IS ELECTROPHILIC SUBSTITUTION POSSIBLE IN THE COORDINATED CYCLOPENTADIENYL LIGAND OF EARLY TRANSITION-METAL METALLOCENE COMPLEXES - MOLECULAR-STRUCTURE OF BINUCLEAR NITRENE NIOBOCENE COMPLEX [(C5H4C(COOET)=N-MU-N)-(C5H5)NB](2)

Diazoacetates N(2)CHCOOR (a: R = Et; b: R = t-Bu) readily react with C p(2)NbH(3) to give Cp(2)NbH(eta(1)-N(2)CHCOOR) (1) as the first identi fiable product, characterized by means of H-1 NMR spectroscopy. Comple x la converts stepwise into a unique binuclear nitrene niobocene compl ex [(C5H4-C(CooEt)=N-mu-N)(C5H5)Nb](2) (2a). The latter is in fact a p roduct of the Cp-ligand functionalized by the diazoacetate group. Mech anistic aspects of the formation of 2a are discussed with the results of an X-ray diffraction study.