Identification of a spin-coupled Mo(<scp>iii</scp>) 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(<sc>iv</sc>). We demonstrate that only Mo(<sc>iii</sc>) fits Mo HERFD XAS data, the first example of Mo(<sc>iii</sc>) in biology. Theoretical calculations further reveal an unusual spin-coupled Mo(<sc>iii</sc>).