Electron Deficient Compounds. II. Relative Energies of ``Half-Bonds''

It is suggested that the tendency to make use of all low energy orbitals is the underlying principle of electron-deficient bonding. Simple theoretical arguments, limited to the special case of two electrons and three orbitals, are given to show that this principle is a direct consequence of quantum-mechanical theories of valence. Two half-bonds are shown to be of lower energy than a single bond plus an unused low energy orbital in most cases. The tendency to form half-bonds should be most important in symmetrical cases, and particularly where the electronegativities of all atoms involved are equal, e.g., the boron hydrides and H3+. Certain consequences of the proposal to the fields of organo-metallic compounds, catalysis, and molecular rearrangements are pointed out. It is suggested that at low temperatures complexes may exist between molecules with excess orbitals and molecules with no unshared pairs.