Synthesis and reactivity of α,β-unsaturated alkylidene and cumulenylidene group 8 half-sandwich complexes

The present review reports on the chemistry and catalytic applications of Group 8 half-sandwich complexes containing non-heteroatom star bilized alpha,beta-unsaturated alkylidene and cumulenylidene, groups. These include bis-carbenes [MLn]=C(R-1)-C(R-2)=C(R-3)-C(R-4), mono- and polyhapto alkenyl-alkylidenes [MLn]=C(R-1 )-C(R-2)=CR3 (R-4), as well as substituted vinylidene [MLn]=C=CR1 (R-2) and allenylidene [MLn]=C=C=CR1(R-2) complexes (R-1 and/or R-2 = unsaturated hydrocarbon substituent). Synthetic methodologies and stoichiometric reactions as well as the involvement of these species in a series of catalytic transformations are presented. Important recent developments in catalytic studies reveal the role of bis-carbene and alkenyl-vinylidene species as intermediate active species in C-C coupling reactions of alkynes. (C) 2004 Elsevier B.V. All rights reserved.