HIGHER-VALENT DERIVATIVES OF THE D-METAL ACIDS .12. ALPHA-PHOSPHONIO(METHYLIDENE) COMPLEXES OF NIOBIUM AND TANTALUM CONTAINING THE METALLOCENE-LIKE [(ETA(5)-C5R5)M(N(T)BU)] CORE

The halfsandwich complexes [eta5-C5H5)Nb(N(t))Cl2] (1) and [(eta5-C5H5 )Ta(N(t))Cl2] (4) react with two equivalents of Ph3P=CH2 to give produ cts of a transylidation reaction [(eta5-C5H5)Nb(N(t)Bu)(CH-PPh3))Cl] ( 2) and [(eta5-C5H5)Ta(N(t)Bu)(CH-PP3)Cl] (5). The X-ray diffraction st udy on 2 confirms a three-legged piano stool structure. The Nb-C-dista nce of the alpha-phosphonio(methylidene) ligand is comparable with cor responding bond lengths of Schrock-type niobium alkylidene complexes. The trigonal planar [M=CH-PR3] moiety adopts a synperiplanar conformat ion with respect to the imido ligand, allowing both pi-donor functions a minimum of competition in their interaction with empty metal d-orbi tals. Treatment of 2 with an excess (4 equiv.) of Ph3P=CH2 leads to an equilibrium mixture of 2 and [(eta5-C5H5)Nb(N(t)Bu)(CH-PPh3)2] (3). C ontrastingly, by reacting lithiated ylide Li(CH2)2PPh2 With [(eta5-C5M e5)Ta(N(t)Bu)Cl2](6), the chelate complex [(eta5-C5Me5)Ta(NtBu){(CH2)2 PPh2)Cl] (7) is isolated. 7 does not thermally rearrange to [(eta5-C5M e5)Ta(N(t)Bu)(CH-PPh2Me)Cl]. However, the reaction of 7 with a second equivalent of the lithiated ylide leads to highly ''pi-loaded [(eta5-C 5Me5)Ta(N(t)Bu)(CH-PPh2Me)2](8). The results are discussed in the cont ext of the isolobal relationship of the fragments [(eta5-C5R'5)M(NR)] (M = V, Nb, Ta), [(eta5-C5R'5)2M] (M = Ti, Zr, Hf) and [M(NR)2] (M = C r, Mo, W).