The amenability of homogeneous systems to small molecule characterization techniques has driven the investigation of organometallic compounds capable of modelling the homologation of CO via C-C bond formation. Prominent examples include d-block metal compounds (mirroring the use of heterogeneous transition metal catalysts such as iron and cobalt in the Fischer-Tropsch process), with examples derived from low-valent s-, p-, and f-block compounds having also been reported (e.g. I-VII, Figure ). Prevalent among the homologation products formed via reduction processes are cyclic systems of the type [CnOn] 2-(n = 3, 4, 6) and the related ethynediolate system [C2O2] 2-, formed by a formal two-electron reduction process, although a number of systems featuring longer linear chains of carbon atoms have also been reported (Figure ). Within this sphere, a small number of studies have emerged which demonstrate the potential for control of homologation processes at a constant oxidation level: the selective formation of [CnOn] 2-(n = 2, 3 or 4) by the cooperative action of U(III) or Mg(I) centres has been shown to be influenced by the steric bulk of ancillary ligands, and the stepwise growth of C3 chains by the addition of CO has been demonstrated at d-block metal systems derived from metal carbonyl precursors. Very recently the formation of C4 or C5 chains using aluminium(I) systems has been demonstrated with the product formed appearing to reflect the nuclearity of the metal reagent.