The decision of which elements belong in what groupings will always be subjective. This Author places the greater weight on the species under strongly oxidizing conditions. To illustrate this point, Table 8.3 shows that, under strongly oxidizing conditions, manganese completes the set of isoelectronic highly oxidizing anions. Whereas vanadium, chromium, and manganese all form soluble tetroxo-anions, titanium forms an insoluble oxide. For this reason, on balance, titanium is the “weakest link” in this set. In addition, the tetroxo-anions of the [V–Cr–Mn] triad form a series of increasing acid strength.
Table 8.3 Comparative species for the [Ti–V–Cr–Mn] tetrad over the pH range under strongly oxidizing conditions
The Trouble with Titanium
Without an aqueous chemistry over most of a pH range, titanium does not appear to fit with its 4th Period neighbors of Group 5, 6, and 7. In fact, titanium is a troublesome element in the context of placement. Should it be considered as being the beginning member of the 4th Period transition series or as the top member of Group 4? Table 8.4 shows the similarity over the pH range with the other members of Group 4.
But before deciding this to be the definitive solution, it is necessary to look at the comparative species at very low pH while reducing the potential. Titanium, but not zirconium or hafnium, has a significant chemistry of its +3 and even +2 oxidation states.
In fact, from a redox perspective, titanium chemistry matches more with that of vanadium and less with zirconium. As can be seen in Table 8.5, there is a remarkable similarity in oxidation states and species, taking into consideration that the maximum oxidation state of vanadium is +5 while that for titanium is +4.
Thus, titanium lays claim both to be the top member of the Group 4 triad [Ti–Zr–Hf] and the first member of the early 3d transition series tetrad [Ti–V–Cr–Mn]. On balance, because of the dominance of the insoluble +4 oxidation-state oxide, the link of titanium with zirconium and hafnium seems to be the stronger.
Table 8.4 Comparative species for the [Ti–Zr–Hf] triad over the pH range under strongly oxidizing conditions
Table 8.5 A comparison of species for the [Ti–Zr–Hf] triad at very low pH over the redox range
Manganese Muddies Things
Just as titanium has two allegiances at the beginning of the 3d row, so manganese in the middle also presents a dilemma. Table 8.3 shows that, under highly oxidizing conditions, manganese completes the set of isoelectronic highly oxidizing anions. However, under normal conditions of aqueous chemistry, manganese favors the +2 oxidation state and its species match well with the subsequent members of the 3d series. And, as alluded to earlier, manganese forms Mn3O4 — part of the mixed oxidation-state oxide series running from manganese to nickel. So manganese, like titanium, has a “dual identity.”
Categorizing the Early 4d–5d Elements
Before dividing up these elements, there is actually at least one isoelectronic series that spans all the 5d elements; that is, the hexacarbonyls [26]. The series is shown in Table 8.6.
Table 8.6 The isoelectronic hexacarbonyl-complexes of the 5d transition elements
Table 8.7 The isostructural octofluoro-complexes of the early 4d–5d transition elements
However, there is a fundamental difference between the early 4d–5d elements and the later ones: size. The early heavier transition metal ions are significantly larger, enabling them to have coordination numbers up to eight. An excellent example is the series of isostructural (though not all valence-isoelectronic) octafluoro-complexes as shown in Table 8.7.
On this basis, should [Zr–Hf–Nb–Ta–Mo–W–Tc–Re] be considered an octad of elements? One pattern does not make a cluster, but on the other hand, if we took every single piece of evidence, we would consider each element completely unique.
A defining difference between the [Zr–Hf–Nb–Ta] tetrad and [Mo–W–Tc–Re] tetrad is the difference in aqueous chemistries of the two tetrads. The simple chemistry of the first tetrad across most of the pH range is defined by the insoluble oxides: ZrO2, HfO2, Nb2O5, and Ta2O5. For the second tetrad, it is the soluble tetraoxo-anions that dominate and
The Platinum Metals
The platinum metal group consists of the [Ru–Os–Rh–Ir–Pd–Pt] hexad of elements. Livingstone has commented that a distinction between the Fe–Co–Ni series and the lower members of the respective Groups is that they form hexahydrated dipositive ion, while the platinum metals do not. However, he cautioned that [27]:
Because of this difference between iron, cobalt and nickel, on the one hand, and the platinum metals on the other, it must not be overlooked that the relationships within the Group VIII are vertical.
Livingstone provided some examples:
•Iron, ruthenium, and osmium (Group 8) all form carbonyls of formula M(CO)5.
•Cobalt, rhodium, and iridium (Group 9) all form carbonyls of formula M2(CO)8.
•Nickel, palladium, and platinum (Group 10) all form tetracyano-anions [M(CN)4]2−.
Nevertheless, there is a general acceptance that the six platinum metals form a cluster. Their simple chemistry is characterized by insoluble oxides under strongly oxidizing conditions.
•For [Ru–Os], the +8 oxidation state is favored: RuO4 and OsO4.
•For [Rh–Ir–Pd–Pt], the +4 oxidation state is favored: RhO2, IrO2, PdO2, and PtO2.
As these six elements are found naturally in their elemental state — and often alloyed together — they have been as much an interest of geologists and metallurgists as chemists. Darling has commented [28]:
In 1860 Claus announced his view that the platinum metals formed “an isolated metallic group, inseparable and solidly constituted.” The physical and metallurgical evidence that has since been accumulated fully confirms the validity of this early chemical generalization.
Geochemists and metallurgists subdivide the platinum metals into the iridium–platinum group elements (IPGEs), [Os–Ir–Ru], and the palladium–platinum group elements (PPGEs), [Rh–Pt–Pd]. The distinction within the platinum metal bloc arises from the IPGEs existing almost exclusively in elemental form (siderophiles), while the PPGEs can also be found as metal sulfides (chalcophiles) [29].
Is There, in Fact, a Group 11?
Using the word “Group” implies similarity between members. For example, Group 4 was simple. The oxidation state of +4 dominated all of the three elements.
