Covalent bonds can contain two, four, six or eight electrons – or just one
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It is not often that chemists create a new kind of chemical bond, but they have just done it. A covalent bond that relies on a single electron has been made almost a century after it was first proposed.
Takuya Shimajiri at the University of Tokyo and his colleagues have been testing the limits of chemical bonds for years. Previously, they experimented with unusually long and flexible bonds, and now they have taken on an idea first proposed in 1931 by chemist Linus Pauling: a chemical bond formed by just one electron.
All known covalent bonds, where atoms connect by sharing electrons, contain two, four, six or eight electrons – but Pauling theorised a covalent bond could exist with a single electron shared between two atoms. To create this, the researchers used a chemical reaction to remove an electron from an existing two-electron covalent bond between two carbon atoms. They used a large hydrocarbon that has exceptionally long bonds between its carbon atoms, which means it would be energetically costly for an electron from elsewhere in the molecule to replace the one they removed.
Shimajiri says past experiments that attempted such electron subtraction left behind weak bonds which broke too quickly for a definitive chemical analysis. But his team’s molecule remained stable enough they could analyse it with X-rays and several kinds of light. Based on how this radiation bounced off or was absorbed, they determined it had a stable one-electron bond.
“It’s not often that you find a molecule with a new kind of bond,” says Henry Rzepa at Imperial College London. He says the molecule had a total of 278 electrons, so it was a real feat to both remove the correct one and prevent all the others from immediately replacing it. Rzepa says this is a “major discovery” that could lead chemists to create whole new families of molecules.
Chemists can now study how one-electron covalent bonds may change chemical reactions, says Shimajiri. But he and his colleagues have bigger questions, too.
“We aim to clarify what a covalent bond is – specifically, at what point does a bond qualify as covalent, and at what point does it not? Our goal is to explore a wide range of bonds that have yet to be discovered,” he says.
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