A new interpretation of interstitial compounds-metallic carbides, nitrides and oxides of composition MX

Evidence is cited to show that interstitial monocarbides, mononitrides and a few monoxides tend to have the sodium chloride structure irrespective of metal structure and metal radius. The need for important metal-non-metal bonds to explain the physical properties of the interstitial phases is pointed out. Interstitial phases are regarded as electron-deficient structures, where the non-metal forms more bonds than it has bond orbitals. It is suggested that hybrid sp-orbitals combined with p-orbitals are used by the non-metal in bonds to metal. A single p-orbital is used to form two bonds at 180°, but the two bonds or half-bonds contain only one electron pair. Resonance of sp and p bonds leads to octahedral bonding by the non-metal. The concept of half-bonds is used to explain the structure, hardness, brittleness, conductivity and high melting-points of interstitial phases with the sodium chloride-type structure, and a few other interstitial types. The bond distances are consistent with this picture. The conditions for forming half-bonds are discussed, and it is concluded that they may be formed between the non-metals H, B, C, N and possibly O and the less electropositive A-group metals. Only in the case of C, N and O is octahedral bonding and the sodium chloride-type structure to be expected on the basis of the proposal. The proposal also accounts for the limitation of metallic MX compounds with the sodium chloride structure to the third, fourth and fifth group A-metals. The metal-non-metal distances in the interstitial phases agree well with predicted distances for 2/3 or ½ bonds, as given by Pauling's rule.