Term: biradical
https://doi.org/10.1351/goldbook.B00671

Definition:
An even-electron molecular entity with two (possibly delocalized) radical centres which act nearly independently of each other, e.g. B00671.png Species in which the two radical centres interact significantly are often referred to as 'biradicaloids'. If the two radical centres are located on the same atom, the species are more properly referred to by their generic names: carbenes, nitrenes, etc. The lowest-energy triplet state of a biradical lies below or at most only a little above its lowest singlet state (usually judged relative to kB.T, the product of the Boltzmann constant kB and the absolute temperature T). The states of those biradicals whose radical centres interact particularly weakly are most easily understood in terms of a pair of local doublets. Theoretical descriptions of low-energy states of biradicals display the presence of two unsaturated valences (biradicals contain one fewer bond than permitted by the rules of valence): the dominant valence bond structures have two dots, the low energy molecular orbital configurations have only two electrons in two approximately nonbonding molecular orbitals, two of the natural orbitals have occupancies close to one, etc. Although this term has been recommended in the past for diradicals, specialists working in the field prefer the latter term.

Related Terms:
1) carbenes (http://doi.org/10.1351/goldbook.C00806).
2) nitrenes (http://doi.org/10.1351/goldbook.N04145).
3) molecular entity (http://doi.org/10.1351/goldbook.M03986).
4) configurations (http://doi.org/10.1351/goldbook.C01248).
5) diradicals (http://doi.org/10.1351/goldbook.D01765).
6) triplet state (http://doi.org/10.1351/goldbook.T06503).
7) singlet state (http://doi.org/10.1351/goldbook.S05699).
8) boltzmann constant (http://doi.org/10.1351/goldbook.B00695).
9) valence (http://doi.org/10.1351/goldbook.V06588).
10) molecular orbital (http://doi.org/10.1351/goldbook.M03996).

Image: Not defined (https://goldbook.iupac.org/img/inline/B00671.png) 

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1089 (https://doi.org/10.1351/pac199466051077)

Citation: 'biradical' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.B00671

License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.

Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.