Recent developments in the reactivity of allenylidene and cumulenylidene complexes

Allenylidene and higher cumulenylidene complexes [M]= C(=C)(n)=(CRR2)-R-1 ( n = 1, 2, 3) have continuously gained significance in the context of transi tion metal carbene chemistry. Important developments which have been disclo sed during the last two years are reviewed. These include a variety of stoi chiometric and catalytic reactions of allenylidene complexes and their util ity in organic synthesis. The related chemistry of butatrienylidene (n = 2) and pentatetraenylidene (n = 3) complexes as well as theoretical studies a re also reviewed.