PAIR POTENTIALS, ELECTRON-PAIR REPULSIONS, AND THE GEOMETRIES OF MOLECULES

The utility of a new parameter, the ligand-ligand pair potential, phi, and its variation with electron count (x) is used to understand the geometries of small molecules and the origin of electron-counting rules. Although derived from molecular orbital calculations and defined as representing the potential between pairs of atoms coordinated to a central atom, phenomenologically phi-equally well represents the interactions between the electron pairs linking these atoms to the central one. This provides a way to extract an electron-pair repulsion picture from a one-electron molecular orbital theory and is able to provide some insights into why the VSEPR scheme works. We show how knowledge of the sign and magnitude of phi-determines angular geometries, relative bond lengths, and critical electron counts for the electronic stability of molecules with certain structures. In contrast to the ideas of the VSEPR scheme, the model is equally applicable to main-group and transition-metal systems.