In a vacuum-sealed flask on a lab bench in Germany sits an emerald-green crystal that will cause some jaws to drop. The crystal is the first stable compound containing a triple chemical bond between two boron atoms, a feat that had previously been limited to only two other non-metal elements – carbon and nitrogen.
Boron is more aloof than these neighbours [carbon and nitrogen], though. So much so that in its stable solid state at room temperature, it barely even reacts with boiling acid. “The chemistry of boron truly is different from the chemistry of all other elements,” says [Holger] Braunschweig [of the Julius Maximilian University of Würzburg in Germany], who built his career coaxing boron into strange bonds.
[T]o coax boron atoms to form triple bonds at room temperature, Braunschweig and colleagues had to make sure all the spaces where it could hold outer electrons were occupied.
Boron has four outer slots that can hold up to two electrons each – but in atomic boron, one of these slots is completely empty and the other three are only half full, with one electron apiece.
First the team filled up the completely empty slot by bonding each boron atom with a carbon-and-nitrogen-containing molecule called an N-heterocyclic carbene, which donated two electrons. Boron atoms bound to the molecule then completed the filling of their slots by pairing up and pooling their original three electrons. Each boron atom then has a full outer suite of eight electrons… [.]
As long as it is not exposed to air or moisture, with which it is known to react, the resulting green crystal with the rare triple bond should stay stable indefinitely, Braunschweig says.
Triple-bonding is not the only way the [Braunschweig team] got boron to mimic its superstar neighbour, carbon, though. They also coaxed boron atoms into forming a chain. Previous attempts to do this with boron failed and resulted in messy clusters. “It is extremely difficult to form chains of boron atoms,” says Braunschweig. The secret was to attach the boron atoms to an iron scaffold, which allowed up to four to form a chain[.]
Source: Lisa Grossman, Triple-Bonded Boron Opens New Chemical World, New Scientist (June 14, 2012); Lisa Grossman, Brand New Bonding Skills Make Boron More Like Carbon, New Scientist (June 22, 2012); see also James Mitchell Crow, Boron Finally Gets a Triple Bond, Nature (June 14, 2012).