Coordination Number Chemistry, Calculations, Examples, and Geometry

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Coordination Number is defined as the number of atoms, ions, or molecules attached to the central atom. The atoms, ions, or molecules attached to the central atom, ion, or molecule are ligands. In other words, the coordination number is equal to the total number of donor atoms/ligands that are attached to the central metal atom.

Alfred Werner coined the term “coordination Number” to describe the total number of bonds between the ligands and the metal atom. Coordination numbers typically range from 2 to 12, with the most prevalent being 4 (tetracoordinate) and 6 (hexacoordinate).

Calculation of Coordination Number

For crystals molecules, the value of the coordination number is determined in different ways. The value is determined by the relative sizes of the central atom and ligands, as well as the charge derived from an ion’s electronic configuration.

Some of the steps to identify the coordination number of complexes are:

  • Determine central atom in the complex.
  • Find the atom, molecule, or ion that is the closest to the central metal atom.
  • Add the number of atoms of the closest atom/molecule/ions together.
  • Find the total number of atoms that are closest to the central atom. Add the two values if the metal has two bonded atoms.

The coordination number in polyatomic molecules or ions is calculated by simply calculating the number of ligands or atoms, ions, or molecules attached to the central metal atom (except the number of chemical bonds).

In coordination complexes, only the sigma bond of ligands coordinated with central atoms are considered.

Coordination Number Examples

Coordination number, coordination number examples, bcc coordination number, coordination number of hcp, coordination number of fcc

Geometry based on Coordination Number

The molecular geometry of the coordination complex can be deduced using the coordination number of the central metal atom/ion. Based on the coordination number, the molecular geometry of some of the coordination compounds are given as follows:

Coordination Number (CN)Molecular Geometry/StructureExamples
2Linear[Hg(NH3)2]+
3Trigonal planar[AgI3]
4TetrahedralNi(CO)4
4Square Planar[Cu(NH3)4]2+
5Trigonal bipyramidalFe(CO)5
6Octahedral [Mn(H2O)6]2+
6Simple Cubic Packing NaCl
8Body-centered cubic (BCC)CsCl
12Hexagonal close packing (HCP)The hcp structure is very common for elemental metals and some examples include beryllium, cadmium, magnesium, titanium, zinc, and zirconium.
12Face centered cubic (FCC)Metals with the fcc structure include aluminum, copper, nickel, gamma iron, gold, and silver.

Similarly, some of the structures based on the coordination Number are:

Coordination NumberStructureExamples
8:4Fluorite structureCaF2
4:8Anti-fluorite structureNa2O
6:3Rutile structureTiO2

Coordination Number Video

MCQs/FAQs

Coordination number of face-centered cubic (fcc)

The coordination number of face-centered cubic (fcc) is 12.

Coordination number of bcc

The coordination number of bcc is 8.

Coordination number of hcp (hexagonal close packing)

The Coordination number of hcp (hexagonal close packing) is 12.

Coordination number of NaCl

The Coordination number of NaCl is 6.

Which metallic crystal structure has a coordination number of 8?

Body-Centered Cubic (BCC) has a coordination number of 8.

The coordination number of copper in cuprammonium sulphate is

The coordination number of copper in cuprammonium sulphate is 4.

rock salt coordination number

The coordination number of rock salt is 6.

Simple cubic lattice coordination number

The Simple cubic lattice has a coordination number of 6.

Coordination number of iron in ferrocene

The Coordination number of iron in ferrocene is 10.

Coordination number of the central metal atom

The coordination number of the central metal atom is the number of ions, atoms,s, or molecules attached to it.

CaF2 crystal structure coordination number

Fluorite coordination number, coordination number

CaF2 crystal structure coordination number is 8:4

coordination number of rutile

Rutile, rutile coordination number, coordination number

The coordination number of rutile is 6:3

coordination number of ethylenediamine

The coordination number of ethylenediamine is 6 as it is a bidentate ligand.

ruthenium coordination number

The most common coordination number of ruthenium is 6.

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