Nitrogen Adsorption and Hydrogenation on a ${\mathrm{MoFe}}_6{S}_9$ Complex

The enzyme nitrogenase catalyzes the biological nitrogen fixation where ${\mathrm{N}}_2$ is reduced to ${\mathrm{NH}}_3$. Density functional calculations are presented of the bonding and hydrogenation of ${\mathrm{N}}_2$ on a ${\mathrm{MoFe}}_6{\mathrm{S}}_9$ complex constructed to model aspects of the active site of nitrogenase. ${\mathrm{N}}_2$ is found to bind end on to one of the Fe atoms. A complete energy diagram for the addition of hydrogen to the ${\mathrm{MoFe}}_6{\mathrm{S}}_9$ complex with and without ${\mathrm{N}}_2$ is given, and a mechanism for ammonia synthesis is proposed on this basis.