EQUILIBRIUM BETWEEN YLIDENE AND ISOCYANIDE COMPLEXES - SYNTHESIS AND CRYSTAL-STRUCTURE OF -YLIDENE)(TRIPHENYLPHOSPHINE)TETRACARBONYLTUNGSTEN

Reaction of 2-(trimethylsiloxy)phenyl isocyanide (1) with [W(THF)(CO)( 5)] gives the complex [W(1)(CO)(5)](2a). Hydrolysis of the Si-O bond l eads to the formation of an equilibrium between complexes with a 2-hyd roxyphenyl isocyanide (3a) or a 1,2-dihydrobenzoxazol-2-ylidene ligand (4a), which lies mostly on the side of the ylidene complex 4a. Substi tution, of one cis-CO ligand in 2a for P(C6H5)(3) enhances metal-to-li gand (d-->p)pi-back-bonding and shifts the equilibrium toward the isoc yanide complex 3e. The equilibrium can again. be shifted toward the yl idene complex by deprotonation and alkylation of the ylidene nitrogen to give -2-ylide)(triphenylphosphine)tetracarbonyltungsten (4f) which was characterized by single-crystal X-ray structure analysis.