ON THE GEOMETRY OF 1,3-DIAZADIPHOSPHETIDINES - THE CIS-TRANS ISOMERISM

The electronic structure of various substituted title compounds has been investigated by ab initio (3-21G*) and MNDO molecular orbital calculations. The preference for the cis or trans isomers can be traced in the behavior of the HOMO of the parent cyclodiphosphazane under the influence of the ring substituents. Thus, alkyl groups promote folding of the ring and tend to occupy the cis positions on phosphorus, while aryl groups at nitrogen make the ring planar and orient the substituents on phosphorus to the trans positions. In case of electron pair bearing substituents the conformation is determined by a more subtle interplay of their donor acceptor properties.