Misconception of reductive elimination of H2, in the context of the mechanism of nitrogenase.

The elimination of H2 from an M(H)2 component of a coordination complex is often described as reductive elimination, in which the H atoms are regarded as hydride ions, and the product complex after elimination is regarded as reduced by two electrons. The concept is M(n+2)(H(-))2 → M(n) + H2 (with oxidative addition as its reverse). This interpretation contravenes Pauling's electroneutrality principle, and a number of researchers of metal-hydrogen systems have warned against literal acceptance of the formalism. A mechanism suggested by others for the chemical catalysis occurring at the Fe7MoS9C active site cluster of nitrogenase has invoked reductive elimination of H2 from Fe as a central premise. I report here calculations of atom partial charges for the relevant nitrogenase steps, as well as atom partial charges for some well-studied Fe complexes that model the nitrogenase chemistry. Fe-coordinated H atoms are <20% hydridic, and during the H2 elimination process the charge on Fe is essentially invariant. The argument for literal reductive elimination of H2 as part of the mechanism of nitrogenase is not sustained.