Question

Oxidation state of $Co$ in $\left[Co\right(CN{)}_6{]}^{3-}$ is :

Answer 1

Let $x$ be the oxidation state of $Co$ in $\left[Co\right(CN{)}_6{]}^{3-}$.
Since the overall charge on the complex is $-3$, the sum of oxidation states of all elements in it should be equal to $-3$.
Therefore, $x+6(-1)=-3$
or, $x=+3$.