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Physical Chemistry

Boron’s Ambidextrous Ways

Chemical Bonding: A boron dicarbonyl donor-acceptor compound has the main-group element behaving like a transition metal

by Stephen K. Ritter
June 22, 2015 | APPEARED IN VOLUME 93, ISSUE 25

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). Until now, no elements outside of the transition metals have 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 prepared a molybdenum pentacarbonyl complex bearing a borylene ligand, RBMo(CO)5, where R is 2,6-di(2,4,6-triisopropylphenyl). 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 quirky ways it can form 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.

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