Coordination Chemistry Reviews 248 (2004) 1627–1657 Review Synthesis and reactivity of ,-unsaturated alkylidene and cumulenylidene Group 8 half-sandwich complexes Victorio Cadierno, M. Pilar Gamasa, José Gimeno ∗ Departamento de Qu´ ımica Orgánica e Inorgánica, Instituto Universitario de Qu´ ımica Organometálica “Enrique Moles” (Unidad Asociada al CSIC), Facultad de Qu´ ımica, Universidad de Oviedo, E-33071 Oviedo, Spain Received 18 February 2004; accepted 3 May 2004 Available online 3 July 2004 Contents Abstract ............................................................................................................................... 1627 1. Introduction ....................................................................................................................... 1628 2. α,β-Unsaturated alkylidene complexes ............................................................................................... 1628 2.1. Monohapto alkylidenes ....................................................................................................... 1628 2.2. Polyhapto alkylidenes ........................................................................................................ 1629 2.3. α,β-Unsaturated bis-carbenes .................................................................................................. 1633 3. α,β-Unsaturated vinylidene complexes ............................................................................................... 1634 3.1. Alkenyl–vinylidene complexes ................................................................................................ 1634 3.1.1. Synthesis ................................................................................................................ 1634 3.1.2. Reactivity ............................................................................................................... 1637 3.2. Other α,β-unsaturated vinylidene complexes .................................................................................... 1644 4. α,β-Unsaturated allenylidene complexes .............................................................................................. 1645 4.1. By activation of alkynols ...................................................................................................... 1645 4.2. By nucleophilic additions to highly reactive [Ru] + =C(=C) 2 =CR 1 R 2 species ........................................................ 1646 4.3. By nucleophilic addition of acetylides and hydride to amino-allenylidene complexes ................................................ 1648 5. Catalysis .......................................................................................................................... 1649 5.1. α,β-Unsaturated alkylidenes ................................................................................................... 1649 5.2. α,β-Unsaturated bis-carbenes (ruthenacyclopentatrienes) ......................................................................... 1649 5.3. α,β-Unsaturated vinylidenes ................................................................................................... 1651 5.4. α,β-Unsaturated allenylidenes ................................................................................................. 1654 6. Conclusions ........................................................................................................................ 1654 Acknowledgements .................................................................................................................... 1654 References ............................................................................................................................ 1654 Abstract The present review reports on the chemistry and catalytic applications of Group 8 half-sandwich complexes containing non-heteroatom sta- bilized α,β-unsaturated alkylidene and cumulenylidene groups. These include bis-carbenes , mono- and poly- hapto alkenyl-alkylidenes [ML n ]=C(R 1 )–C(R 2 )=CR 3 (R 4 ), as well as substituted vinylidene [ML n ]=C=CR 1 (R 2 ) and allenylidene [ML n ]=C= C=CR 1 (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. © 2004 Elsevier B.V. All rights reserved. Keywords: Alkylidene complexes; Vinylidene complexes; Allenylidene complexes; Cumulenylidene complexes; Carbene complexes; Iron, Ruthenium and osmium complexes; Half-sandwich complexes ∗ Corresponding author. Tel.: +34 985 103 461; fax: +34 985 103 446. E-mail address: jgh@fq.uniovi.es (J. Gimeno). 0010-8545/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2004.05.008