Werner proposed the term "Secondary valence" for the number of groups bound directly to the metal ion in a coordination complex.

Explanation:
- Coordination complexes are compounds that consist of a central metal ion or atom surrounded by ligands. Ligands are molecules or ions that donate electron pairs to the metal ion to form coordinate bonds. The number of ligands bound directly to the metal ion is known as the coordination number.
- Alfred Werner was a Swiss chemist who made significant contributions to the field of coordination chemistry. He proposed the concept of primary and secondary valence to explain the bonding in coordination complexes.
- The primary valence refers to the oxidation state or the charge of the metal ion, which determines its ability to accept or donate electrons. This is equivalent to the oxidation number of the metal ion.
- The secondary valence, on the other hand, refers to the number of groups directly bound to the metal ion. These groups can be ligands or other atoms or ions attached to the metal ion.
- Werner observed that the coordination number of a metal ion is often equal to its primary valence. For example, a metal ion with an oxidation state of +2 would typically have a coordination number of 6, meaning it is surrounded by 6 ligands.
- However, there are cases where the coordination number does not match the primary valence. Werner proposed that this discrepancy can be explained by the presence of secondary valence. The secondary valence represents the additional bonds formed by the metal ion with ligands or other groups.
- The concept of secondary valence helped to explain the existence of different coordination numbers and geometries in coordination complexes. It provided a more comprehensive understanding of the bonding in these compounds.
- Werner's work laid the foundation for the modern understanding of coordination chemistry and earned him the Nobel Prize in Chemistry in 1913. His proposed term, "Secondary valence," is used to this day to describe the number of groups bound directly to the metal ion in a coordination complex.