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| Identifiers | |
|---|---|
3D model (JSmol) | |
| |
| |
| Properties | |
| HgF4 | |
| Molar mass | 276.58 g/mol |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Mercury(IV) fluoride, HgF4, is a purported compound, the first to be reported with mercury in the +4oxidation state. Mercury, like the othergroup 12 elements (cadmium andzinc), has an s2d10electron configuration and generally only forms bonds involving its 6s orbital. This means that the highest oxidation state mercury normally attains is +2, and for this reason it is sometimes considered apost-transition metal instead of atransition metal. HgF4 was first reported from experiments in 2007, but its existence remains disputed; experiments conducted in 2008 could not replicate the compound.[1][2]
Speculation about higher oxidation states for mercury had existed since the 1970s, and theoretical calculations in the 1990s predicted that it should be stable in the gas phase, with asquare-planar geometry consistent with a formal d8 configuration. However, experimental proof remained elusive until 2007, when HgF4 was first prepared using solidneon andargon formatrix isolation at a temperature of 4 K. The compound was detected usinginfrared spectroscopy.[3][4]
However, the compound's synthesis has not been replicated in other labs, and more recent theoretical studies cast doubt on the possible existence of mercury(IV) (and copernicium(IV)) fluoride.Dirac–Hartree–Fock computations including both relativistic effects andelectron correlation suggest that an HgF4 compound would be unbound by about 2 eV (and CnF4 by 14 eV).[5]
Theoretical studies suggest that mercury is unique among the natural elements of group 12 in forming atetrafluoride, and attribute this observation torelativistic effects. According to calculations, the tetrafluorides of the "less relativistic" elements cadmium and zinc are unstable and eliminate a fluorine molecule, F2, to form the metal difluoride complex.[citation needed] On the other hand, the tetrafluoride of the "more relativistic" synthetic element 112,copernicium, is predicted to be more stable.[6][failed verification]
Subsequentdensity functional theory andcoupled cluster calculations indicated that bonding in HgF4 (if it really exists) involvesd orbitals. This has led to the suggestion that mercury should be considered a transition metal (the group 12 metals are sometimes excluded from the transition metals because they do not oxidize beyond +2).[7] Chemical historianWilliam B. Jensen has argued that the compound alone is insufficient to reclassify the metal, because HgF4 represents at best anon-equilibrium transient state.[8]
HgF4 is produced by the reaction of elemental mercury withfluorine:
HgF4 is only stable in matrix isolation at 4 K (−269 °C); upon heating, or if the HgF4 molecules touch each other, it decomposes tomercury(II) fluoride and fluorine:
HgF4 is adiamagnetic, square planar molecule. The mercury atom has a formal 6s25d86p6 electron configuration, and as such obeys theoctet rule but not the18-electron rule. HgF4 isisoelectronic with thetetrafluoroaurate anion,AuF−
4, and is valence isoelectronic with thetetrachloroaurate (AuCl−
4),tetrabromoaurate (AuBr−
4), andtetrachloroplatinate (PtCl2−
4) anions.
