The Mixed-Metal Oxochromates(VI) Cd(Hg I2 ) 2 (Hg II ) 3 O 4 (CrO 4 ) 2 , Cd(Hg II ) 4 O 4 (CrO 4 ) and Zn(Hg II ) 4 O 4 (CrO 4 )—Examples of the Different Crystal Chemistry within the Zinc Triad

: The three mixed-metal oxochromates(VI) Cd(Hg I2 ) 2 (Hg II ) 3 O 4 (CrO 4 ) 2 , Cd(Hg II ) 4 O 4 (CrO 4 ), and Zn(Hg II ) 4 O 4 (CrO 4 ) were grown under hydrothermal conditions. Their crystal structures were determined from single-crystal X-ray diffraction data. The crystal-chemical features of the respective metal cations are characterised, with a linear coordination for mercury atoms in oxidation states +I and +II, octahedral coordination spheres for the divalent zinc and cadmium cations and a tetrahedral conﬁguration of the oxochromate(VI) anions. In the crystal structures the formation of two subunits is apparent, viz. a mercury-oxygen network and a network of cadmium (zinc) cations that are directly bound to the oxochromate(VI) anions. An alternative description of the crystal structures based on oxygen-centred polyhedra is also given.


Introduction
The three elements of the zinc triad have a closed-shell nd 10 (n + 1)s 2 electronic configuration with n = 3, 4, and 5 for zinc, cadmium, and mercury, respectively. In compounds of these elements with ionic or predominantly ionic character, zinc exclusively exhibits oxidation state +II, cadmium with very few exceptions has an oxidation state of +II (Cd 2 (AlCl 4 ) 2 being one of them with an oxidation state of +I [1,2]), whereas a multitude of mercuric (oxidation state +II), mercurous (oxidation state +I) and mixed-valent mercury compounds are known. The crystal-chemical features of all three elements are remarkably different. The most frequently observed coordination numbers for zinc in its compounds are 4, 5, and 6 with (distorted) tetrahedral, trigonal-bipyramidal, and octahedral coordination environments, respectively. The larger cadmium cation has a coordination number of four only in combination with larger anions (like in CdS), and in the majority of cases exhibits coordination numbers of six, or higher. For most of the latter cases, the coordination spheres are considerably distorted and difficult to derive from simple polyhedra. In many aspects, including structural characteristics, zinc and cadmium compounds resemble their alkaline earth congeners magnesium and calcium, respectively, which likewise have a closed shell electronic configuration. Mercury, on the other hand, is unique amongst all metals (cf. the low melting point) and has a peculiar crystal chemistry, showing a preference for linear coordination by more electronegative elements (coordination number of two). To a certain extent, these features can be related to relativistic effects that are pronounced for this element [3,4]. While a number of review articles devoted to the crystal chemistry of mercury have been published over the years [5][6][7][8][9][10][11], to the best of the author's knowledge, apart from chapters in a compendium on coordination chemistry [11,12], special reviews on the crystal chemisty of zinc or cadmium did not appear thus far.

Results and Discussion
Three mixed-metal oxochromates(VI) were obtained under the given hydrothermal conditions, viz. Cd(Hg I 2 ) 2 (Hg II ) 3  The formation of mixed-valent mercury(I,II) compounds, i.e., wattersite in both batches and Cd(Hg I 2 ) 2 (Hg II ) 3 O 4 (CrO 4 ) 2 in the cadmium-containing batch, indicates that complex redox equilibria between different mercury species (Hg(0) Hg(I) Hg(II)) must have been present under the chosen hydrothermal reaction conditions. Such redox equilibria are easily influenced by the presence of additional redox partners, here, for example Cr(VI) Cr(III), and other interacting parameters like temperature, pressure, pH, concentration of the reactants, etc. Such a complex interplay between different adjustable parameters not only makes a prediction of solid products difficult, but can also lead to multi-phase formation and the presence of element species with different oxidation states in one batch. This kind of behaviour is not only exemplified by the three title compounds but also for other mixed-valent mercury oxocompounds that were obtained under similar hydrothermal conditions [16,[28][29][30].
The presence of two distinct structural subunits in each of the Cd(Hg I 2 ) 2 (Hg II ) 3 , Zn), respectively. The alternative formulae also emphasize the "basic" character (in an acid/base sense) of these compounds which is associated with the presence of oxygen atoms that are exclusively bonded to metal cations, here, those of mercury, cadmium (zinc), or mixtures thereof. Since these oxygen atoms do not belong to a chromate anion they are defined as "basic". In the vast majority of cases, such "basic" oxygen atoms are surrounded by four metal cations in the form of distorted tetrahedra. Krivovichev and co-workers have resumed the use of such oxygen-centred [OM 4 ] tetrahedra for a rational structure description and classification of mineral and synthetic lead(II) oxo-compounds [32]. A general review of anion-centred [OM 4 ] tetrahedra in the structures of inorganic compounds with different metals M has been published some time ago, including [OHg 4 ] tetrahedra [33]. However, mixed [OM 4 ] tetrahedra with M = Hg and Cd or Zn are unknown so far.
The presence of two distinct structural subunits in each of the Cd(Hg I 2)2(Hg II )3O4(CrO4)2 and M(Hg II )4O4(CrO4) structures, viz., a mercury-oxygen network and cadmium/zinc cations bound directly to [CrO4] 2− anions, allows to reformulate them as [{(Hg I 2)2(Hg II )3O4} 2+ {Cd(CrO4)2} 2− ] and MCrO4·4HgO (M = Cd, Zn), respectively. The alternative formulae also emphasize the "basic" character (in an acid/base sense) of these compounds which is associated with the presence of oxygen atoms that are exclusively bonded to metal cations, here, those of mercury, cadmium (zinc), or mixtures thereof. Since these oxygen atoms do not belong to a chromate anion they are defined as "basic". In the vast majority of cases, such "basic" oxygen atoms are surrounded by four metal cations in the form of distorted tetrahedra. Krivovichev and co-workers have resumed the use of such oxygen-centred [OM4] tetrahedra for a rational structure description and classification of mineral and synthetic lead(II) oxo-compounds [32]. A general review of anion-centred [OM4] tetrahedra in the structures of inorganic compounds with different metals M has been published some time ago, including [OHg4] tetrahedra [33]. However, mixed [OM4] tetrahedra with M = Hg and Cd or Zn are unknown so far.
In the structure of Cd(Hg I 2)2(Hg II )3O4(CrO4)2, the "basic" oxygen atoms are represented by O4 and O5, both being bound to three mercury cations and one cadmium cation. The two types of      Bond valence sums (BVS) [36], using the bond valence parameters of Brese and O'Keeffe [37], were calculated for the three structures. The results are reasonably close to the expected values (in valence sums) of 1 for mercurous Hg, 2 for mercuric Hg, 2 for Cd and Zn, 6 for Cr and 2 for O ( Table 2). The global instability index GII was used as a measure of the extent to which the valence sum rule is violated [36]. The resultant GII values of 0.14 v.u. for Cd(Hg I 2)2(Hg II )3O4(CrO4)2, 0.14 v.u. for Cd(Hg II )4O4(CrO4) and 0.11 v.u. for Zn(Hg II )4O4(CrO4) indicate stable structures with some lattice-induced strain [38]. (1) For oxygen atoms the type and number of atoms they are bound to are indicated in brackets.

