PHOSPHAZENES AND PHOSPHAZANES

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Abstract
Three types of phosphazene and four types of phosphazane structural units are classified on the basis of the coordination number of the phosphorus atoms involved. Phosphorus-nitrogen bond lengths are discussed in acyclic compounds and this information is used in analysing ring systems consisting wholly or partially of phosphorus-nitrogen skeletons. Four-, five-, six-, seven-, eight-, ten-, and twelve-membered rings are surveyed, as are polycyclic systems. Methods of synthesis for cyclophosphazanes, and mixed cyclo-phosphazene-phosphazanes are briefly considered, as are some of their reactions. The minimum ring size where stable phosphazenes have been observed is referred to. The effect of protonation, methylation, or coordination to a metal moiety on adjacent phosphorus-nitrogen bonds is discussed. The conformations of dimethylamino and phenyl substituents on phosphorus are considered, as is the conformations of the ring skeletons. The flexibility of a cyclic and cyclic phosphorus-nitrogen skeletons is noted, and factors governing phosphorus-nitrogen bond lengths are analysed. On the basis of this and by analogy with sulphur-nitrogen chemistry predictions are made about conditions favouring very long and very short phosphorus-nitrogen bonds.