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Non-metal atoms or groups of atoms (ligands or complexing agents) surround a core metal atom. These atoms share a chemically link between themselves.
Coordination complexes are metal-containing compounds that contain transition metals (mostly d block elements).
Werner published a theory in the year 1893 that explained the structures, production, and nature of bond formation in coordination complexes. According to Werner’s hypothesis, during the formation of a metal complex, the central or core metal seeks to satisfy both its valencies, i.e., primary and secondary.
What is the primary valency in a coordination compound?
Primary valencies are those that a metal shows in the synthesis of simple salts. It also represents the metal’s charge and oxidation state. They are written outside the coordination or ionisation sphere.
The primary valencies are ionisable and non-directional. They do not have any specific effect on the geometry of the coordination complex.
The primary valency can sometimes function as a secondary valency. The primary valency also gives the charge present on the central metal atom in the complex.
Calculation of Primary Valency
To calculate the primary valency, find out the oxidation state of the central or core metal atom. It will give the primary valency. Finding the number of negatively charged ions outside the coordination sphere will also provide the primary valency.
What is secondary valency in a coordination compound?
The secondary valency is the number of positive or negative ions or neutral molecules coordinated to the core metal atom. The coordination number is another name for the secondary valency. Negative ions or neutral compounds can satisfy secondary valencies.
The secondary valency is the number of ligand molecules. These cationic, anionic, or neutral molecules are bound to the central metal atom (present inside the coordination sphere). The coordination number of the core metal ion or atom determines the geometry of the complex compound or ion in three dimensions.
Suppose the central metal atom has a coordination number 6. In that case, the complex exhibits octahedral geometry. In contrast, if the coordination number is 4, the geometry will be tetrahedral or square planar.
Calculation of Secondary Valency
For a metal, the secondary valence will be equal to the coordination number of the metal atom and is constant. The secondary valencies are non-ionisable valencies. Neutral molecules or negative ions satisfy these needs.
A metal has a fixed secondary valency.
The number of sigma bonds that the ligand is forming with the central atom/ion determines the legacy of the central atom/ion, which also determines the secondary valency.
Some examples to show the primary and secondary valencies of coordination compounds
- [Cr(H2O)6]Cl3
The given complex is cationic. The central metal atom, chromium, is in a +3 oxidation state in this complex. The primary valency will be equal to 3 (there are 3 Cl- ions present). The central metal atom is bound to six H2O molecules, and hence, its secondary valency is 6.
- CoCl3.3NH3
Here, the Cl- ion contributes the primary valency, and it is 3. The secondary valency of the complex is 6, as it has the coordination number of 6. The central cobalt atom is in a +3 oxidation state in this complex.
- [Co(NH3)4Cl2]Cl –
The oxidation state of cobalt is +3, and hence, the primary valency will be 3. The secondary valency is 6, as there are 4 NH3 molecules and 2 Cl- ions linked to the cobalt atom.
- K2[Ni(CN)4]
It is an anionic complex. The central metal atom has a +2 oxidation state. The primary valency will be 2, and the secondary valency will be 4.
Difference between Primary and Secondary Valency in Coordination Compound
| Primary Valency | Secondary Valency |
| The primary valency is a mostly positive value, and in some cases, it is zero. | The secondary valency will be the same as the ligancy of the central metal atom or ion. |
| Primary valency is only satisfied by the negatively charged ligand species. | The secondary valency can be satisfied by a neutral molecule, a negatively charged species, or a positively charged species. |
| These valencies are directional. | These valencies are non-directional. |
| E.g., In [Co(NH3)6]Cl3, the primary valency is 3 | E.g., In [Co(NH3)6]Cl3, the secondary valency is 6 |
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Conclusion
Werner states that the formation of a coordination complex fulfils the conditions of primary and secondary valencies. The primary valency will be the same as the oxidation number of the central metal atom, and the secondary valency will be the same as the coordination number.
The number of molecules coordinated to the metal atom is secondary valence. Negative ions or neutral compounds can satisfy secondary valencies. The number of the secondary valency is fixed for a specific metal atom or ion. The coordination number refers to the value of the secondary valency of the concerned atom or ion.
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