What is the oxidation number of the metal of $[\text{Co}(\text{H}_2\text{O})(\text{CN})(\text{ox})_2]^{2-}$ coordination entity?

+3

The correct answer is Option (3) → +3

Oxidation state of the central metal is calculated by considering the charges of all ligands and the overall charge of the complex.

Neutral ligands contribute 0 charge, while anionic ligands contribute their respective negative charges.

Let oxidation state of $\text{Co} = x$

Ligand charges:

$\text{H}_2\text{O}$ is neutral $\rightarrow 0$

$\text{CN}^- \rightarrow -1$

Oxalate $(\text{ox}^{2-}) \rightarrow -2$ each

Since there are two oxalate ligands:

Total ligand charge =

$\text{CN}^- = -1$

$2 \times \text{ox}^{2-} = -4$

Total ligand charge $= -5$

Overall charge of complex $= -2$

So,

$x + (-5) = -2$

$x = +3$