The prediction of new species, in particular in the field of high-oxidation-state transition-metal complexes, has a long-standing history in our group. The prediction in 1993 of the existence of HgF₄ [1,2] has been the starting point. An oxidation state +IV for mercury in HgF₄ would correspond to a low-spin square-planar d⁸ system (Figure 1) and turns this group 12 “post-transition element” into a true transition metal. While over the years the studies have been extended to higher and higher computational levels [3] and to other Hg^IV species [4], it took almost 15 years before experimental confirmation by low-temperature matrix-isolation IR spectroscopy was achieved, supported by extensive high-level vibrational-frequency calculations [5].

We may put this into the wider perspective of the highest oxidation states of the 5d transition metal elements, where high oxidation states are favored by relativistic effects [6]. Further computational studies led, among other things, to a refutation of previous claims of the existence of AuF₇ [7], and to the prediction of the new oxidation state Ir^+VII in iridium fluoride species [8] (the first experimental confirmation of such a species is to be published soon; S. Riedel, personal communication). Subsequently, it was shown, again by a combination of matrix IR spectroscopy and high-level computations, that iridium even features an oxidation state +VIII in form of its tetraoxide [9]. This leads to the current situation of the highest known oxidation states in the 5d-series shown in Figure 2. Subsequently, we have shown that the cation [Hg(cyclam)]³⁺ observed already in the 1970s as a spectroscopic intermediate, is not a true Hg^III species but features an oxidized cyclam ligand [10]. It will be difficult to prepare M^III species of the group 12 elements [11]. In contrast, TcF₇ has a reasonable chance for observation by matrix techniques [12], and high-oxidation-state osmium fluorides may also be suitable targets [13].

[1] Gaseous Mercury(IV)fluoride, HgF₄: An ab Initio Study M. Kaupp, H. G. v. Schnering Angew. Chemie, Int. Ed. Engl. 1993, 32, 861-863.

[2] The Oxidation State +IV in Group 12 Chemistry. Ab Initio Study of  Zn^IV, Cd^IV, and Hg^IV Fluorides M. Kaupp, M. Dolg, H. Stoll, H. G. v. Schnering Inorg. Chem. 1994, 33, 2122-2131.

[3] Validation of Density Functional Methods for Computing Structures and Energies of Mercury(IV) Complexes S. Riedel, M. Straka, M. Kaupp Phys. Chem. Chem. Phys. 2004, 6, 1122-1127.

[4] Can Weakly Coordinating Anions Stabilize Mercury in its Oxidation State +IV? S. Riedel, M. Straka, M. Kaupp Chem. Eur. J. 2005, 11, 2743-2755.

[5] Mercury is a Transition Metal: The First Experimental Evidence for HgF₄  X. Wang, L. Andrews, S. Riedel , M. Kaupp Angew. Chem., Int. Ed. Engl. 2007, 46, 8371-8375.

[6] The highest oxidation states of the transition metal elements S. Riedel, M. Kaupp Coord. Chem. Rev. 2009, 253, 606-624.

[7] Has AuF₇ been made? S. Riedel, M. Kaupp Inorg. Chem. 2006, 45, 1228-1234.

[8] Revising the highest oxidation states of the 5d elements: The case of iridium(+VII) S. Riedel, M. Kaupp Angew. Chem. Int. Ed. Engl. 2006, 45, 3708-3711.

[9] Formation and Characterization of the Iridium Tetraoxide Molecule with Iridium in the Oxidation State +VIII Y. Gong, M. Zhou, M. Kaupp, S. Riedel Angew. Chem. Int. Ed. Engl. 2009, 48, 7879-7883.

[10] Is Allred’s [Hg(cyclam)]³⁺ a True Hg^III Complex? P. Hrobárik, M. Kaupp, S. Riedel Angew. Chem. Int. Ed. Engl. 2008, 47, 8631-8633.

[11] Quantum chemical study of trivalent group-12 fluorides S. Riedel, M. Kaupp, P. Pyykkö Inorg. Chem. 2008, 47, 3379-3383.

[12] High-valent technetium fluorides. Does TcF₇ exist? S. Riedel, M. Renz, M. Kaupp Inorg. Chem. 2007, 46, 5734-5738.

[13] Where is the limit of highly fluorinated high-oxidation state osmium species? S. Riedel, M. Kaupp Inorg. Chem. 2006, 45, 10497-10502.