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

Abstract

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 Mⁿ⁺²(H⁻)₂ → Mⁿ + 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.