The breaking of the very strong NO bond by the enzyme nitric oxide reductase has been explored using density functional theory calculations of a model cluster. A five-step mechanism is proposed. We find a novel mechanism for the formation of the N-N bond in N2O, involving the addition of NO to the intermediate Fe-N(H)OH, followed by a conformational change on a flat potential energy surface to give the strong N-N bond. (c) 2005 Elsevier B.V. All rights reserved.