Boron makes quadruple bond with rhodium

Abstract

Main group elements—those in the s- and p-blocks of the periodic table—are generally thought to max out at triple bonds because of the number and geometry of their orbitals. Researchers assumed that quadruple or higher-order bonds were restricted to transition metals. But now, Lai-Sheng Wang and colleagues at Brown University have defied conventional wisdom by reporting that diatomic rhodium boride (RhB) contains a quadruple bond (J. Phys. Chem. Lett. 2020, DOI: 10.1021/acs.jpclett.9b03484). Wang’s team stumbled on boron’s quadruple bond by accident, but when the researchers began to study RhB, they found that the molecule’s vibrational frequency pointed to a bond that is shorter and stronger than the triple bond of its protonated form, RhBH+. In addition, their theoretical calculations of the molecular orbitals in RhB showed that the atoms were connected by two sigma bonds and two pi bonds (shown). The combined evidence suggests that RhB is the first diatomic