Near the end of the 19th century, the periodic system of the elements was developed by the Russian Dmitri Mendelejev and the German Viktor Meyer. From then on, every single element had its proper place and the behaviour of still unknown elements could be predicted.
Shortly afterwards, an entire group of new elements - the so-called noble gases - were found, which had not been foreseen. However, they fitted perfectly between the neighbouring elements, the halogens and the alkali metals, bringing the number of main groups in the periodic system from the mathematically unpleasing count of seven to the much nicer eight.
The neighbouring elements are very reactive, but in quite opposite ways. One group can be described as extremely electropositive, the other group as extremely electronegative. So it was nice that nature put a series of elements between these two groups that did not seem to react with any other element. They were, therefore, named the noble gases. In 1962, however, it was shown by Neil Bartlett that some noble gases were capable of reacting with other elements, but only under tedious, experimental conditions.
Xenon (Greek: the strange one) makes most such chemical compounds; krypton (Greek: the hidden one) only a few; argon (Greek: the lazy one) possibly only one; radon (Latin: the radiating one) is too radioactive to be investigated in detail; while neon (Greek: the new one) and helium (Greek: the sun-like one) have formed none so far. All of the compounds of the noble gases made so far have been in combination with a few non-metallic elements. However, the bulk of the elements, approximately 80 per cent, are metallic. This is where we came in. In an attempt to prepare a novel gold (Au) compound, we reacted gold trifluoride with xenon (Xe) and immediately a red-black material was formed. This, after its composition had been established, turned out to be a gold-xenon complex. It may sound simple, but the reaction takes place only in the very aggressive solvent hydrogen fluoride, a substance so corrosive that it cannot even be handled in glassware because it eats it up. The reaction takes place below room temperature and under a certain pressure of xenon gas, which further complicates its handling.
It is ironic that the first metal that a noble gas has been reacted with is a metal that is also called noble. The atomic structure of the AuXe4 2+ cation (positively charged ion) is aesthetically pleasing, as it is a perfectly regular star, with four corners of xenon atoms around a central gold atom. Now the second taboo of noble gas chemistry is broken, namely that these gases do not make bonds with metals (remember: the first taboo was that they did not form compounds at all).
The next stage is to find out if metals other than gold do the same. A more academic question is, why do just four xenon atoms surround the gold centre, and not one, two, or three? Chemistry often works like human history. First there is the period of hunting and collecting - chemical compounds are found accidentally. Then comes the period of crop growing and cattle raising - chemical compounds are planned and synthesised systematically. After this there is modern history - chemical compounds are tailored for special applications.
The history of metal-noble gas compounds is only at the beginning of the first step. It remains to be seen if few or many related compounds can be found before the field can go on to the next step, and long before any use can be made of them.
Konrad Seppelt is professor of inorganic chemistry at the Freie Universitat, Berlin.