The structures of hypervalent phosphorus(III) anions P(CN)4–nBrn–. Transition from ψ-trigonal-bipyramidal to ψ-octahedral co-ordination and deviation from Valence Shell Electron Pair Repulsion Theory

Abstract

Salts of the title anion series have been prepared with the sodium–[18]crown-6 cation (n= 1 and 2)([18]crown-6 = 1,4,7,10,13,16-hexaoxacyclo-octadecane) or tetra-alkylammonium cations (n= 4) by addition of Br^– to P(CN)₃ or PBr₃, or by Br₂ or BrCN oxidation of P(CN)₂^–. Contrary to this, CN^– addition to P(CN)₃, in a reductive elimination, gives P(CN)₂^–. X-Ray crystal-structure determinations of the above mentioned three salts are reported and their anion structures are discussed. The P(CN)₂Br₂^– anion shows the ψ-trigonal-bipyramidal co-ordination expected on the basis of Valence Shell Electron Pair Repulsion Theory (V.S.E.P.R.), P(CN)₃Br^– has ψ-octahedral co-ordination in a dimeric, bromide double-bridge structure, and PBr₄^– a structure intermediate between the two. Here the trigonal-bipyramidal co-ordination is distorted towards a tetrahedral geometry. This is in contradiction to V.S.E.P.R. and presumably the result of ligand–ligand repulsion. The structures are of interest as transition-state models of phosphorus(III) nucleophilic substitution.