Boron makes quadruple bond with rhodium

When it comes to extreme multiple bonds, the transition metals have long ruled the roost. Quadruple bonds? They’ve got thousands of them. Quintuple bonds? Right there in a landmark organometallic dichromium complex made in 2005. Even sextuple bonds have been found, in Mo₂ and W₂.

In contrast, the main-group elements—those in the s- and p-blocks of the periodic table—have lagged behind in the multiple-bonding stakes. But now Lai-Sheng Wang and colleagues at Brown University have defied expectations by reporting that diatomic rhodium boride (RhB) contains a quadruple bond (J. Phys. Chem. Lett. 2020, DOI: 10.1021/acs.jpclett.9b03484).

Main-group elements are generally thought to max out at triple bonds because of the number and geometry of orbitals they can use for bonding. In 2012, researchers calculated that diatomic C₂ could contain a quadruple bond, but the claim was controversial because it invoked a weak bonding interaction between electrons on opposite sides of the molecule in orbitals that pointed directly away from each other. And whereas a quadruple bond should be stronger than a triple bond between the same atoms, calculations suggested that the connection in C2 was actually weaker than the triple bond in ethyne (C₂H₂).

Wang’s team stumbled on boron’s quadruple bond by accident. A few years ago the researchers made a compound containing a triple bond between bismuth and boron, and wondered if they could swap bismuth for a transition metal. They started their search with rhodium, largely because it only has one naturally occurring isotope, which makes it easier to work with experimentally and computationally.

So they fired a laser at a disk containing powdered rhodium and boron to produce a stream of vaporized compounds, and separated them in a mass spectrometer. Then they studied selected compounds by photoelectron spectroscopy, which measures the kinetic energies of electrons ejected from ionized samples and serves as a fingerprint of the structure and bonding in the molecules.

This process allowed the researchers to identify two curious compounds: a boronyl-coordinated rhodium boride, RhB(BO^-), and RhB itself. The vibrational frequency of RhB suggests that its bond is shorter and stronger than the triple bond in its protonated form, RhBH⁺. The team also used theoretical calculations to study how the orbitals of each atom in RhB overlapped to form molecular orbitals. This showed that the atoms were connected by two sigma bonds and two pi bonds.

Taken together, Wang says, all their evidence suggests that RhB is the first diatomic molecule containing a quadruple bond to a boron atom. “Next time I teach a physical chemistry class, this will change how I lecture about the chemical bond,” he says.

RhB is not a new molecule, though. It was made more than a decade ago (Mol. Phys. 2007, DOI: 10.1080/00268970701390172), and was regarded as triply bonded. This was because researchers assumed that one of the sigma-bonding molecular orbitals was a non-bonding lone pair on the boron atom, Wang says.

“It’s definitely the most strongly bound of all the transition metal borides, based on our own results, so that’s consistent with the quadruple bond,” says Michael D. Morse at the University of Utah, who has previously worked with RhB. “I think their interpretation makes sense.”

Overcoming preconceptions about RhB’s triple bond was a big challenge, adds Wang, and their paper was submitted to several journals before it was finally published. “I have never had an article with so many reviewers,” he says. That delay meant they were unable to claim the first ever quadruple bond to boron—another team reported quadruple bonding in BFe(CO)₃^- late last year (Nat. Commun. 2019, DOI: 10.1038/s41467-019-12767-5). “In science, you win some, you lose some,” Wang says.