Identification of a spin-coupled Mo( iii ) in the nitrogenase iron–molybdenum cofactor

The molybdenum atom in FeMoco is imperative to the high activity of the enzyme and has been proposed to be Mo( iv ). We demonstrate that only Mo( iii ) fits Mo HERFD XAS data, the first example of Mo( iii ) in biology. Theoretical calculations further reveal an unusual spin-coupled Mo( iii ).