Charge number of complex ion and oxidation state of metal ion

The charge number of a complex ion is simply its net overall charge -- the algebraic sum of the oxidation state (charge) of the central metal ion and the total charge contributed by all its ligands together. Formally, charge number = (oxidation state of metal) + (sum of ligand charges). Since the metal's oxidation state is one of the two quantities in this sum, the equation can equally be rearranged to calculate the metal's oxidation state whenever the complex ion's overall charge and the ligands' individual charges are already known -- which is exactly how the oxidation state of iron (+2) was determined for Fe(CN)6 in section 9.3.1, and is the standard method used for every oxidation-state question in this chapter (e.g. determining O.S. of Co in [Co(NH3)5Br]SO4, or O.S. of Pt in [Pt(NH3)Cl5]-). Neutral ligands like NH3, H2O, en and CO co …