Coordination number

Coordination number refers to the number of atoms, ions, or molecules that a central atom or ion can bind to in a coordination compound or complex. This concept is fundamental in the field of inorganic chemistry, particularly in the study of crystal structures and molecular geometry. The coordination number primarily depends on the size, charge, and electron configuration of the central atom or ion as well as the ligands that it binds to.

Definition

The coordination number of a central atom in a molecule or crystal is the number of atoms, ions, or molecules directly bonded to it. This includes the number of ligand atoms to which the central atom is directly bonded in a coordination complex. The ligands are usually molecules like water or ammonia, or ions such as chloride or hydroxide.

Determining Coordination Numbers

Coordination numbers can be determined through various methods, including X-ray crystallography, Neutron diffraction, and Electron diffraction. These techniques allow for the visualization of the spatial arrangement of atoms within a molecule or crystal, thereby revealing the coordination geometry and number.

Common Coordination Numbers

While coordination numbers can range from 2 to 16, the most common coordination numbers are 2, 4, and 6. For example, in a tetrahedral coordination, the coordination number is 4, whereas in an octahedral coordination, it is 6. The coordination number can provide insight into the stability and reactivity of the coordination compound.

Factors Affecting Coordination Number

Several factors influence the coordination number of a central atom or ion, including:

• Size of the central atom or ion: Larger atoms or ions can accommodate more ligands due to their larger size.
• Charge of the central atom or ion: Higher charges can attract more ligands due to the increased electrostatic attraction.
• Electron configuration: The availability of empty orbitals in the central atom or ion for bonding with ligands.
• Size and charge of the ligands: Larger or highly charged ligands may occupy more space or provide stronger electrostatic attractions, affecting the coordination number.

Coordination Geometry

The coordination number is closely related to the coordination geometry, which describes the shape formed by the points where the ligands are attached to the central atom. Common coordination geometries include linear, tetrahedral, square planar, and octahedral.

Applications

Understanding coordination numbers is crucial in various fields such as Biochemistry, where it plays a role in the structure and function of Enzymes and Metalloproteins; in Material science, for the design of new materials with specific properties; and in Catalysis, where coordination compounds often serve as catalysts in industrial processes.

See Also

• Ligand
• Complex
• Crystal field theory
• Ligand field theory
• Metal ions in aqueous solution