Question

What is the oxidation number of underlined element?

$\left[\underset{–}{Ni}\right(CO{)}_4]$

Answer 1

Let $x$ be the oxidation number of $Ni$ in $\left[\underset{–}{Ni}\right(CO{)}_4]$.
Since the overall charge on the compound is $0$, the sum of oxidation states of all elements in it should be equal to $0$.
Therefore, $x+4\left(0\right)=0$
or, $x=0$
Hence, the oxidation number of $Ni$ in $\left[\underset{–}{Ni}\right(CO{)}_4]$ is $0$.