Reactions of R2P–P(SiMe3)Li with [(R′3P)2PtCl2]. A general and efficient entry to phosphanylphosphinidene complexes of platinum. Syntheses and structures of [(η2-PPiPr2)Pt(p-Tol3P)2], [(η2-PPtBu2)Pt(p-Tol3P)2], [{η2-PP(NiPr2)2}Pt(p-Tol3P)2] and [{(Et2PhP)2Pt}2P2]