Periodic Table, The: Past, Present, And Future
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56.G. J. Schrobilgen, “The Fluoro(hydrocyano)krypton(II) Cation [HC≡N–Kr–F]+; the First Example of a Krypton–Nitrogen Bond,” J. Chem. Soc. Chem. Commun. 863–865 (1988).
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Chapter 8
Patterns among the Transition Metals
In this chapter, we will deconstruct the monolithic block of transition metals. Traditionally, classification has been by Group, but there are far richer patterns and trends to be found. By using chemical criteria for assignment, a hybrid approach to categorizing and clustering transition metals offers many advantages. Nevertheless, ambiguities arise as to the assignments that we choose.
The first task is to define the transition metals, which is not quite as obvious as it may seem. The main group elements are always identified as the members of Groups 1 to 2, and 13 to 18; that is, the s-block and p-block elements. In addition, about half of textbook sources include Group 12 as main group elements [1]. One might justifiably conclude by deduction that Groups 3 to 11, or Groups 3 to 12, would be the transition metals. As with much of inorganic chemistry, it is not that simple.
What Is a Transition Metal?
The terms “d-block metal” and “transition metal” are not synonymous. Identifying a “transition metal” is not simply a question of location in the Periodic Table, but also one of chemical behavior. A definition common among inorganic chemists is a transition metal is an element that has at least one simple ion with an incomplete outer set of d electrons.
Exclusion of Group 3
Using the criterion earlier, the Group 3 metals are excluded as their common chemistry is all based on the d0 3+ ion, Sc3+ and Y3+. In fact, the chemistry of these two metals more closely resembles that of the lanthanoids. Patterns among the Group 3 elements, therefore, will be discussed in Chapter 12. Of note, the classic series by Sneed and Brasted, Comprehensive Inorganic Chemistry, combined scandium and yttrium with the lanthanoids [2].
Exclusion of Group 12
The Group 12 metals are also excluded. For them, the predominant ions, Zn2+, Cd2+, and Hg2+ have d10 configurations. The isolation of d8 mercury(IV) fluoride, HgF4, at very low temperatures [3], initially provoked the claim that mercury should be redesignated as a transition metal. As ever more exotic and fragile species are identified [4], there is the potential for a never-ending expansion of claimed members of the transition metal series.
However, such an eventuality can be avoided by using a definition of:
A transition metal is an element that has at least one simple ion with an incomplete outer set of d electrons, which is stable under ambient conditions.
Here, this definition will be utilized, though it will be shown that there can be considered one exception to the rule. Before doing so, it is appropriate to review the previous categorizations of transition metals.
Exclusion of Honorary Transition Metals
A new term entering the vocabulary of inorganic chemistry is that of honorary d elements [5] or honorary transition metals [6]. These terms have been devised to describe organometallic compounds of Group 1 or Group 2 elements that, it is claimed, are using their inner d-orbitals in bonding. Such compounds have been identified by computational studies and/or by synthesis under extremely low temperatures. As such, they are excluded by the earlier definition from study here.
Previous Classifications of Transition Metals
One of the modern authoritative works on inorganic chemistry, Greenwood and Earnshaw’s Chemistry of the Elements [7] treats each of the transition metal groups as individual entities, devoting a chapter to Group 4, one to Group 5, and so on (Figure 8.1).
The species that seem to be Group-specific are the simple carbonyls [8]. The pattern is shown in Table 8.1.
Another common approach is adopted in Advanced Inorganic Chemistry (Cotton, Wilkinson, Murillo, and Bochmann) [9], with each of the elements of the first transition series being treated individually, then the 4d–5d pairs of elements being covered in a subsequent section. This arrangement of material is also adopted in Inorganic Chemistry (Housecroft and Sharpe) [10] and in Descriptive Inorganic Chemistry (Rayner-Canham and Overton) [11].
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 to nickel.
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 accep
tance 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. The three elements, copper, silver, and gold of so-called “Group 11,” can scarcely be said to form a “group.” Thompson has pointed out [30]: