Diradicals are even-electron molecules but have one fewer bond than the number permitted by the octet rule.
The triplet state has total spin quantum number S=1 and is paramagnetic.
When the unpaired electrons with opposite spin are antiferromagnetically coupled, diradical species can display a singlet state (S=0) and be diamagnetic.
Other important diradicals are certain carbenes, nitrenes, and their main-group elemental analogues.
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.