Title: coordination number
Long Title: IUPAC Gold Book - coordination number
DOI: 10.1351/goldbook.C01331
Status: current

Definitions
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Definition (1)
The coordination number of a specified atom in a chemical species is the number of other atoms directly linked to that specified atom. For example, the coordination number of carbon in methane is four, and it is five in protonated methane, $\ce{CH5^{+}$. (The term is used in a different sense in the crystallographic description of ionic crystals.)

Related Terms
- chemical species: https://goldbook.iupac.org//terms/view/CT01038
- coordination: https://goldbook.iupac.org//terms/view/C01329

Source
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1100 (https://doi.org/10.1351/pac199466051077)

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Definition (2)
In an inorganic coordination entity, the number of $\ce{\upsigma\!\mbox{-}bonds}$ between ligands and the central atom. $\ce{\uppi\!\mbox{-}bonds}$ are not considered in determining the coordination number.

Related Terms
- central atom: https://goldbook.iupac.org//terms/view/C00930
- coordination: https://goldbook.iupac.org//terms/view/C01329
- coordination entity: https://goldbook.iupac.org//terms/view/C01330
- ligands: https://goldbook.iupac.org//terms/view/L03518

Source
- Red Book, 3rd ed., p. 146

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Other Outputs
- html: https://goldbook.iupac.org/terms/view/C01331/html
- json: https://goldbook.iupac.org/terms/view/C01331/json
- xml: https://goldbook.iupac.org/terms/view/C01331/xml

Citation: Citation: 'coordination number' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.C01331
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Accessed: 2026-06-07T05:13:23+00:00