Insertion of activated acetylenes into the metal-hydride bond of [(η 5-C 5H 5) 2M(CO)H] (M = Nb, Ta)

Abstract

Reactions of [Cp 2M(CO)H] (M = Nb, Ta; Cp = η 5-C 5H 5) with various acetylenes RCCR having electron-withdrawing groups were investigated. They give the σ-al- kenyl complexes [Cp 2M(CO)(CRCHR)] via insertion of the alkyne into the MH bond. On the basis of 1H and 19F NMR data the reactions were shown to be: (i) regioselective, monosubstituted alkynes giving only the α-R metallated complex; (ii) stereoselective, exclusive formation of the Z-isomer being observed with hexafluorobut-2-yne. The Z isomer has been shown to exist as two conformers, the steric requirements of the ligands creating a barrier to rotation of the alkenyl group around the MC σ bond.