Preparation
For the hydrothermal experiments, Teflon containers with an inner volume of 5 ml were used. The metal oxides HgO, CrO3 and ZnO (CdO), all purchased from Merck (Darmstadt, Germany), were used without further purification. 1 mmol HgO, 0.5 mmol CrO3, and 0.5 mmol ZnO (CdO) were mixed, placed in a Teflon container and poured with 3 ml water. The container was sealed with a Teflon lid, placed in a steel autoclave, heated at 215 °C for one week and cooled within 12 h to room temperature. In both cases (cadmium-and zinc-containing batches) the final supernatant solution was colourless (pH ≈ 8), and the different crystal colours and forms indicated multi-phase formation. The Bond valence sums (BVS) [36], using the bond valence parameters of Brese and O'Keeffe [37], were calculated for the three structures. The results are reasonably close to the expected values (in valence sums) of 1 for mercurous Hg, 2 for mercuric Hg, 2 for Cd and Zn, 6 for Cr and 2 for O ( Table 2). The global instability index GII was used as a measure of the extent to which the valence sum rule is violated [36]. The resultant GII values of 0.14 v.u. for Cd(Hg I 2 ) 2 (Hg II ) 3   (1) For oxygen atoms the type and number of atoms they are bound to are indicated in brackets.

Preparation
For the hydrothermal experiments, Teflon containers with an inner volume of 5 mL were used. The metal oxides HgO, CrO 3 and ZnO (CdO), all purchased from Merck (Darmstadt, Germany), were used without further purification. 1 mmol HgO, 0.5 mmol CrO 3 , and 0.5 mmol ZnO (CdO) were mixed, placed in a Teflon container and poured with 3 mL water. The container was sealed with a Teflon lid, placed in a steel autoclave, heated at 215 • C for one week and cooled within 12 h to room temperature. In both cases (cadmium-and zinc-containing batches) the final supernatant solution was colourless (pH ≈ 8), and the different crystal colours and forms indicated multi-phase formation. The solid reaction products were filtered off with a glass frit, washed with water, ethanol, and acetone and air-dried. In both the cadmium-and the zinc-containing batch, dark-red crystals of wattersite [22] were identified as the main product. In the cadmium-containing batch the two title compounds, Cd(Hg I 2 ) 2 (Hg II ) 3 O 4 (CrO 4 ) 2 and Cd(Hg II ) 4 O 4 (CrO 4 ), were obtained as dark-red rods and orange plates, respectively, in an estimated ratio of 1:2. In the zinc-containing batch, orange plates of Zn(Hg II ) 4 O 4 (CrO 4 ) could be isolated as a minor product.

Single Crystal X-ray Diffraction
Prior to the diffraction measurements, crystals were separated from wattersite crystals and checked for optical quality under a polarizing microscope. Selected crystals were fixed with superglue on the tip of thin silica glass fibres. Intensity data were measured at room temperature with Mo-Kα radiation, using either a SMART CCD three-circle diffractometer (Bruker, Madison, WI, USA) or a CAD-4 four-circle diffractometer with kappa geometry (Nonius, Delft, The Netherlands). After data reduction, a numerical absorption correction was performed for each data set with the aid of the HABITUS program by optimizing the crystal shape [39]. The crystal structures were solved by Direct Methods [40] and were refined using SHELXL-97 [41].
Numerical details of the data collections and structure refinements are gathered in Table 3, selected bond lengths are given in Table 1. Structure graphics were produced with ATOMS [42]. Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum (Karlsruhe, Eggenstein-Leopoldshafen, Germany, Fax: +49-7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de, https://www.fiz-karlsruhe.de/) on quoting the depository numbers listed at the end of Table 3.