## Chemistry: The Central Science (13th Edition)

Coordination number: 5 Oxidation number: $+2$
The ligand ($CN_{5}$) that surround the central metal atom $Ni$ are 5. Thus, the coordination number is 5. Let us suppose the oxidation number of $Ni$ is X. To find the oxidation number, we sum the known oxidation numbers and equal it to the charge on the complex. $CN_{5} = 5\times(-1)$ The overall charge is $-3$ on the complex, thus: $X + 5\times(-1) = -3$ $X -5 = -3$ $X =-3+5$ $X= +2$ The oxidation number of Ni is +2.