Coordination number refers to the number of ligands that are directly bonded to a central metal atom or ion in a coordination complex. It defines the number of coordination sites available on the metal for ligand attachment. The coordination number determines the overall geometry of the complex and has implications for its physical and chemical properties.

Here are a few key points about coordination number:

1. Range of Values: Coordination numbers can range from 2 to more than 12, depending on the metal, ligands, and the overall structure of the complex.
2. Common Values: Common coordination numbers include 2, 4, 6, and 8, with other values less commonly observed.
3. Geometries: The coordination number influences the geometry of the complex. For example, a coordination number of 2 often leads to a linear geometry, while coordination numbers of 4 and 6 can result in tetrahedral or octahedral geometries, respectively.
4. Steric and Electronic Factors: The choice of coordination number is influenced by both steric (geometric) and electronic factors. Steric factors involve the spatial arrangement of ligands around the metal, while electronic factors consider how the ligands’ electron-donating or -withdrawing properties affect the metal’s d-orbital splitting.
5. Chelating Ligands: Ligands that can bind to a metal at multiple sites can impact the coordination number. A chelating ligand can effectively occupy more than one coordination site, leading to coordination numbers higher than the ligand’s count.
6. Influence on Properties: Coordination number affects properties such as color, magnetic behavior, and reactivity of the complex. For instance, a complex with a coordination number of 4 may have different properties than a complex with a coordination number of 6.
7. Coordination Polyhedra: The arrangement of ligands around the metal can form various geometric shapes known as coordination polyhedra. These shapes include tetrahedral, square planar, trigonal bipyramidal, and octahedral polyhedra.
8. Biological Relevance: Coordination numbers are significant in biological systems where metal ions play essential roles in enzymes, cofactors, and other biomolecules. The coordination number influences the metal’s reactivity and binding affinity.
9. Determining Coordination Number: The coordination number can often be determined experimentally through techniques like X-ray crystallography, NMR spectroscopy, and other structural analyses.

In summary, the coordination number is a fundamental aspect of coordination chemistry that determines how ligands are arranged around a central metal atom or ion in a coordination complex. It has a profound impact on the complex’s geometry, properties, and reactivity.