Difference Between Primary and Secondary Valency

The key difference between primary and secondary valency is that primary valency is the oxidation state of the central metal atom of a coordination complex whereas secondary valency is the coordination number of the central metal atom of a coordination complex.

The terms primary and secondary valency fall under coordination chemistry. Valency is the combining power of an element, especially as measured by the number of hydrogen atoms it can displace or combine with.

CONTENTS

1. Overview and Key Difference
2. What is Primary Valency
3. What is Secondary Valency
4. Side by Side Comparison – Primary vs Secondary Valency in Tabular Form
5. Summary

What is Primary Valency?

Primary valency is the oxidation state of the central metal atom of a coordination complex. A coordination complex is a complicated compound that has a metal ion at the centre, which is surrounded by several atoms or groups of atoms. These surrounding chemical species are called ligands. The central metal atom binds with a certain number of ligands depending on the electron configuration of that atom. The number of ligands that binds to the central metal atom is called the coordination number.

Besides, the central metal atom has its own oxidation state. We can calculate the oxidation state using the chemical formula of the complex. Here, if we know the net electrical charge of the complex, the charges and the number of ligands attached to the metal atom, we can easily calculate the oxidation state. In other words, primary valency is the number of ligands we need to satisfy the charge on the metal ion.

What is Secondary Valency

Secondary valency is the coordination number of the central metal atom of a coordination complex. The coordination number is the number of ligands attached to the central metal atom. Let us consider an example to understand both primary and secondary valences. In the coordination complex K₄[Fe(CN)₆] the central metal atom is iron (Fe).

We can calculate the primary valency as below:

• The charge of the potassium ligand is always +1.
• The charge of the cyanide ligand (CN) is always -1.
• There are four potassium ligands which equal +4 charge.
• There is six cyanide (CN) ligands which equals -6 charge.
• Then we can calculate the oxidation state of Fe as follows:

The total charge of the complex = 0
0 = [(charge of potassium ligand) x 4] + [charge of the Fe ion] + [(charge of cyanide ligand) x 6]
0 = [(+1) x 4] + [charge of the Fe ion] + [(-1) x 6]
0 = 4 + [charge of the Fe ion] – 6

charge of the Fe ion = +2
oxidation state of Fe = +2

What is the Difference Between Primary and Secondary Valency?

The terms primary and secondary valency fall under the field of coordination chemistry. Here, the key difference between primary and secondary valency is that primary valency is the oxidation state of the central metal atom of a coordination complex. But, secondary valency is the coordination number of the central metal atom of a coordination complex. Moreover, primary valency is the number of ligands we need to satisfy the charge on the metal ion, while secondary valency is the number of ligands attached to the central metal atom.

Below infographic summarizes the difference between primary and secondary valency.

Summary – Primary vs Secondary Valency

The terms primary and secondary valency fall under the coordination chemistry. The key difference between primary and secondary valency is that primary valency is the oxidation state of the central metal atom of a coordination complex. But, whereas secondary valency is the coordination number of the central metal atom of a coordination complex.

Reference:

1. “Transition Metals.” Coordination Complexes and Ligands, Available here.
2. “Orbital Hybridisation.” Wikipedia, Wikimedia Foundation, 14 Nov. 2019, Available here.
3. “Boundless Chemistry.” Lumen, Available here.

Image Courtesy:

1. “Di xeno” By Officer781 – Own work (Public Domain) via Commons Wikimedia
2. “Tetra sulf” By Officer781 – Own work (Public Domain) via Commons Wikimedia