Concomitant with the displacement of two N-H protons, porphyrins bind metal ions in the N4 "pocket". The metal ion usually has a charge of 2+ or 3+. A schematic equation for these syntheses is shown:
:Hporphyrin + [ML] → M(porphyrinate)L + 4 L + 2 H, where M = metal ion and L = a ligand