Many chemical connections might be more accurately portrayed as one-electron connections. This is the conclusion drawn by researchers in Spain, whose computational research uncovered instances wherein, instead of a pair of delocalized electrons, the bonding is influenced by a solitary ‘active’ electron, while a second ‘spectator’ electron stays predominantly localized on one component. Investigating straightforward systems typically characterized as covalent, ionic, or dative, the team discovered indications of one-electron connections across all three types. This challenges the commonly accepted notion of electron pairs as the foundational element of bonding, with researchers suggesting that these insights could transform our understanding and teaching of chemical bonding. The team is eager for researchers across inorganic chemistry and further fields to test this viewpoint.
Dative bonds are generally regarded as a form of electron pair bond where both electrons are provided by a single atom. Nevertheless, Ángel Martín Pendás from the University of Oviedo, who directed the study, states that this definition raises unresolved questions regarding the characteristics of such systems. ‘One of the inquiries that remains unresolved over the years is: what precisely constitutes a dative bond? The donor–acceptor definition has played a significant role in recent decades, and comprehending dative bonds is very beneficial but not extensively understood.’
While it is commonly recognized that dative bonds are weak, Martín Pendás is convinced that his team’s one-electron viewpoint could elucidate the reasons behind this.
He mentions that the discovery that one electron ‘does not move at all’ during the bond formation in various dative, covalent, and ionic systems is ‘quite fascinating and remarkable’. By employing a blend of quantum chemistry methodologies, including Monte Carlo simulations and the electron distribution function, the researchers noted one electron bonding across multiple systems known for their weak, elongated bonds, which had previously been categorized using various bonding models.
NH₃BH₃ has historically been perceived as a donor–acceptor complex, formed when the lone electron pair from ammonia is transferred into an unoccupied orbital on borane. However, upon mapping the electron distribution within the molecule, rather than detecting the expected electron pair, the researchers identified an ‘active’ delocalized electron, which contributes to bonding the NH₃ fragments, alongside a ‘spectator’ electron that remained localized at the ammonia.
The researchers observed parallel behavior in LiH, an ionic system typically described as a two-centre, two-electron interaction. Their computations again suggested that only one electron in the pair is actively engaged in bonding.
Discovering that one electron remains with the donor, while the other delocalized partner engages in bonding, corroborates a 1989 study by Arne Haaland, which demonstrated that dative bonds cleave heterolytically, producing species where the acceptor fragment becomes more reactive for subsequent chemical conversions.
Cina Foroutan-Nejad, a theoretical and computational chemist from the Polish Academy of Sciences, characterizes the research as ‘very comprehensive’ and is curious to see its practical applications beyond theoretical frameworks. He points out that it is ‘fundamentally’ different from the previously noted one-electron covalent bond between two carbons, which involved merely a single electron. He asserts that this research provides a novel basis for not only grasping bonding but also understanding how electrons interact: ‘It [the electron entangled in bonding across these systems] moves more freely compared to the other electron. However, this enhanced mobility does not imply it is not paired with the other localized electron. Similar to a married couple: if one of them goes on a trip abroad, they remain a couple.’
Nonetheless, Foroutan-Nejad perceives challenges in adapting the one-electron concept for reaction mechanism design. He suggests that investigations in inorganic complexes and x-ray structures could yield ‘a snapshot’ of charge accumulation. Additional research employing time-resolved spectroscopy might provide insights into electron exchange, but he is uncertain which reactions would be the most suitable to test this system.
A challenge to convention
With this fresh perspective, Martín Pendás aims to shift the focus away from orbital theory: ‘Everyone thinks in terms of orbitals, so if we can convince people that alternate ways of thinking can be beneficial, that would be advantageous for me.’
He asserts that the active and spectator electrons cannot be illustrated by orbital theory, as both electrons are depicted by the same function. Employing generalized valence bond theory (GBV) – not the most precise on its own – in conjunction with electron distribution and density matrix systems enabled the team to characterize electrons ‘in real space’.
Martín Pendás states that with real-space theories of chemical bonding like those applied in this research, ‘you can perform chemistry without orbitals’, which may create challenges for chemists and educators who depend on orbital theory, but could assist in addressing numerous issues: ‘I’ve been conversing with colleagues who have synthesis systems based on lone pairs and dative bonds, where they ultimately end up with radical pairs. If you begin to think in this manner, you may arrive at