Nitrogen gets friendly

When it comes to nitrogen compounds breaking up is easy to do, it is well known as being rather an unsociable element, not entirely noble nor aloof but forming bonds, especially with more than four atoms at a time is not its wont. Now, chemists in Poland have predicted that contrary to the normal rules of nitrogen etiquette, there are conditions when nitrogen might actually form six chemical bonds simultaneously.

Of course, there are structures in which nitrogen will display a valency of five, although unlike its periodic neighbor in this regard, phosphorus, rather than bonding to five different other elements with single bonds, nitrogen will more commonly form multiple bonds. Two Polish scientists, Dominik Kurzydlowski and Patryk Zaleski-Ejgierd, have now investigated whether there might be conditions in which a pentavalent nitrogen would combine with five neighbors through standard covalent interactions. They searched through thousands of crystal structures of nitrogen compounds with fluorine that could form at high pressures in the hope of finding nitrogen pentafluoride (NF5) but unexpectedly they found that the most stable structure contained hexafluoronitrogen(V) anions (NF6-).

"Two electrons are typically required for the formation of a single covalent bond. The problem with nitrogen lies in the fact that when creating various compounds it 'trades' electrons so as to always be surrounded by eight of them," explains Dominik Kurzydlowski. "This constrains the total number of atoms bonded to nitrogen to no more than four. We were the first to propose a stable crystal in which nitrogen breaks the octet rule, i.e. the requirement to possess exactly eight electrons, and forms bonds involving a total of up to twelve electrons."

Nitrogen thus could turn out to be hypervalent after all, like phosphorus, sulfur and various metals. If it proves to be true, then the number of possible compounds of nitrogen could be expanded considerably. "The compounds we tested, as well as the conditions under which these compounds were formed, were very exotic. The accuracy of the calculations was therefore our absolute priority which is why we decided to use the hybrid functional for the calculations," explains Kurzydlowski

The team's detailed analysis of their computer simulations allowed them to identify the unique crystal structure that with an increase in pressure at some point automatically ionizes in a very particular way. The reorganization of bonds between nitrogen trifluoride and fluorine leads to ionic species, among them the negative nitrogen hexafluoride, at 400 to 500 thousand times atmospheric pressure. The simulations point to metastability at much lower pressures, although still much higher than atmospheric pressure. [Kurzydlowski et al. Sci Rep 6, 36049 (2016); DOI: 10.1038/srep36049]

"The next step in this work is performing the actual experiment although this will take some time as fluorine, one of the reagents needed for obtaining the NF6-containing compound, is a very toxic and corrosive gas," Kurzydlowski told Materials Today.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".