Structures, vibrational spectra, and ligand behaviour of tris(2-cyanoethyl)phosphine and its oxide, sulphide, and selenide

Abstract

The crystal vibrational spectra of tris(2-cyanoethyl)phosphine, P(CH₂CH₂CN)₃, and its derivatives P(CH₂CH₂CN)₃E (E = O, S, or Se) are discussed in the light of their known crystal structures. The CN stretching modes in P(CH₂CH₂CN)₃ are affected by intermolecular coupling effects but the spectra are otherwise interpreted in terms of the local molecular (site) symmetries. The difference between ν(PS) in P(CH₂CH₂CN)₃S and ν(PSe) in P(CH₂CH₂CN)₃Se is 161 cm^–1, much greater than the differences ν(PS)–ν(PSe) usually observed in related PR₃S and PR₃Se molecules. It is suggested that the PSe stretch may be severely affected by interaction with skeletal bending modes. The oxide, and more particularly, the sulphide and selenide, are much less effective ligands than the parent phosphine and the addition of an O, S, or Se atom to the phosphine molecule appears to have a pronounced deactivating effect on the donor abilities of the cyano-nitrogen atoms.