Alkyl and Hydride-Olefin Complexes of Niobocene

Reactions of Cp2NbCl2 with RMgCl (R = n-C3H7, i-C3H7, n-C4H9, s-C4H9 and n-C5H11) give niobocene hydride olefin complexes Cp2Nb(H)L (L = C3H6, C4H8 and C5H10). The last step of the reaction probably proceeds via a stereospecific β-H elimination from the monoalkyl species Cp2NbR. Decomposition of n-alkyl products initially leads to endo-hydride olefin complexes exclusively, whereas iso-alkyls give predominantely the exo-isomers. The endo-hydride olefin complexes of niobocene are readily converted into alkylniobocenes by treatment with carbon monoxide or isocyanides. Carbonylation of the exo-isomers gives only Cp2Nb(CO)H. It is not as easy to produce the endo- or exo-hydride olefin complexes, selectively as in the case of tantalum, but the conversion of niobocene hydride olefin complexes into alkylniobocenes proceeds much more easily than does the corresponding reaction for tantalum derivatives.