The structure of the NH + 3 intermediate has short distances between both the Mo and NH 3 group, and the Mo and carbonyl oxygens on the ligands which lengthen significantly upon reduction (from ca 2.1 to 2.5 Å), indicating a decrease in the strength of these interactions. We propose that these interactions increases the positive charge on Mo compared to a methyl ligand (with Mullliken charges of 1.92 vs 1.66), which increases the electrostatic repulsion with the NH + 3 moiety, leading to a destabilization relative to neutral NH 3 .