Question

# The primary and secondary valency of a central metal ion in $\left[ { Co }{ \left( { NH }_{ 3 } \right) }_{ 4 }{ CO }_{ 3 } \right] { Cl }$ complex is ………… and ………….. respectively.A.) 2,6B.) 4,6C.) 4,5D.) 3,6

Hint: Primary valence is the oxidation number and secondary valence is the coordination number. Oxidation number is the charge carried by the central metal with the consideration of charges of surrounding ligands and counterions whereas the coordination number is the number of bonds formed by the central metal with the ligand.

For this we need to know what this primary valency and secondary valency is, which is explained by Werner’s Coordination theory.
Werner’s coordination theory is proposed by Alfred Werner. According to him,
(i) In a coordination compound, central metal has two types of valencies- Primary valency and the secondary valency.
(ii) Primary valencies are normally ionisable and are satisfied by anions/neutral ligands
It is fixed for a particular metal.
(iii) Primary valence is the oxidation number and secondary valence is the coordination number.
(iv) Secondary valency determines the geometry of the complex.
From the above we understood how to determine the primary valence and secondary valence of a coordination compound.
Here when we calculate the oxidation state of the metal ion(Co), we have the following information from the formula of the compound ${ \left[ { Co }{ \left( { NH }_{ 3 } \right) }_{ 4 }{ CO }_{ 3 } \right] }^{ + }$ that :
NH3 has an oxidation number of 0, ${ CO }_{ 3 }^{ 2- }$ has an oxidation number of -2 and the overall charge of the complex is +1. So the oxidation state of Co, is determined by:
Let ‘x’ be the oxidation number of central metal,
x + ( 4)+ (-2) = +1
x = 1 + 2= +3
and hence +3 is the primary valence.

Now, to find the secondary valence, we need to find the coordination number of the central metal. As we have already mentioned in the hint, coordination number is the number of bonds formed by the central metal with the ligand.
Here, the number of bonds formed by Co, with the ligands are 4 monodentate ligands ${ NH }_{ 3 }$ and one bidentate ligand (carbonate ion) i.e, total number of ligands are 4 +2 =6 and hence, the secondary valence is 6.
Therefore the primary valence and secondary valence of $\left[ { Co }{ \left( { NH }_{ 3 } \right) }_{ 4 }{ CO }_{ 3 } \right] { Cl }$ is +3 and 6 respectively.
The correct answer is 3 , 6.
So, the correct answer is “Option D”.

Note: In some molecules, the same ion (anion) may satisfy both primary and secondary valence. For example, in the case of $\left[ { Co }{ \left( { NH }_{ 3 } \right) }_{ 5 }Cl \right] { Cl }_{ 2 }$ , all the three chlorine atoms are involved in both primary as well as secondary valence.