Question

What is the oxidation number of underlined element?

$\left[\underset{–}{Co}\right(N{H}_3{)}_{5}Cl]C{l}_2$

Answer 1

Let $x$ be the oxidation number of $Co$ in $\left[\underset{–}{Co}\right(N{H}_3{)}_{5}Cl]C{l}_2$.
Since the overall charge on the complex is $0$, the sum of oxidation states of all elements in it should be equal to $0$.
Therefore, $x+5\left(0\right)+3(-1)=0$
or, $x=+3$
Hence, the oxidation number of $Co$ in $\left[\underset{–}{Co}\right(N{H}_3{)}_{5}Cl]C{l}_2$ is $+3$.