The possibility of activating N2 at the molybdenum centre of the FeMo-cofactor of nitrogenase is investigated using EHMO calculations on functional and structural models of the enzyme active site. We consider a diverse range of structurally defined complexes and elicit the EHMO characteristics of these sites. We then compare these with EHMO calculations on model clusters and FeMo-co with N2 bound at Mo. Our focus is on both the binding of dinitrogen and its activation towards protonation. The latter requires a substantial occupation of the π* molecular orbitals of N2 which is experimentally achieved in some mononuclear MN2 complexes. Binding and activation are also theoretically possible at the Mo centre of MoFe3S4 clusters provided they have the right oxidation state and geometry. However, structural deformations of the FeMo cofactor cluster core do not appear to provide a favourable situation for N2-activation at Mo.