Synthesis and property of phosphenium complexes containing double bond character between a transition metal and a phosphorus atom

In this review, a new and general method for the preparation of phosphenium complexes of transition metals has been reported. The stability of phosphe nium complexes increases with going to a heavier congener among group 6 tra nsition metals, and with increasing the number of amino substituents on the phosphenium phosphorus. The X-ray structures, P-31 NMR and Mo-95 NMR studi es, and theoretical calculations have revealed that pi back donation from a filled d orbital of a transition metal to an empty p orbital of a phosphen ium phosphorus is a dominant contribution. Unique reactivities of phospheni um complexes have been developed: (i) heteroatom migration from a tertiary phosphorus to a phosphenium phosphorus in a coordination sphere, (ii) migra tory insertion of a phosphenium ligand into a carbon-transition metal bond, (iii) 1,3-migration of an alkyl group from a stannyl ligand to a phospheni um phosphorus. Activation parameters concerning phosphenium ligand rotation along the P-M axis have been estimated.