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Chemical Bonding

Aluminum triple bond made for first time

Sodium ions enable elusive species in theoretical and experimental studies

by Sam Lemonick
September 16, 2018 | A version of this story appeared in Volume 96, Issue 37


Pi and sigma orbitals of an Al≡Al complex.
Credit: Katie Lundell
Calculated π and σ orbitals (purple and pink) forming an aluminum-aluminum triple bond. Aluminum is yellow and sodium is blue.

Chemists have in the past succeeded in creating compounds containing triple bonds between two gallium or two boron atoms, species that are considered chemical oddities. An equivalent version made with aluminum—gallium and boron’s group 13 periodic table sibling—has so far remained elusive. Ivan A. Popov at Los Alamos National Laboratory proposed attempting Al≡Al as a student in Alexander I. Boldyrev’s group at Utah State University. Now, a few years later, along with Kit H. Bowen of Johns Hopkins University and Xinxing Zhang of Nankai University, who had been working toward the same goal independently, Popov and Boldyrev report experimental and computational confirmation of the bond in gas-phase clusters with sodium ions (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201806917). An ideal triple bond between two identical atoms, Boldyrev says, involves 10 electrons: six of them in π and σ bonds between the atoms and four in lone pairs. Elements like nitrogen, with five valence electrons, are well suited to the task, but aluminum falls two electrons short. So the researchers used sodium ions as electron donors to add four electrons to the aluminum pair, a process they call electronic transmutation. Boldyrev’s group computationally demonstrated that Na4Al2 clusters are stable, and Bowen’s used a pulsed arc cluster ionization source to generate Na3Al2 clusters. The researchers note that the sodium ions pull some electron density from the triple bond, making it less complete than something like a N≡N bond. “Now that the concept is proved, synthetic chemists can go and make it,” Boldyrev says.


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