The elusive ethenediselone, Se=C=C=Se

The neutral ethenediselone, Se=C=C=Se, has been characterised by neutralisation-reionisation mass spectrometry, which implies a minimum lifetime of the order of microseconds. Tetraselenafulvalene 1 and tetramethyltetraselenafulvalene 2 were used as precursor molecules. Flash vacuum thermolysis (FVT) of these compounds with isolation of the products in Ar matrices permitted the identification of ethyne, 2-butyne, CSe₂, and selenoketene, H₂C=C=Se, but at best traces of Se=C=C=Se survived the FVT/matrix isolation experiment. Multiconfigurational calculations indicate that Se=C=C=Se is a ground state triplet molecule with a very small singlet-triplet gap.