Question

How to calculate oxidation number of coordination compounds?

Answer 1

Consider$[Co(NH_3)_6]Cl_3$. We know (or should know) that the chloride counterions each have −1 charge. Thus we can separate the charges.

$[Co(NH_3)_6]3$++3×Cl−. We have to preserve the charge here: we should start with a neutral complex, and it's still neutral as required. Then we should dismember the metal complex: $Co_3++6×NH_3$ we have to recognize the ammine ligand as a neutral entity; the same as I would for a phosphine, $PR_3$, or carbonyl. C≡O, ligand.

Another example, $Pd(PR_3)2Cl_2$. Splitting this up I get Pd2++2×PR3+2×Cl−. And then we have a Pd2+ centre.