One-electron bonds may be more common than chemists thought

Many chemical bonds could be better described as one-electron bonds. That’s according to researchers in Spain whose computational study identified situations where, rather than a pair of delocalised electrons, the bonding is driven by a single ‘active’ electron, while a second ‘spectator’ electron remains largely localised on one fragment.¹ Examining simple systems conventionally described as covalent, ionic or dative, the team found evidence for one-electron bonds across all three categories. Challenging the widely held belief of electron pairs as the fundamental unit of bonding, researchers say the findings could redefine how we think about and teach chemical bonding. The team is keen to see researchers spanning inorganic chemistry and beyond put the perspective to the test.

Dative bonds are broadly considered to be a type of electron pair bond in which both electrons are supplied by the same atom. However, Ángel Martín Pendás from the University of Oviedo, who led the study, says this description leaves outstanding questions when it comes to the properties of such systems. ‘One of the fights yet to be solved over the years is: what exactly is a dative bond? The donor–acceptor definition has been influential in the last decades and understanding dative bonds is very useful but not very well understood.’

And while it is well-known that dative bonds are weak, Martín Pendás believes his team’s one-electron perspective could help explain why.

He says the finding that one electron ‘does not move at all’ during the bond formation process in several dative, covalent and ionic systems is ‘pretty interesting and amazing’. By using a combination of quantum chemistry approaches, including Monte Carlo simulations and the electron distribution function, the researchers observed one electron bonding across several systems known for their weak, elongated bonds, which had previously been classified using different bonding models.

NH₃BH₃ has traditionally been viewed as a donor–acceptor complex, formed when the lone pair of electrons on ammonia is donated into an empty orbital on borane. However, when the researchers mapped the electrons’ distribution within the molecule, rather than the expected electron pair, they observed an ‘active’ delocalised electron, which is involved in bonding the NH₃ fragments, and a ‘spectator’ electron that remained localised at the ammonia.

The researchers observed similar behaviour in LiH, an ionic system whose bond is conventionally described as a two-centre, two-electron interaction. Their calculations again indicated that only one electron in the pair is actively involved in bonding.

Finding that one electron stays on the donor, while the other delocalised partner participates in bonding aligns with a 1989 study from Arne Haaland,² which showed that dative bonds break heterolytically, forming species where the acceptor fragment is then more reactive for subsequent chemical transformations.

Cina Foroutan-Nejad, a theoretical and computational chemist from the Polish Academy of Sciences, describes the research as ‘very thorough’ and is intrigued to see how it might be applied beyond theory. He notes that it differs ‘fundamentally’ from the previously observed one-electron covalent bond between two carbons, which involved just a single electron. He says this research provides a new framework for not just understanding bonding, but also how electrons couple: ‘It [the electron involved in bonding across these systems] is moving more freely compared to the other electron. But its freedom of movement doesn’t mean it is not paired with the other, localised electron. Like a married couple: if one of them goes for some mission overseas, they remain a couple.’

Still, Foroutan-Nejad sees difficulties in term of translating the one-electron concept into reaction mechanism design. He says testing in inorganic complexes and x-ray structures could provide ‘a snapshot’ of the accumulation of charge. Further work using time-resolved spectroscopy could offer insights into the electron exchange, but he’s unsure which reactions would be best to test this system on.

A challenge to convention

With this new perspective, Martín Pendás seeks to move away from orbital theory: ‘Everybody thinks in terms of orbitals, so if we manage to convince people that other ways of thinking can be useful, that would be good for me.’

He says the active and spectator electrons cannot be shown by orbital theory because the two electrons are described by the same function. Using generalised valence bond theory (GBV) – not the most accurate alone – in combination with electron distribution and density matrix systems allowed the team to describe electrons ‘in real space’.

Martín Pendás says with real-space theories of chemical bonding like adopted in this study, ‘you can do chemistry without orbitals’, which may cause issues for chemists and teachers who rely on orbital theory, but could help tackle many problems: ‘I’ve been talking with colleagues who have synthesis systems based on lone pairs and dative bonds in which they end up with radical pairs. If you start thinking in this way, you may come to new reaction pathways and provide a new way to prepare radicals. I guess the orbital community will not move much from their position. If applied in radical chemistry, it could be impactful and I think we’d be closer to convincing wider communities to think outside of the box.’

But Frank Neese, director of molecular theory and spectroscopy at the Max Planck Institute for Coal Research, says the study ‘didn’t capture’ him. He says the GBV model used by the team presents an interpretation that may not withstand further scrutiny: ‘It wasn’t clear if I was looking at an artefact of GBV or fundamental physics. I may have missed something important, but I find the picture they’re painting pretty bizarre. I’d be surprised if it left a big mark. I’m really happy to be wrong 10 years down the road.’

Neese adds that CASSCF calculations – which map active electrons across active orbital space and testing on larger molecules – could provide more meaningful insight.

However, Foroutan-Nejad says using CASSCF on the relatively simple molecules examined in the research would be overkill: ‘From the perspective of computational chemists, using CASSCF to solve the electronic structure of something this is like killing a mosquito with a tank. You don’t need to do that. This is a very well-behaved, simple system.’

Martín Pendás welcomes wider conversations, noting that the study builds on decades of research but is unlikely to settle the debate: ‘There shouldn’t be such a myriad of different ways to understand what a chemical bond is. I don’t know where we’re going, but we’re putting our grains of sand in.’