Figure 8.1 The traditional group-by-group study of the transition metals.
Table 8.1 Simple carbonyls of the middle transition metal series
Figure 8.2 Combining the Fe–Co–Ni triad together; and considering the platinum group metals as separate entity.
A third approach is to cover Groups 4 to 7 and 11 individually, then the [Fe–Co–Ni] ferromagnetic triad together, and the platinum metals as a separate entity (Figure 8.2). This format was adopted, among others, by Partington’s General and Inorganic Chemistry for University Students [12], the series Pergamon Texts in Inorganic Chemistry [13, 14], and by Textbook of Inorganic Chemistry (Gopalan) [15]. Historically, the platinum group metals, since their discovery, have always been considered as a related “cluster” [16]. Lee has commented that, for the Groups 8, 9, and 10 [17]:
. . . the horizontal similarities between these elements are greater than anywhere in the periodic table except among the lanthanides.
He also noted that the similarities:
. . . are sometimes emphasized by considering these nine elements as two horizontal groups: the three ferrous metals Fe, Co and Ni, and the six platinum metals Ru, Rh, Pd, Os, Ir and Pt.
Each of these classification systems has one flaw — that they organize the transition metals largely according to one strategy and they define the trends according to that organization. Thus linkages, relationships, patterns, or similarities outside of that framework are ignored. Two exceptions have been the proposals by Habashi and by Schweitzer and Pesterfield.
Habashi’s Categorizations
Habashi [18] has identified three categories of transition metals (excluding the Group 11 metals) and named them as follows:
•The vertical similarity transition metals: [Zr–Hf]; [Nb–Ta]; [Mo–W]; and [Tc–Re].
•The horizontal similarity transition metals: [Ti–V–Cr–Mn–Fe–Co–Ni].
•The horizontal–vertical transition metals: [Ru–Os–Rh–Ir–Pd–Pt].
That is, Habashi considered the 3d metals as separate entities from the 4d and 5d metals (Figure 8.3). There is much chemical evidence for the 4d and 5d metals, as a set, being very different to those of the 3d metals. In particular, the chemistry of zirconium and hafnium is almost identical, yet significantly different to that of titanium [19].
One of the vertical similarity transition sets has a biochemical basis. Certain bacteria, which normally utilize molybdenum in some of their enzymes, utilize tungsten instead when the bacteria are in high-temperature environments. It is believed that the tungsten-containing enzymes can survive and function as thermophiles [20].
Figure 8.3 The transition metal classification according to Habashi [18].
Schweitzer and Pesterfield’s Categorizations
The reference work, the Aqueous Chemistry of the Elements (Schweitzer and Pesterfield), includes a series of Pourbaix diagrams [21]. The authors assign the 3d transition metals to two horizontal triads: [V–Cr–Mn] that form compounds in the maximum oxidation states; and [Fe–Co–Ni] for which +2 and +3 oxidation states predominate [22]. Schweitzer and Pesterfield treat copper with the other two Group 11 metals as the [Cu–Ag–Au] vertical triad, while titanium is placed in a chapter with all the 4d and 5d transition metals. They divide the heavy transition metals (plus titanium) into three subcategories:
•The elements for which insoluble oxides dominate [Ti–Zr–Hf–Nb–Ta].
•The elements with high oxidation-state oxo-anions [Mo–W–Tc–Re].
•The platinum metals [Ru–Os–Rh–Ir–Pd–Pt].
This scheme is shown in Figure 8.4.
Other Categorizations
In recent years, using chemotopological methods, there have been new attempts at classifications of the elements. Sneath’s study [23] divided the heavy transition metals into one cluster and the later ones (plus gold) into a separate cluster. The analysis by Leal et al. [24] suggested that, for the 3d metals, there was a [Cr–Fe–Co–Ni] linkage, while titanium belonged to Group 4 as [Ti–Zr–Hf] and manganese and vanadium were unique in their chemistry. For the 4d–5d transition metals, they proposed the following linkages: [Zr–Hf (with Ti)]; [Mo–W (with Ge)]; [Nb–Ta–Tc–Re]; [Ru–Os]; [Rh–Ir–Pd–Pt]; and [Ag–Au].
Figure 8.4 The transition metal classification according to Schweitzer and Pesterfield [21].
Categorizations of the Transition Metals
In this chapter, a schema will be deduced from first principles using chemical criteria that have similarities to, but also differences from, those of Habashi and of Schweitzer and Pesterfield. Though copper and gold fit the chemical transition metal criteria, silver does not. In developing these criteria, flexibility in any classification is necessary and, indeed, at least two elements might be considered as having “secondary allegiances.”
The 3d Period Patterns
Just as the 2nd Period main group elements differ from those of the subsequent Periods (see Chapter 7), it has always been recognized that the metals of the 3d transition series differ significantly from those of the 4d and 5d series. The 3d metals are more commonly found in lower oxidation states and they can form high-spin compounds as a result of the lower crystal field stabilization energy. But the 3d metals do not form a homogeneous series. This differentiation can be seen by looking at the formulas of the common oxides (Table 8.2) [25].
Table 8.2 The common oxides of the 3d transition metals
Among the many oxides, for the Group 4 to Group 7 metals, there is an oxide series for which the metal has its maximum oxidation state (TiO2, V2O5, CrO3, and Mn2O7). For the same set of metals, there is also an oxide series MO2 (with M = Ti, V, Cr, and Mn). On the basis of the oxides, it could be considered that there is a [Ti–V–Cr–Mn] tetrad subgroup of the 3d metals. Likewise, the next three of the 3d transition metals, the [Fe–Co–Ni] triad, are characterized by having +2 and +3 oxidation states in their common oxides. Copper is unique among the 3d metals in exhibiting an oxidation state of +1 in an oxide (and in its chemistry in general). Of course, the divisions are not clear-cut. For example, there is the oxide series of M3O4 that encompasses manganese through
