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Hypervalent MoleculeA hypervalent molecule is a molecule that contains one or more typical elements (group 1, 2, 13-18) formally bearing more than eight electrons in their valence shells. PCl5, SF6, ICl2-, and I3- are examples of hypervalent molecules. Hypervalent molecules were first defined by Jeremy I. Musher in 1969 as molecules of the elements of group 15-18 in any oxidation state other than the lowest. Since the hypervalent molecules do not seem to obey the octet rule, some models have been proposed to describe their bonding properties. At first, hypervalent bonding was believed to be described as dsp3 and d2sp3 hybrid orbitals composed of s, p, and d-orbitals at higher energy levels. However, advances in the study of ab initio calculations have revealed that the contribution of d-orbitals to hypervalent bonding is too small to describe the bonding properties, and this hybrid orbital description is now regarded as much less important. As another description of hypervalent molecules, modifications of the octet rule have been attempted to involve ionic characteristics in hypervalent bonding. As one of these modifications, in 1951, the concept of the 3-center-4-electron (3c-4e) bond, which described hypervalent bonding with a qualitative molecular orbital, was proposed. The 3c-4e bond is described as three molecular orbitals given by the combination of a p orbital on a central atom and two ligand orbitals: an occupied bonding orbital, an occupied non-bonding orbital (HOMO), and an unoccupied anti-bonding orbital (LUMO).
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