1. K.A.R. Mitchell, Chem. Rev., 69, 157 (1969).
2. W.G. Salmond, Quart. Rev., 22, 253 (1968).
3. C.C. Price and S. Oae, “Sulfur Bonding” (Ronald Press, New York, N.Y., 1962).
4. G. Cilento, Chem. Rev., 60, 147 (1960).
5. C. K. Jørgensen, Struct, and Bonding, 6, 94 (1969);