Coordination Number
Definition and meaning of Coordination Number in chemistry.
Coordination number is the number of donor atoms (ligand atoms) directly bonded to a central metal atom or ion in a coordination complex, or the number of nearest-neighbor ions surrounding a given ion in a crystal lattice.
In more detail
In complexes, coordination number is set mainly by the size and charge of the central metal ion and by the size and electronic properties of the ligands, and it dictates the geometry around the metal (4 gives tetrahedral or square planar; 6 gives octahedral). A polydentate ligand can supply several donor atoms from a single molecule, so coordination number is not simply the count of ligand molecules. In ionic solids, coordination number instead reflects the cation-to-anion radius ratio and determines the packing arrangement of the lattice.
Key facts
| Common values | 2, 4, and 6 are most frequent for transition-metal complexes |
|---|---|
| Example ion | [Co(NH3)6]3+ has coordination number 6 |
| Ionic solids | NaCl has coordination number 6 for both Na+ and Cl- |
| Field | Inorganic Chemistry |
In the complex ion [Co(NH3)6]3+, the cobalt(III) ion is bonded to six nitrogen atoms from six ammonia ligands, giving a coordination number of 6 and an octahedral geometry.
Frequently asked questions
Is coordination number the same as the number of ligand molecules?
Not necessarily. A monodentate ligand contributes one donor atom per molecule, but a polydentate ligand like EDTA can occupy six coordination sites while remaining a single ligand molecule.
What determines coordination number in an ionic crystal?
The radius ratio of the cation to the anion largely determines coordination number, since it governs how many oppositely charged ions can pack around a central ion without touching each other.