Expanded Octet - Bonding in Hypervalent Molecules

Bonding in Hypervalent Molecules

Early considerations of the structure of hypervalent molecules, returned familiar arrangements that were well explained by the VSEPR model for atomic bonding. Accordingly, AB₅ and AB₆ type molecules would possess a trigonal bi-pyramidal and octahedral geometry, respectively. However in order to account for the observed bond angles, bond lengths and apparent violation of the Lewis octet rule, several alternative models have been proposed.

In the 1950s molecular orbital treatment of hypervalent bonding was adduced to explain the molecular architecture. According to MO theory, the central atom of penta- and hexacoordinated molecules would be sp3d and sp3d2 hybridized, which requires the promotion of central atom electrons to unoccupied d-orbitals. 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. It was shown that in the case of hexacoordinated SF₆, d-orbitals are not involved in S-F bond formation, but charge transfer between the sulfur and fluorine atoms and the apposite resonance structures were able to account for the hypervalency.

Additional modifications to 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 atomic orbital on the central atom and an atomic orbital from each of the two ligands on opposite sides of the central atom. Only one of the two pairs of electrons is occupying a molecular orbital that involves bonding to the central atom, the second pair being non-bonding and occupying a molecular orbital composed of only atomic orbitals from the two ligands. This model in which the octet rule is preserved was also advocated by Musher.

An example of this is the hexacoordinated SF₆, which has been proposed to be composed of three 3c-4e bonds. In this model each bond is equivalent, linear and orthogonal with one lying along each of x, y and z axes. These interactions are F(p1)-S(3p_x2)-F(p1), F(p1)-S(3p_y2)-F(p1), and F(p1)-S(3p_z2)-F(p1). Together these data account for both the octahedral symmetry of the molecule as well as observed molecular structure.

A more complete description of hypervalent molecules arises from consideration of molecular orbital theory through quantum mechanical methods. A LCAO in, for example, sulfur hexafluoride, taking a basis set of the one sulfur 3s-orbital, the three sulfur 3p-orbitals, and six octahedral geometry symmetry-adapted linear combinations (SALCs) of fluorine orbitals, a total of ten molecular orbitals are obtained (four fully occupied bonding MOs of the lowest energy, two fully occupied intermediate energy non-bonding MOs and four vacant antibonding MOs with the highest energy) providing room for all 12 valence electrons. This is a stable configuration only for SX₆ molecules containing electronegative ligand atoms like fluorine, which explains why SH₆ doesn't form. In the bonding model, the two non-bonding MOs (1e_g) are part of the 3-center 4-electron bonding, localized equally on all six fluorine atoms.