A computational study of the molecular structures, conformational preferences and anomeric effects in mono- and bisaminophosphanes

The molecular structures of a number of mono- and bisaminophosphanes, A(2)PNB(2) or AP(NB2)(2), have been optimised by ab initio calculations at the MP2/6-31G* level. The equilibrium structures of the aminophosphanes H2PNH2, H2PNMe2, Me=CH3, Cl2PNMe2 and HFPNH2 are characterised by nonplanar N and P atoms and equilibrium conformations where the electron lone pair on the N atom is trans to the most electronegative of the remaining substituents on the P atom (X). It is suggested that these conformations are stabilised by anomeric effects, i.e. through delocalisation of the N electron lone pair into the antibonding sigma*(P-X) orbital. The IV atom in F2PNMe2 is found to be planar or near-planar, and the equilibrium conformation is such as to allow interaction between the N lone pair and both P-F antibonding orbitals. The equilibrium conformations of HP(NH2)(2) and HP(NMe2)(2) are such that the electron lone pair on each N atom is trans to the other P-N bond. (C) 1998 Elsevier Science B.V.