The comparative chemistry of indium(III) and bismuth(III) is more extensive. The parallels between indium(III) and bismuth(III) are particularly strong as +3 is the more common oxidation state for both elements. Bismuth exemplifies the phenomenon that the elements of the later 6th Period tend to favor the lower oxidation states over the highest. Some of the similarities are as follows:
Table 10.4 Some corresponding copper(I) and indium(I) compounds
•All tripositive halides and chalcogenides are known for both elements.
•Indium(III) and bismuth(III) form corresponding tetra-and hexa-coordinate halo-complex ions: and
•Indium(III) and bismuth(III) form isostructural alums: MIMIII(SO4)2.12H2O, where MI is a large monopositive ion and MIII is indium(III) or bismuth(III).
•Indium(III) and bismuth(III) form stable oxo-halides of matching formula, such as InOCl and BiOCl.
Zinc–Tin(II) and Tin(IV)–Polonium(IV) Double T-M Links
Though here we focus on knight’s move resemblances, it is important to note that an element in this region can also possess similarities to elements elsewhere in the Periodic Table. Zinc may hold the record in this context. In Chapter 9, the similarity of zinc (Group 12) to magnesium (Group 2) by the (n) and (n + 10) relationship is discussed, a linkage also reported by Laing [17]; while Massey has pointed out similarities of zinc with beryllium (Group 2) in compound formulas [18]. Massey also found similarities of zinc with gallium in chemical behavior (though not formula) [19].
The comparative chemistry of the zinc–tin(II) certainly provides several similarities.
•Zinc and tin(II) exhibit amphoteric behavior, their hydroxides dissolving in excess hydroxide ion to form zincates and stannates, respectively.
•Aqueous solutions of their divalent chlorides hydrolyze to give insoluble Zn(OH)Cl and Sn(OH)Cl.
•In the presence of high chloride ion concentrations, the chlorides give parallel chloro-complex ions: and and and
•Zinc and tin(II) form dialkyls of the form R2Zn and R2Sn (though the zinc series tend to be monomeric while the tin(II) compounds are polymeric).
Though some compounds of polonium(VI) are known, polonium, like bismuth, prefers lower oxidation states. There are a wide range of compounds of polonium(IV) together with some compounds of polonium(II). It was noted by Brasted over 45 years ago [20] that polonium bore little resemblance in its chemistry to tellurium and instead that polonium(II) behaved more like lead(II) of Group 14. Curiously, in one respect, polonium(II) has a resemblance to zinc(II): that is, polonium forms volatile dimethylpolonium(II), (CH3)2Po [21] analogous to (CH3)2Zn.
Despite Brasted’s claim of a link between polonium(II) and lead(II), there seem to be more similarities between polonium(IV) and tin(IV), than polonium(IV) and lead(IV). Following are examples of some matching formulas.
•There are matching chlorides and hexachloro-ions, SnCl4 and PoCl4, and [SnCl6]2− and [PoCl6]2−.
•The only solid stable nitrates of both metals correspond: Sn(NO3)4 and Po(NO3)4.
•There are matching oxides in the +4 oxidation state: SnO2 and PoO2.
Melting Points of Some Copper(I)–Indium(I) and Indium(III)–Bismuth(III) Halides
Laing [1] noted close-matching melting points among the following halide pairs: AgCl/TiCl; AgBr/TlBr; CdI2/PbI2; ZnCl2/SnCl2; and GaCl3/SbCl3. The question arises whether such patterns are pervasive, or just found for a few selected cases. Here, as an example, the melting point series are provided for the halides of two double pairs, copper(I)–indium(I) (Table 10.5) and indium(III)–bismuth(III) (Table 10.6).
Table 10.5 Melting points of corresponding copper(I) and indium(I) halides
Table 10.6 Melting points of corresponding indium(III) and bismuth(III) halides
As can be seen from the data earlier, there are no clear patterns among these K-M pairs. Nor could any consistent pattern be found for any other K-M pairs.
The Knight’s Move Relationship and the “Inert Pair” Effect
Having established that there is indeed a K-M relationship, the question needs to be asked as to the reason for it. Laing attempted to answer the question [1]:
Is the “knight’s move” merely a special case of the “inert pair effect” applied to metals with a d10 electron configuration?
The Inert Pair Effect
First, a digression onto the definition of the “inert pair” effect. This phenomenon was first described by Sidgwick in 1927 [22]. The observation was concisely explained by Orgel [23]:
Many B subgroup [main-group] metals exhibit a stable valency two smaller than the group valency. This tendency is most pronounced for thallium, lead, and bismuth and is also important for many lighter elements such as tin and antimony.
The contention then, is that, in the cases of ionic bonding, the ns2 electrons are significantly more tightly bound than the npx electrons. Or to reverse the statement, the npx electrons are more easily removed. For example, the noble gas core electron configuration of tin is: [Kr]5s24d105p2; the configuration of the more common ion, Sn2+, would be [Kr]5s24d10; and that of the less common ion, Sn4+, would be [Kr]4d10. The prevalence of the tin(II) ion would therefore be attributable to the inert pair effect.
However, with many of these compounds, the bonding is believed to be more covalent than ionic. Drago developed an explanation of the inert pair effect in terms of the low bond enthalpies of the heavy p-block metals [24]. Laing, himself, recognized this problem [1]:
There is more behind the knight’s move than meets the eye. We are dealing here with an extremely complex phenomenon, not easy to explain. … Nevertheless, application of the idea of the knight’s move among metals with d10 configurations on the bottom right hand side of the Periodic Table leads to many correct predictions that would not be made by applying the usually accepted trends in the Periodic Table.
“Inert Pair” as a Relativistic Effect
It is relativistic effects, first mentioned in Chapter 2, that provide the most logical explanations for most of the inert pair phenomenon [25–27]. When electron relativistic effects are considered, the energy of the electrons in the s-orbital drops significantly, that is, the electrons are more tightly bound to the nucleus. This pattern is shown in Figure 10.5 for tin and lead.
