Not previously identified, there are similarities in the chemistry of yttrium and indium. For example, their aqueous chemistry is dominated by the soluble 3+ cation in acid and by the insoluble hydroxide at neutral and basic pH (Table 9.4).
As a final note, the bottom member of Group 13, thallium, has very different chemistry to either yttrium or indium. The chemistry of thallium is more appropriately linked to that of silver through the “knight’s move” relationship (see Chapter 10).
Table 9.4 A comparison of yttrium and indium species under oxidizing conditions
Group 4 and Group 14
This Group–pair (Figure 9.5) seems to be unique in that, although there are similarities between titanium(IV) and silicon(IV), there is a much greater resemblance of titanium(IV) with tin(IV), farther down Group 14.
In some ways, the chemistry of titanium(IV) resembles that of all the Group 14 elements in their +4 oxidation state. In particular, all five form tetrahedrally coordinated chlorides that are hydrolyzed to give the dioxide and hydrogen chloride.
Interestingly, even though they have significantly different molar masses, the chlorides of titanium(IV) and tin(IV) have remarkably similar melting and boiling points (Table 9.5). By contrast, both zirconium(IV) chloride and hafnium(IV) chloride are high-melting solids with a polymeric, six-coordinate structure.
As another example of the similarity of titanium(IV) and tin(IV), the most common form of crystal structure of titanium(IV) oxide is rutile, and tin(IV) oxide adopts the same structure. Also, titanium(IV) oxide and tin(IV) oxide share the rare attribute of thermochromism by turning from white to yellow reversibly on heating.
Figure 9.5 Members of Group 3 and Group 13.
Table 9.5 A comparison of melting points for the Group 4 and 14 chlorides
Although the emphasis in this chapter is on links near the top of the respective groups, there are also relationships among the lower members. For example, the zirconates, M2Zr2O7, and the stannates, M2Sn2O7, adopt similar structures [17].
Group 5 and Group 15
In this Group–pair (Figure 9.6), the major resemblance seems to be between vanadium(V), phosphorus(V), and arsenic(V). There is also a similarity in aqueous species between niobium and antimony.
Figure 9.6 Members of Group 5 and Group 15.
Table 9.6 A comparison of aqueous vanadium, phosphorus, and arsenic species in dilute solution under oxidizing conditions
In terms of the simple oxo-anions, vanadium resembles both phosphorus and arsenic. Vanadate, phosphate, and arsenate are all strong bases with similar pKa values. The only significant difference is that at low pH, vanadium forms the vanadyl ion, not the undissociated acid as do phosphorus and arsenic (Table 9.6).
There are a significant number of parallel compounds between vanadium, phosphorus, and arsenic, as can be seen from Table 9.7. Of the two, vanadium more closely resembles phosphorus as there are several examples, two of which are listed in the following, for which there is no known arsenic analogue.
Table 9.7 Some parallel compounds and ions of vanadium(V) with phosphorus(V) and arsenic(V)
Table 9.8 A comparison of aqueous antimony and niobium species under oxidizing conditions
There is also an interesting parallel in aqueous species between antimony and niobium as is shown in Table 9.8. Though the aqueous antimonate ion is usually represented as [Sb(OH)6]−(aq) and the aqueous niobate ion as [NbO3]− (aq), Greenwood and Earnshaw [18] have pointed out that for both of them, isopolymeric species predominate over most of the soluble range. By contrast, tantalum forms insoluble tantalum(V) oxide across the full pH range, while bismuth(III) dominates that element’s aqueous chemistry.
The 4th Period Anomaly Revisited
In Chapter 7, the concept of the 4th Period anomaly was introduced. This anomaly was characterized by some aspects of the chemistry of the 4th Period member of a specific group differing from the pattern for the other group members [19]. Dasent listed some of the 4th Period examples [20]. In this context, he noted that while PCl5, NbCl5 (of Group 5), and SbCl5 (of Group 15) are stable and well characterized, members of the 5th Period, both VCl5 (Group 5) and AsCl5 (Group 15) were elusive. They are now known, AsCl5 decomposing above −50°C [21] and VCl5 decomposing above −40°C [22], but certainly not “stable” species like the other matching chlorides.
Group 6 and Group 16
Just as vanadium(V) resembles phosphorus(V) and arsenic(V), so chromium(VI) resembles both sulfur(VI) and selenium(VI) (Figure 9.7).
Again there are parallels in the acid–base behavior of the oxo-anions, the only difference in this case being that chromic acid is a weaker acid than either sulfuric acid or selenic acid (Table 9.9).
There are also several formula similarities between chromium(VI) and both sulfur(VI) and selenium(VI). A few examples are given in Table 9.10.
Figure 9.7 Members of Group 6 and Group 16.
Table 9.9 A comparison of aqueous chromium, sulfur, and selenium species under oxidizing conditions
Table 9.10 Some parallel compounds and ions of chromium(VI) with sulfur(VI) and selenium(VI)
Group 7 and Group 17
Once again, there seems to be a triangular relationship, this time between manganese(VII), chlorine(VII), and bromine(VII) (Figure 9.8). There are also parallels in formulas between rhenium(VII) and iodine(VII).
The most obvious similarity between the three at the top of their Groups are the strongly oxidizing oxo-anions: permanganate, perchlorate, and perbromate. All three elements form corresponding trioxofluorides: MnO3F, ClO3F, and BrO3F. There seems to be a slightly greater similarity between manganese(VII) and chlorine(VII) in that only those two form oxides in the +7 oxidation state: Cl2O7, and Mn2O7, both of which are highly explosive liquids at room temperature.
Figure 9.8 Members of Group 7 and Group 17.
Table 9.11 Some parallel compounds and ions of rhenium(VII) and iodine(VII)
Equally interesting in this group–pair are the similarities of rhenium with iodine.
Some of the parallel compounds are shown in Table 9.11. Of note, ReOF5 has a melting point of 44°C while that of
