one or more lone pairs on the central atom (SF4, IF6-, ClF3, etc.), we provide a classification of hypervalent (and many nonhypervalent) molecules. The simple, qualitative bonding concepts for hypervalent molecules developed here supercede the inaccurate and misleading dsp3 and d2sp3 models that are still in widespread use. © 1990, American Chemical Society. All rights reserved.