Diradical

In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate. The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and non-Kekulé molecules that are rarely isolated. Diradicals are even-electron molecules but have one fewer bond than the number permitted by the octet rule.

Examples of diradical species can also be found in coordination chemistry, for example among bis(1,2-dithiolene) metal complexes.

Spin states

Diradicals are usually triplets. The phrases singlet and triplet are derived from the multiplicity of states of diradicals in electron spin resonance: a singlet diradical has one state (S=0, Ms=2*0+1=1, ms=0) and exhibits no signal in EPR and a triplet diradical has 3 states (S=1, Ms=2*1+1=3, ms=-1; 0; 1) and shows in EPR 2 peaks (if no hyperfine splitting). The triplet state has total spin quantum number S=1 and is paramagnetic. Therefore, diradical species display a triplet state when the two electrons are unpaired and display the same spin. When the unpaired electrons with opposite spin are antiferromagnetically coupled, diradical species can display a singlet state (S=0) and be diamagnetic.

Examples

Stable, isolable, diradicals include singlet oxygen and triplet oxygen. Other important diradicals are certain carbenes, nitrenes, and their main-group elemental analogues. Lesser-known diradicals are nitrenium ions, carbon chains, and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms. N-heterocyclic-carbene derived singlet diradicals have been used for solution-phase singlet fission via self-assembly. Some isolable Kekulé diradicals are stable on air and room-temperature luminescent with large Stokes-shifts and solvatochromism. Main-group cyclic structures can also exhibit diradicals, such as disulfur dinitride, or diradical character, such as diphosphadiboretanes. In inorganic chemistry, both homoleptic and heteroleptic 1,2-dithiolene complexes of d8 transition metal ions show a large degree of diradical character in the ground state.

Diradicals in which the unpaired electrons nevertheless interact are sometimes referred to as diradicaloids.

References

  1. ↑ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Diradicals". doi:10.1351/goldbook.D01765
  2. ↑ Abe M (September 2013). "Diradicals". Chemical Reviews. 113 (9): 7011–7088. doi:10.1021/cr400056a. PMID 23883325.
  3. 1 2 Aragoni MC, Caltagirone C, Lippolis V, Podda E, Slawin AM, Woollins JD, et al. (December 2020). "Diradical Character of Neutral Heteroleptic Bis(1,2-dithiolene) Metal Complexes: Case Study of [Pd(Me2timdt)(mnt)] (Me2timdt=1,3-Dimethyl-2,4,5-trithioxoimidazolidine; mnt2-=1,2-Dicyano-1,2-ethylenedithiolate)". Inorganic Chemistry. 59 (23): 17385–17401. doi:10.1021/acs.inorgchem.0c02696. PMC 7735710. PMID 33185438.
  4. ↑ Ray K, Weyhermüller T, Neese F, Wieghardt K (July 2005). "Electronic structure of square planar bis(benzene-1,2-dithiolato)metal complexes [M(L)(2)](z) (z=2-, 1-, 0; M=Ni, Pd, Pt, Cu, Au): an experimental, density functional, and correlated ab initio study". Inorganic Chemistry. 44 (15): 5345–5360. doi:10.1021/ic0507565. PMID 16022533.
  5. ↑ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Triplet State". doi:10.1351/goldbook.T06503
  6. ↑ Bachler V, Olbrich G, Neese F, Wieghardt K (August 2002). "Theoretical evidence for the singlet diradical character of square planar nickel complexes containing two o-semiquinonato type ligands". Inorganic Chemistry. 41 (16): 4179–4193. doi:10.1021/ic0113101. PMID 12160406.
  7. ↑ Sharma, Mahendra K.; Ebeler, Falk; Glodde, Timo; Neumann, Beate; Stammler, Hans-Georg; Ghadwal, Rajendra S. (2021-01-13). "Isolation of a Ge(I) Diradicaloid and Dihydrogen Splitting". Journal of the American Chemical Society. 143 (1): 121–125. doi:10.1021/jacs.0c11828. ISSN 0002-7863. PMID 33373236. S2CID 229719653.
  8. ↑ Seenithurai S, Chai JD (July 2017). "Effect of Li Termination on the Electronic and Hydrogen Storage Properties of Linear Carbon Chains: A TAO-DFT Study". Scientific Reports. 7 (1): 4966. arXiv:1702.03055. Bibcode:2017NatSR...7.4966S. doi:10.1038/s41598-017-05202-6. PMC 5504039. PMID 28694445.
  9. ↑ Ullrich, T.; Pinter, P.; Messelberger, J.; Haines, P.; Kaur, R.; Hansmann, M. M.; Munz, D.; Guldi, D. (2020). "Singlet Fission in Carbene Derived Diradicaloids". Angewandte Chemie International Edition. 59: 7906. doi:10.1002/anie.202001286.
  10. ↑ Messelberger, J.; Grünwald, A.; Pinter, P.; Hansmann, M.; Munz, D. (2018). "Carbene Derived Diradicaloids – Building Blocks for Singlet Fission?". Chemical Science. 9: 6107. doi:10.1039/C8SC01999A. PMC 6053972.
  11. ↑ Hansmann, M. M.; Melaimi, M.; Munz, D.; Bertrand, G. (2018). "Modular Approach to Kekulé Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes". Journal of the American Chemical Society. 140: 2546. doi:10.1021/jacs.7b11183.
  12. ↑ Bevilacqua, M.; Reato, M.; Cilento, F.; Graiff, C.; Antonello, S.; Schio, L.; Aliprandi, A.; Tubaro, C.; Franco, L.; Dell'Angela, M.; Munz, D.; Baron, M. (2026). "Hidden Diradical: Conformational Switch for Solvatochromic NIR Emission with Unity Quantum Yield in Thiele's Hydrocarbon". Angewandte Chemie International Edition e202524042. doi:10.1002/anie.202524042. hdl:11577/3586820.
  13. ↑ Punzi, A.; Ullrich, T.; Orza, M.; Mesto, D.; Moliterni, A.; Olieric, V.; Engilberge, S.; Giannini, C.; others (2026). "Twist and Shine: The Impact of Halogen Substitution on Thiele Hydrocarbon's Optical Properties". Angewandte Chemie International Edition e24043. doi:10.1002/anie.202524043. PMC 12865251.

Further reading