Acetyl-coenzyme A synthases/carbon monoxide dehydrogenases are bifunctional enzymes that catalyze the synthesis of acetyl-CoA and the reversible reduction of CO(2) to CO. The active site for the first reaction, called the A-cluster, consists of a [Fe(4)S(4)] cubane bridged to a dinuclear nickel subcomponent. The active site for the second reaction, the C-cluster, consists of a [Fe(3)S(4)] subsite linked to a Ni-Fe dinuclear site. There is evidence for the formation of five Ni-C bonds, involving methyl, acetyl, carbonyl, and carboxylate groups. In this review, the current evidence for each of these bonds is described. The mechanism of catalysis is discussed, highlighting the role of these species. The unique coordination environments of nickel that may facilitate the formation of organometallic species is discussed. Current puzzles in the field and future research directions that might resolve them are outlined.