Bonding in the hypervalent molecules SF4, BrF5, PF5, and SF6 was studied using multicenter bond order indices and examination of the eigenvalues and the eigenvectors of the Fermi holes of the constituent atoms. Diagonalization of the Fermi holes provided quantitative validation of Musher's categorization of hypervalency with SF4 and BrF5 representative of type I, and PF5 and SF6 belonging to type II. The eigenvalues and eigenvectors of type I molecules distinguished between classic two-center two-electron bonds and three-center four-electron bonds, whereas the results of diagonalization for type II molecules demonstrated the presence of substantial reorganization of the valence state of the central atom leading to equivalent bonds and the highest expected symmetry of the molecule.
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Ponec et al. (1999) studied this question.