The “shocking” Xe-Au bond.

Chemistry rarely makes it to the cover of popular science magazines. Thus when this week, the New Scientist ran the headline “Forbidden chemistry. Reactions they said could never happen“, I was naturally intrigued. The examples included Woodward and Hoffmann’s “symmetry-forbidden” reactions, which have been the subject of several posts here already. But in the section on nobel gas chemistry, the same Hoffmann is reported as having been shocked to hear of a compound of xenon and gold, both of which in their time were thought of as solidly inert, and therefore even more unlikely to form a union.

Science magazines are often fearful of showing molecular structures on their pages (much like mathematical equations) and so the “shocking” compound is not illustrated in the article; you can see the essential feature above (the counterion is the otherwise unremarkable Sb₂F₁₁^–). Here, I include the 3D crystallographic coordinates (published in 10.1126/science.290.5489.117) which you can explore by clicking on the above graphic or view below as a rotatable 3D model.

Since the year 2000, when the above was reported, there have been very few additional examples, and so it remains a rare phenomenon. I thought it might be amusing to also give an example of the reverse phenomenon, a reaction that chemists said should happen, but which they had great difficulty in inducing, namely the formation of perbromate salts. The reaction was eventually induced by resorting to nuclear physics, and the radioactive transmutation of Se⁸³ to Br⁸³. Once chemists had been persuaded it really could be made, lots of more conventional ways of preparing perbromates were discovered.

Finally, I ponder how long it might take for magazines such as New Scientist to include interactive diagrams such as the below on the pages of the (increasingly) electronically delivered editions? Or indeed, when mainstream chemistry journals might start incorporating “HTML5” into their own production processes.