Group 8 and Group 18
Somewhat surprisingly, there are two elements near the bottom of each group that share several similarities in chemical formulas: “noble metal” osmium(VIII) and “noble gas” xenon(VIII) (Figure 9.9).
Figure 9.9 Members of Group 8 and Group 18.
Table 9.12 Some parallel compounds of osmium(VIII) and xenon(VIII)
Group 8
Group 18
OsO
4
XeO
4
OsO
3
F
2
XeO
3
F
2
OsO
2
F
4
XeO
2
F
4
Some parallels in formula are shown in Table 9.12. Similarities even extend to chemical behavior: osmium(VIII) oxide, OsO4, is a yellow solid and strongly oxidizing; while xenon tetraoxide, XeO4, is a pale yellow explosive compound.
Group 1 and Group 11
Group 1 and Group 11 are the most problematic of the group–pairs. In Chapter 8, it was argued that, in fact, there is no “Group 11” as the three elements: copper, silver, and gold, have so little in common. In the context of this chapter, it is the parallel in the +1 oxidation state, which must be considered. It is only for silver that this oxidation state dominates, thus it will be the focus of the comparison here.
There is one similarity: several of the sodium and silver(I) compounds are isostructural. These pairs include NaNO3 and AgNO3; Na2SO4 and Ag2SO4; and Na2S2O6·2H2O and Ag2S2O6·2H2O.
This brief list provides meager evidence of linkage between these two Groups. Thompson alluded to the lack of any correlations [23]:
Because of these differences in electronic structure comparison of silver with the alkali metals is fruitless even though each possesses a single s electron in the outermost shell. Probably the only similarity between the two is their diamagnetism and lack of colour.
Group 2 and Group 12
Based upon chemical similarities, in 1905, Werner designed a Periodic Table that showed beryllium and magnesium to belong to the zinc group (Figure 9.10) [5]. This was not mere speculation. Over 100 years later, Restrepo has shown on chemical topological grounds, by means of a hypergraph, that the chemistry of beryllium and magnesium fits more closely with that of zinc, cadmium, and mercury, than the lower members of Group 2 [24].
Figure 9.10 Part of Werner’s Periodic Table showing beryllium and magnesium as part of the zinc Group (from Ref. [5]).
Figure 9.11 Members of Group 2 and Group 12.
There are certainly grounds to consider this assignment. All of these elements have +2 as the sole common oxidation state, except for mercury for which it is the higher oxidation state. Thus there are some resemblances for all of the Group 2 elements with zinc and cadmium (Figure 9.11). For example, all six of these elements form hygroscopic anhydrous metal chlorides.
The closer link seems to be between cadmium and calcium. Cadmium oxide has the NaCl structure, as do the Group 2 oxides. Of specific biochemical relevance, high levels of calcium ion inhibit the toxicity of cadmium(II) ion, suggesting that calcium and cadmium ions share the same cellular pathway [25]. Thus overall, cadmium and calcium seem to have the closer resemblance.
A Curious (n + 5) and (n + 10) Case
In this chapter, it has been shown how there are resemblances between scandium in Group 3 and aluminum in Group 13. Curiously, the chemistry of aluminum also resembles that of the iron(III) ion (Figure 9.12). These similarities may be ascribed to the same 3+ charge and near-identical ion radii (and hence charge density). As a result of the high charge density, the [M(OH2)6]3+ ions of both metals are very strongly acidic through hydrolysis.
Figure 9.12 Iron of Group 8 and aluminum of Group 13.
There are some very specific similarities. For example, in the vapor phase, both ions form covalent chlorides of the form M2Cl6. These (anhydrous) chlorides can be used as Friedel–Crafts catalysts in organic chemistry, where they function by the formation of the [MCl4]– ion [26]. In addition, another result of their high charge densities.
There are, however, some significant differences. For example, the amphoteric aluminum oxide reacts with hydroxide ion to give the soluble tetrahydroxoaluminate ion, [Al(OH)4]–, whereas the basic iron(III) oxide remains in the solid phase. It is this difference that enables aluminum oxide to be separated from iron(III) oxide in the commercial Beyer process, prior to the aluminum smelting step [27].
Commentary
Thanks to the work by Laing, the forgotten links between main group elements and transition metals have been rediscovered. Even though the eight-Group Table has long since been consigned to ancient history, these links in formula of compounds and polyatomic ions of elements of Group (n) with pseudo-isoelectronic species of elements of the corresponding Group (n + 10) provide chemists with a different perspective. Who would ever have imagined, for example, that there would be chemical similarities of osmium(VIII) and xenon(VIII)? What other unusual matching pairs are yet to be synthesized?
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