HALOGEN SUBSTITUTION AT PHOSPHORUS IN PHOSPHAALKENES AND PHOSPHASILENES VERSUS BIS(METHYLENE)PHOSPHORANES - A QUANTUM-CHEMICAL EVALUATION OF BONDING PROPERTIES

The bonding properties of/at phosphorus-halogen substituted (F, Cl, Br , I) phosphaalkenes and -silenes were explored with ab initio calculat ions with effective core potential (ECP(d)) optimizing the geometries at the RHF and CISD levels. Compared with the parent compounds HP=CH2 the PC bond is slightly shrunk, indicating the formation of an ion pai r X(-)PCH(2)(+). This is substantiated by an analysis of the natural a tomic orbital populations. They evidence a strong shift of electron de nsity from phosphorus toward the electronegative halogen atom in the a -space, with negligible pi-mesomeric interaction of the lone pair at t he halogen with the PC sc-bond. A similar but slightly less affirmed s ituation results for the P-halogen substituted phosphasilenes as well as for the bis(methylene)phosphoranes. Furthermore, for the latter the halogen substitution enforces a Jahn-Teller distortion, causing symme try reduction from C-2v to C-2 symmetry. d-orbital participation at ph osphorus does not play any role in bonding.