Boron’s Ambidextrous Ways

Chemists have built a new compound in which boron donates and accepts electron pairs when binding two carbon monoxide molecules, a peculiar situation that has the nonmetal main-group element acting like a transition metal (Nature 2015, DOI: 10.1038/nature14489).

Before now, no elements outside of the transition metals in groups 4 to 12 of the periodic table have ever been observed to react directly with two or more CO molecules. Holger Braunschweig of Julius Maximilian University, in Würzburg, Germany, led a team that figured out a way to make it happen.

The researchers first prepared a molybdenum pentacarbonyl complex bearing a borylene ligand, RBMo(CO)₅, where R is a superbulky 2,6-di(2,4,6-triisopropylphenyl) substituent. After refluxing the complex in a CO-saturated benzene solution, they removed Mo(CO)₆ and isolated blue crystals of the borylene dicarbonyl compound, RB(CO)₂.

Boron has only three valence electrons and is known for the sometimes quirky ways it forms bonds. In borylenes, boron uses one electron to bond with the bulky substituent. The remaining two electrons form a lone pair in one orbital, and two orbitals remain vacant. Boron can therefore accept electrons from two CO molecules (σ bonding) as it contributes the lone pair to the CO molecules (π backbonding). This donor-acceptor behavior mimics that of transition-metal carbonyl complexes.

Computational chemist Gernot Frenking of Philipps University, in Marburg, Germany, was part of a team that previously predicted that a borylene with transition-metal-like properties might be possible. “I did not expect that it would be so stable—and so easy to make!” Frenking tells C&EN. Frenking calls the boron bonding situation “mind-boggling,” and he predicts that more boron surprises are in store.