Silver is the middle element of the Group IB series, yet in many of its physical properties it exhibits extreme values rather than values which fall between those of copper and gold, e.g. melting point (min.), boiling point (min.), thermal conductivity (max.), etc. It exhibits a steel grey colour similar to that of a majority of metals similar to that of the majority of metals rather than a colour similar to that of copper or gold.
To extend the argument to the chemical properties, as shown in Table 8.8: for copper, the +2 oxidation state dominates; for silver, the +1 state; and for gold, the +3 state.
Table 8.8 Comparative species for the [Cu–Ag–Au] ions under oxidizing conditions
Table 8.9 Comparative species for the [Fe–Co–Ni–Cu] tetrad under mild oxidizing conditions
In fact, in terms of its coordination chemistry, copper fits better with the later 3d elements. For example, in the species across the oxidizing pH range, we see a strong similarity (Table 8.9).
The dominance of the +1 d10 state for the normal aqueous chemistry of silver [31] make it more appropriately considered as a main group metal. As to be discussed in Chapter 10, silver is the “classic” example of the “knight’s move” linkage, showing a startling similarity to thallium. A unique parallel is that they are the only two metal ions to form brick-red insoluble chromates, Ag2CrO4 and Tl2CrO4. Under oxidizing conditions, we have the parallel shown in Table 8.10.
Gold is a very different element to that of copper and silver. In fact, it has been referred to as the gold anomaly [32]. This anomaly is largely ascribed to the importance of the relativistic effect, as mentioned in Chapter 2. Just as there are unusual species linking silver(I) and thallium(I), so gold(V) shows a strong resemblance to platinum(V). A good example is the pair of compounds: [O2]+[PtF6]− and [O2]+[AuF6]−.
Table 8.10 Comparative species for the [Ag–Tl] diad under oxidizing conditions
Table 8.11 Comparative species for the [Au–Pt] diad under oxidizing conditions
The species for platinum and gold can be compared under oxidizing conditions. Allowing for the fact that +4 is the dominant oxidation state of platinum, and six, its common coordination number, there is again a much closer parallel of gold with platinum than with silver and copper (Table 8.11).
Gold is actually the easiest one of this Group to assign to a cluster: that is, to the platinum metals. It is the platinum metals plus gold that are the only metals found almost entirely in their elemental state [33]. In fact, gold is sometimes found in nature as an alloy with platinum and palladium [34]. The acceptance of this “cluster” was cited by Barnes et al. [35]:
Au is strictly speaking not a platinum-group element but it is a noble metal and will be included with the PGE’s in this paper.
This Author supports the addition of gold to the platinum metals with the designation of “noble metals” (see Chapter 5).
Geochemists often extend this cluster to include rhenium. These eight elements, [Os–Ir–Ru–Rh–Pt–Pd–Re–Au], are referred to as the highly siderophile elements (HSE), that is, elements found throughout the solar system most commonly in elemental form [36].
A Hybrid Solution
So, is there a classification of the transition elements that better reflect the linkages? Obviously, we cannot satisfy all the many similarities, but the argument is made here that the best fit is not accomplished by either the Group or the Period approach. Instead, a hybrid combination generates the clusters of elements that have similarities worthiest of highlighting (Figure 8.5). As one example of the allegiance of titanium to the [Zr–Hf–Nb–Ta] tetrad is that all five of the elements form trisulfides of the form MS3 [37].
Subsequent to deducing the splitting of allegiances for titanium and manganese, Leal and Restrepo have highlighted the same divisions in their “ordered hypograph” [38].
Figure 8.5 A hybrid approach to transition metal classification.
To review, the most logical clustering is listed in the following, showing the “secondary allegiances” of titanium and of manganese:
•The [Ti–Zr–Hf–Nb–Ta] pentad whose simple chemistry is dominated by insoluble oxides.
•The [V–Cr–Mn] triad that exhibits soluble, oxidizing, isoelectronic tetraoxo-anions plus a stable +3 oxidation state, to which Ti can be appended for other aspects of common chemistry.
•The [Fe–Co–Ni–Cu] tetrad for which the +2 aqueous ion is a major component of simple chemistry, to which Mn can be appended for some commonalities.
•The [Mo–W–Tc–Re] tetrad for which nonoxidizing soluble valence-isoelectronic tetraoxo-anions exist.
•The [Ru–Os–Rh–Ir–Pd–Pt–Au] heptad that can be defined as the “noble metals.”
•The [Ag–Tl] diad that reminds us to “think outside the (transition metal) box.”
Commentary
Chemists like smooth patterns — continuities — systematic trends. Sorry, it doesn’t happen with the transition metals! Instead, each of the transition metals flaunts its individuality and refuses to fit neatly into a specific category. The 3d metals are fractured into two halves; zirconium and hafnium behave like twins; silver seems more at home outside the transition metals; gold nestles up to the platinum metals; and titanium and manganese have split allegiances between their Period and their Group. Never let it be said that transition metal chemistry is predictable and boring!
References
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3.X. Wang et al., “Mercury Is a Transition Metal: The First Experimental Evidence for HgF4,” Angew. Chem. Int. Ed. 46, 8371–8375 (2007).
4.W. B. Jensen, “Is Mercury Now a Transition Element?” J. Chem. Educ. 85, 1182–1183 (2008).
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