A Valence Bond Model for Electron-Rich Hypervalent Species: Application to SF sub(n) (n=1, 2, 4), PF sub(5), and ClF sub(3)

Some typical hypervalent molecules, SF sub(4), PF sub(5), and ClF sub(3), as well as precursors SF ( super(4) capital sigma super(-) state) and SF sub(2) ( super(3)B sub(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 super(4) capital sigma super(-) 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 super(3)B sub(1) state of SF sub(2), as well as the ground states of SF sub(4), PF sub(5) and ClF sub(3), 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. Charge-shift bonding is the glue that makes electron-rich hypervalent molecules stable in the framework of the valence bond (VB) version of the Rundle-Pimentel model (see figure). This model emerges directly from high-level VB calculations, thus ensuring the correctness of the interpretations. It provides a unified description for all studied electron-rich hypervalent species, and allows both extra insight and improved predictive power compared with other traditional or recently proposed models for hypervalence.