A Valence Bond Model for Electron-Rich Hypervalent Species: Application to SFn (n=1, 2, 4), PF5, and ClF3

Some typical hypervalent molecules, SF4, PF5, and ClF3, as well as precursors SF ((4)Sigma(-) state) and SF2 (B-3(1) state), are studied by means of the breathing-orbital valence bond (BOVB) method, chosen for its capability of combining compactness with accuracy of energetics. A unique feature of this study is that for the first time, the method used to gain insight into the bonding modes is the same as that used to calculate the bonding energies, so as to guarantee that the qualitative picture obtained captures the essential physics of the bonding system. The (4)Sigma(-) state of SF is shown to be bonded by a three-electron sigma bond assisted by strong pi back-donation of dynamic nature. The linear B-3(1) state of SF2, as well as the ground states of SF4, PF5 and ClF3, are described in terms of four VB structures that all have significant weights in the range 0.17-0.31, with exceptionally large resonance energies arising from their mixing. It is concluded that the bonding mode of these hypervalent species and isoelectronic ones complies with Coulson's version of the Rundle-Pimentel model, but assisted by charge-shift bonding. The conditions for hypervalence to occur are stated.