A diradical based on odd-electron σ-bonds

The concept of odd-electron σ–bond was first proposed by Linus Pauling. Species containing such a bond have been recognized as important intermediates encountered in many fields. A number of radicals with a one-electron or three-electron σ-bond have been isolated, however, no example of a diradical based odd-electron σ-bonds has been reported. So far all stable diradicals are based on two s/p-localized or π-delocalized unpaired electrons (radicals). Here, we report a dication diradical that is based on two Se∴Se three-electron σ–bonds. In contrast, the dication of sulfur analogue does not display diradical character but exhibits a closed-shell singlet.