Phospha-alkenes X–PCTMS2 (X= F, Cl, l; TMS= SiMe3) display different reaction behaviour towards metal complexes (Bu3P) 2Ni (cod)(cod= cyclo-octa-1, 5-diene) or (Ph3P) 2Pt (C2H4), respectively, undergoing either co-ordination to give η2; phospha-alkene complexes or alternatively oxidative addition of the P–X bond, forming phospha-alkenyl- metal (II) complexes,[(R3P) 2M (X)(σ-PCTMS2)].