The geometries and stabilities of the FeFe cofactor at different oxidation states and its complexes with N-2 have been determined by density functional calculations. These calculations support an EPR-inactive resting state of the FeFe cofactor with four Fe2+ and four Fe3+ sites (4Fe(2+)4Fe(3+)). FeFeco(mu(6)-N-2) with a central dinitrogen ligand is predicted to be the most stable complex of the FeFe cofactor with N-2. It is easily formed by penetration of N-2 into the trigonal Fe-6 prism of the FeFe cofactor with an approximate barrier of 4 kcal mol(-1). The present DFT results suggest that an FeFeco(mu(6)-N-2) entity is a plausible intermediate in dinitrogen fixation by nitrogenase. CO is calculated to bind even more strongly than N-2 to the FeFe cofactor so that CO may inhibit the reduction of nitrogen by Fe-only nitrogenase.