Ligand Redox Non-Innocence in Transition-Metal sigma-Alkynyl and Related Complexes

Transition-metal sigma-alkynyl complexes are valuable functional materials that have found application as sructural units in the assembly of polymetallic arrays and large molecular structures, reagents for the transfer, oligomerisation or functionalisation of alkynes, magnetic or optical materials and putative components for use in a future molecular-based electronics platform. Many sigma-alkynyl complexes are redox-active, undergoing facile oxidation (reduction) at moderate potentials to generate radical cations (anions) in which the charge and spin density can be tuned from being largely metal-centred to alkynyl ligand-centred by variation of the nature of the metal, supporting ligands and alkynyl substituents. This review summarises the diverse chemical behaviour of metal-supported sigma-alkynyl radicals, and some selected closely related systems, which can often be rationalised in terms of the distribution of electron-spin density over the metal-alkynyl scaffold.