Mercury Tetrafluoride Synthesized

Mercury, a group 12 element with a valence electron configuration of s²d¹⁰, is generally considered to be limited to the +1 and +2 oxidation states. Theoretical work, however, has long predicted that mercury could be stable in the +4 oxidation state. In a fundamental advance that opens new possibilities for mercury compounds, that prediction has now been confirmed with the successful synthesis of HgF₄ using matrix isolation techniques (Angew. Chem., DOI: 10.1002/anie.200703710).

HgF₄ was prepared in cryogenic conditions by reacting mercury with excess fluorine in either an argon or a neon matrix, with ultraviolet irradiation from a mercury arc lamp. According to chemistry professor W. Lester S. Andrews at the University of Virginia, who did the experimental work with senior research scientist Xuefeng Wang, two factors were critical to the success of the experiments. These were controlling the light intensity, since HgF₄ is photosensitive, and using neon to improve the yield. Neon has a lower melting point than argon and thus is more permeable to fluorine at 4 K and gives higher yields of HgF₄.

Andrews and Wang confirmed the identity of the species by comparing experimental infrared spectra with vibrational frequencies from coupled-cluster and density functional calculations. The theoretical analysis was done by Sebastian Riedel, now a postdoc at the University of Helsinki, in Finland, and chemistry professor Martin Kaupp at the University of Würzburg, in Germany.

"It's a very thorough piece of work," says Gary J. Schrobilgen, a chemistry professor at McMaster University, in Canada. Although identification of HgF₄ hinges on a single infrared spectroscopic band, "the calculations are well done, and the researchers really considered all the angles. I'm sure they've got it right," he adds.

Although the researchers have not yet obtained structural data on HgF₄, prior computations by Kaupp predicted that it would be a low-spin d⁸ species with square-planar geometry and that the mercury d-orbitals would be strongly involved in bonding, just as in other transition metals.

The work is likely to stimulate renewed efforts to synthesize other Hg(IV) species, says Schrobilgen, who has made past attempts to synthesize high-valent mercury fluorides himself. One likely target is HgF₆^2???, since an anion would be better able to stabilize the high oxidation state than a neutral or cationic species.