coordinationnumber

Coordination number is a term used in chemistry and crystallography to describe the number of donor atoms bound to a central atom. In coordination chemistry, it is the number of atoms or ions directly bonded to the central metal atom or ion in a coordination complex. In solid-state chemistry and crystallography, the coordination number can also refer to the number of nearest neighboring atoms bonded to a given atom within a crystal lattice.

Determination and factors: The coordination number is determined by the geometry around the metal center and is influenced by factors such as the ionic size and charge of the metal, the size and denticity of the ligands, and steric constraints. It is important to note that CN counts donor atoms, not ligands. A single multidentate ligand can occupy multiple coordination sites, while several monodentate ligands bound to different donor atoms each contribute one to the CN.

Common coordination numbers and geometries: Typical coordination numbers include 2 for linear arrangements, 4 for tetrahedral or square planar geometries, and 6 for octahedral geometries. Higher or lower CN values can occur, with CN 5 corresponding to trigonal bipyramidal or square pyramidal shapes, and CN values up to 7–9 found in certain complexes or extended solid structures.

Examples: In the hexamminecobalt(III) complex, [Co(NH3)6]3+, the coordination number is 6. In the hexacyanoferrate(III) complex, [Fe(CN)6]3−, the coordination number is also 6. A square-planar complex such as [Ni(CN)4]2− has CN = 4, while Fe(CO)5, with five carbonyl ligands, has CN = 5.

Significance: The coordination number influences the geometry, reactivity, and physical properties of a complex or solid. It helps predict ligand binding patterns, ligand lability, and the overall stability of structures in inorganic chemistry and materials science.