Coordination sphere

The coordination sphere is the central metal ion together with all the ligands directly bonded to it, conventionally enclosed together in square brackets -- it behaves as one discrete structural unit. When this bracketed unit carries a net electrical charge, it is specifically called a complex ion. Any ionisable groups written OUTSIDE the square bracket are counter ions, which balance the complex ion's charge but are not themselves bonded to the metal. For example, in K4[Fe(CN)6], the coordination sphere/complex ion is Fe(CN)6, while the four K+ ions are the counter ions; on dissolving, the compound ionises as $\text{K}_4[\text{Fe(CN)}_6] \rightarrow 4\text{K}^{\oplus} + [\text{Fe(CN)}_6]^{4-}$, and the complex ion Fe(CN)6 stays fully intact rather than breaking down into separate Fe and CN- ions. A worked calculation building directly on this idea: since the complex ion's charge number is -4, and this must equal (oxidation state of Fe) plus (6 x charge of each CN- ligand), i.e. $-4 = (\text{O.S. of Fe}) + 6\times(-1)$, the oxidation state of iron works out to $-4-(-6)=+2$. …