Scientists in Japan have made a groundbreaking discovery: they have identified a covalent bond between two carbon atoms that share only a single electron. ‘This [study was driven] by a curiosity to know where the limits of chemical bonds are,’ says Takuya Shimajiri at the University of Tokyo.

One-electron bonds were first proposed in 1931 by Linus Pauling and have since been reported between heteroatoms. However, no direct evidence of their existence between carbon atoms had ever been observed. ‘We came up with the idea that it would be possible under the proper molecular design,’ says Shimajiri.

The sharing of electron pairs serves an important purpose: it fills an atom’s outer shell of electrons to make it more stable. With one crucial electron missing, one-electron bonds are understandably much weaker and, therefore, break down quickly. As such, they have only been proposed to exist transiently as intermediates in chemical reactions.