Diradical
Chemical species with two electrons occupying degenerate molecular orbitals
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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.
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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.