Coordination Number

In the field of chemistry and materials science, the coordination number is a fundamental concept that describes the number of atoms, ions, or molecules that a central atom or ion is directly bonded to within a molecule or a crystal. This central atom is often referred to as the coordination center, and the surrounding entities are known as ligands. The coordination number is critical in determining the structure and properties of coordination complexes and is key to understanding coordination geometry.

Coordination Number in Molecules and Crystals

For molecules and polyatomic ions, the coordination number is determined by counting the atoms directly bonded to the central atom. This counting includes all types of bonds, whether single, double, or triple. For example, in the complex ([Cr(NH_3)_2Cl_2Br_2]^{-}), the Chromium ion (Cr) serves as the central atom and has a coordination number of 6, described as hexacoordinate.

In crystalline solids, the coordination number is determined by the number of nearest neighbors surrounding an atom in the crystal lattice. This number can vary significantly depending on the crystal structure. For instance, in a body-centered cubic lattice, each atom has a coordination number of 8, while in a face-centered cubic lattice, the coordination number is 12.

Coordination Number in Transition Metal Complexes

In the context of transition metal complexes, coordination numbers can vary widely. They typically range from 2 to more than 9. For example, gold (I) in the complex ([Ph_3PAuCl]) has a coordination number of 2, while rhenium (VII) in ([ReH_9]^{2-}) has a coordination number of 9. The variation in coordination numbers is often due to the availability of d-orbitals which can accommodate multiple bonds with ligands.

High Coordination Numbers: f-Block Elements

Metals belonging to the f-block, such as the lanthanoids and actinoids, can exhibit even higher coordination numbers. This is attributed to their larger ionic radii and the availability of additional orbitals for bonding. These elements can achieve coordination numbers exceeding 9, which is uncommon in other regions of the periodic table.

Theoretical Coordination Extremes

Recent computational chemistry studies have predicted unusually high coordination numbers in theoretical constructs. For instance, a particularly stable ion, ([PbHe_2]^{15+}), has been proposed, consisting of a central lead ion coordinated by up to 15 helium atoms. Moreover, in certain metallic phases, such as the Frank–Kasper phases, coordination numbers can reach as high as 16, revealing the extraordinary versatility and complexity of coordination chemistry.

Related Topics

• Valence (chemistry)
• Orbital hybridization
• Pauling's rules
• Coordination polymers
• Tetrahedral molecular geometry

Understanding coordination number is vital to the study of molecular geometry, bonding, and the overall stability of compounds in both natural and synthetic contexts.