Question

What is the primary valency of $\left[ {Co{{\left( {N{H_3}} \right)}_4}C{l_2}} \right]C{l_2}$ ?

Answer 1

The oxidation number of the central metal ion is known as its primary valency while the coordination number is known as the secondary valency. The charge carried by the central metal with consideration of charges of the attached ligands and counterions is known as the oxidation number.

Complete step by step answer:
The primary and secondary valencies are explained by Werner’s coordination theory. According to it, coordination compounds have two valencies namely primary valency and secondary valency.
The primary valency or the oxidation number in coordination number can be easily determined by the number of negative ions in the complex. Their charge is equivalent to the charge on the metal ion.
In the given compound, there are two chlorine anions outside the coordination sphere. From this we can infer that the charge on the coordination sphere is $+ 2$. In the coordination sphere, the metal ion is bonded to four amine ligands and two chloro ligands. The amine ligands are neutral and do not contribute to the primary valency of the cobalt ion. Each chloro ligand carries a charge of $- 1$.

Thus, the oxidation number or primary valency of cobalt is $+ 4$.

Note:
To find the secondary valency of a central metal atom or ion, we need to find the coordination number of the metal which is equal to the number of bonds formed by the central metal. In the given compound, the cobalt ion forms four bonds with monodentate ammine ligand and two bonds with monodentate chloro ligand. Thus its primary valency is six.