Question

In ${K_4}Fe{(CN)_6}$
A. $(CN)$ are linked with primary valency
B. $(CN)$ are linked with secondary valency
C. $K$ are linked with secondary valency
D. $K$ are linked with non-ionic valency

Answer 1

In coordination compounds, the primary valency of the compound relates to the oxidation state of the central metal atom whereas the secondary valency relates to the coordination number of the complex i.e., number of ligands bonded to the central metal atom.

Complete Step By Step Answer:
Postulates of Werner’s Coordination theory are as follows:
1. The coordination complexes, the metal atoms exhibit two types of valencies i.e., primary valency and the secondary valency. The primary valency is ionizable whereas the secondary valency is non-ionizable.
2. The metal tends to satisfy both its primary as well as secondary valency. The primary valency is satisfied by the negative ions whereas secondary valency is satisfied either by negative ions or by neutral molecules.
3. The secondary valencies are always directed towards the fixed position in space and thus lead to definite geometry of the coordination complex. If the secondary valency is 6, then the geometry of the complex will be octahedral whereas for coordination number 4, the geometry will be tetrahedral or square planar.
For the given coordination complex ${K_4}\left[ {Fe{{(CN)}_6}} \right]$ , the potassium atoms $K$ are linked with primary valency because it can be ionized while cyanide ions $(CN)$ are linked with secondary valency because it is non-ionizable.
Hence, option (B) is the correct answer.

Note:
Remember that if the ligands are neutral molecules, then it only satisfies the secondary valency but if the ligands happen to have a negative ion, then it will not only satisfy the secondary valency, but also primary valency.