Primary Valence and Secondary Valence

Werner's theory of coordination compounds revolutionized the understanding of complex compounds and their behavior. One of the key concepts proposed by Werner was the idea of primary and secondary valence.

1. Primary Valence:
- The primary valence refers to the total number of ligands directly bound to the central metal ion through coordinate covalent bonds.
- It represents the oxidation state or charge of the metal ion.
- It determines the overall charge of the complex and its stability.
- The primary valence is often determined by the periodic table group number of the metal ion.

2. Secondary Valence:
- The secondary valence, also known as coordination number, refers to the number of groups directly bound to the metal ion in a coordination complex.
- It represents the number of bonds formed by the ligands with the central metal ion.
- The coordination number can vary depending on the metal ion and the ligands involved.
- The coordination number can range from 2 to 12, with common coordination numbers being 4, 6, and 8.
- The coordination number is determined by the electron pair accepting capacity of the metal ion and the electron donor capacity of the ligands.

Proposal of Secondary Valence as the Correct Term:
- Werner proposed the term "secondary valence" to describe the number of groups directly bound to the metal ion in a coordination complex.
- This term is more appropriate because it focuses on the bonding between the metal ion and the ligands.
- It helps to distinguish between the primary valence, which represents the overall charge of the complex, and the secondary valence, which represents the coordination number or the number of bonds formed by the ligands.
- The secondary valence concept is widely used in coordination chemistry to describe the geometry and stability of coordination complexes.

In summary, Werner proposed the term "secondary valence" to describe the number of groups directly bound to the metal ion in a coordination complex. This term focuses on the bonding between the metal ion and the ligands and helps to distinguish it from the primary valence, which represents the overall charge of the complex. The concept of secondary valence is widely used in coordination chemistry to describe the geometry and stability of coordination complexes.