A particularly exciting development in 2003 has been the publication of the X-ray crystal structure of acetyl-coenzyme A synthase/CO dehydrogenase from Moorella thermoacetica at a resolution of 1.9 A. The protein contains two Ni-containing active sites that catalyse distinct CO transformations. One of these active sites, the A-cluster, contains an unprecedented Ni–Ni–[Fe4S4] species in which the central bridging Ni ion is essentially square planar. The new crystal structure also forms the basis for new mechanistic proposals for the mode of action of the enzyme. It has been suggested that the bridging Ni centre binds CO and a methyl group, which then inserts into the Ni–CH3 bond to form a Ni-bound acetyl group which is attacked by CoA to yield the product. The distal Ni centre and the [Fe4S4] moiety are suggested to be required for tuning the activity of the bridging Ni atom through the extensive cysteinyl ligation about the bridging Ni centre.A second major development in 2003 has been the conversion of dinitrogen to ammonia by a Mo complex in a series of steps that involves no Mo oxidation state lower than (III). While previous systems have employed Mo centres exhibiting lower oxidation states, this analogue presents reactions that are more compatible with those that might be expected for nitrogenase biochemistry and biology. Furthermore the chemistry refocuses attention on the single Mo atom in the FeMoco active site of nitrogenase as being a strong candidate for the site of the reduction of dinitrogen.A third exciting development has been the isolation of a mononuclear non-haem complex containing a terminal FeIVO unit and its characterisation by X-ray crystallography, UV/Vis absorption, Mossbauer, IR and resonance Raman spectroscopies. Such a species has been proposed as an intermediate in dioxygen activation by non-haem monoiron centres and these results provide the synthetic precedent for invoking such high-valent Fe species in the catalytic cycles of non-haem monoiron enzymes.