In order to gain insight into how the negative protonation energy is achieved we systematically replace negative and neutral ligands by NH - 2 and NH 3 , respectively, keeping the metal-ligand distance unchanged. We start replacing each of the 5 ligand individually and recompute the electronic protonation energy to determine the effect of removing each ligand. Then, we order the ligands from largest positive increase to lowest positive increase in energy, and use this order to subsequently remove one by one ligand and recompute electronic protonation energy.