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Chapter 14
Pseudo-Elements
Most chemists regard the ammonium ion as behaving much like an alkali metal cation. Likewise, cyanide ion shows similarities to halide ions. In this chapter, the similarities and differences of these polyatomic ions to their group analogues will be explored. In addition, some less-common “pseudo-element” ions will be introduced.
… and so, Gentle Reader, the end of this particular voyage has been reached. Rest assured, Gentle Reader, there are other voyages out there to cross hitherto uncharted seas of patterns and trends in the Periodic Table. The Periodic Table is not, and never has been “set in stone.” The Author has endeavored to show that, contrary to public belief and to the passing references to the Periodic Table in chemistry textbooks, it is a living evolving organism. The White Queen in Alice Through the Looking Glass had it correct (Figure 14.1).
Yes, Alice, there are indeed fantastical — and once thought impossible — aspects to the Periodic Table. For example, until 1962, it was known that it was impossible to make any compound of the noble gas elements. There was the “proof” of the octet-rule limit — so how could they? In some chapters, it has been proposed that elements belonged in more than one place in the Periodic Table — or in a place other than their atomic number/electron configuration mandated location — heresy in the past. The awesomeness of the Periodic Table continues in this final chapter: compounds and polyatomic ions that “behave” like elements and element ions.
Figure 14.1 Alice and the White Queen, from Alice Through the Looking Glass [1].
Pseudo-Elements
Some polyatomic ions resemble element ions in their behavior, and, in a few cases, there is a molecule that corresponds to the matching element. We can define this unusual category as:
A pseudo-element is the parent of a polyatomic ion whose behavior in many ways mimics that of an ion of an element or of a group of elements.
In this chapter, the focus will be on the ammonium ion as a pseudo-alkali metal ion, and on the cyanide ion as a pseudo-halide ion (Figure 14.2).
The Ammonium Ion as a Pseudo-Alkali Metal Ion
Even though the ammonium ion is a polyatomic cation containing two nonmetals, it behaves in many respects like an alkali metal ion. The similarity results from the ammonium ion being a large low-charge cation just like the cations of the alkali metals. In fact, the radius of the ammonium ion (151 pm) is very close to that of the potassium ion (152 pm). However, the chemistry of ammonium salts more resembles that of rubidium or cesium ions, perhaps because the ammonium ion is not spherical, and its realistic radius is larger than its measured value. The similarity to the heavier alkali metals is particularly true of the crystal structures. Ammonium chloride, like rubidium chloride and cesium chloride, has a CsCl crystal lattice at high temperatures and a NaCl crystal lattice at low temperatures.
Figure 14.2 Relationship of ammonium to the Group 1 elements and of cyanide to the Group 17 elements.
The ammonium ion also resembles an alkali metal ion in its precipitation reactions. Although all simple sodium compounds are water soluble, there are insoluble compounds of the heavier alkali metal ions with very large anions. The ammonium ion gives precipitates with solutions of these same anions. A good example is the hexanitritocobaltate(III) ion, [Co(NO2)6]3−, which is commonly used as a test in qualitative analysis for the heavier alkali metals. With ammonium ion, a bright yellow precipitate of (NH4)3[Co(NO2)6] is obtained analogous to that of K3[Co(NO2)6] with potassium ion.
However, the similarity does not extend to all chemical reactions that these ions undergo. For example, gentle heating of alkali metal nitrates typically gives the corresponding nitrite and oxygen gas, but heating ammonium nitrate results in decomposition of the cation and anion to give dinitrogen oxide and water.
Other Cations as Pseudo-Alkali Metal Ions
To stabilize large mononegative ions in the solid phase, a larger monopositive cation is required. One of the commonly used cations is the tetramethylammonium ion, [N(CH3)4]+. With an ionic radius of 234 pm, this ion is even larger than the cesium ion (181 pm). As a result, the tetramethylammonium ion will form a stable solid compound with the very large pentaiodide(1−) ion, [2], while the cesium ion will only form a solid cesium triiodide(1−), CsI3.
In addition to the ammonium ion, the phosphonium ion, had been used as a large monopositive cation [3]. Now the methyl-substituted phosphorus (and arsenic) analogues have become more popular (and safer) for this function [4]. With the very big tetraphenylphosphonium ion, [P(C6H5)4]+, the extremely large heptaiodide(1−) ion, ion can be stabilized, forming [P(C6H5)4]I7.
Ammonium as a Species
A major weakness of the parallel between ammonium ion and the heavier alkali metal ions is that the parent pseudo-element of the ammonium ion, “NH4,” cannot be isolated. However, claims of the existence of “ammonium” in the form of a mercury amalgam date back to the early 1800s. The discovery of this species was claimed first by Davy, while experiments with ammonium amalgam were described by Wetherill in 1865 [5]. An electrolysis experiment using mercury and ammonium ion published in 1929 concluded that the only explanation for the cell reaction was [6]:
… the ammonium group can reach the cathode only by diffusing
