Are noble gases representative elements

Mercury can also be with four partners

It will certainly take some time before the school and textbooks for chemistry are rewritten. But all pupils and students can now prepare for the fact that they will have to look at the familiar periodic table of the elements with different eyes in the future. Chemists from Würzburg and Charlottesville (USA) are to blame for this: They have detected a new oxidation state for the element mercury, as the journal Angewandte Chemie reports in its latest issue.

In 1993, Professor Martin Kaupp from the Institute for Inorganic Chemistry at the University of Würzburg predicted in the same journal that there would have to be a further oxidation stage for mercury. Normally the element is bivalent at most, that is, it only combines with two other atoms. Kaupp claimed, however, that the tetravalent level in the form of mercury tetrafluoride, HgF4, should also be experimentally accessible: In it, four fluorine atoms are arranged in a square around one mercury atom. “I carried out quantum chemical calculations and suggested synthetic routes,” says Kaupp.

Experiment close to absolute zero

But as much as experimental chemists tried, until recently they had not succeeded in making the crucial experiment a success. Lester Andrews from the University of Virginia had repeatedly been in discussions with the Würzburg theorists on this question. Together with his colleague Xuefang Wang, he succeeded in demonstrating the HgF4 beyond any doubt, with the decisive support of the theorists from Würzburg, who made the identification possible by calculating the vibration spectra.

The experiment is not that easy. It has to take place at very low temperatures, namely at a frosty four to ten Kelvin, i.e. at less than minus 260 degrees Celsius. In this cold, noble gases turn into solids. The chemists used neon - a very "lazy" material that does not like to react chemically - and equipped it with mercury atoms and fluorine molecules. Then they heated the whole thing up a few degrees and exposed it. This causes the fluorine molecules to break down into atoms, which in turn combine with the mercury. This resulted in 90 percent of the well-known combination of one mercury and two fluorine atoms (HgF2), but also around ten percent of the previously unprecedented HgF4. This provided experimental proof that Kaupp's prediction was correct.

Special position in the periodic table

“There is still no practical application for it,” says the chemist. However, the experiments have consequences for the periodic table of the elements: The elements in group 12 (zinc, cadmium and mercury) have so far been regarded as so-called post-transition metals or “representative elements”. Now, however, mercury has a special position in this group: "It can now be viewed as a transition metal because it uses electron shells - so-called d orbitals - in HgF4, which are only available to transition metals," says Kaupp.

That this research result is important is also shown by the fact that it was accepted by Angewandte Chemie, the most important international chemistry journal, in a record time of three days, and classified there as a “very important paper”. And the findings of Kaupp, Riedel and their US colleagues are by no means commonplace: “The last time a new oxidation state was experimentally found for an element in chemistry was almost 20 years ago,” says the Würzburg professor.

(University of Würzburg, October 11, 2007 - NPO)

October 11, 2007