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 through the mercury in the form of a dissolved metal, since ammonia and ammonium salts are quite insoluble in mercury.
A reported electrolytic synthesis in 1951 [7], together with an article titled “Is There a Neutral Ammonium Radical?” in 1968 [8] marked a revival of interest in the possibility of such a species. Whiteside, Xantheas, and Gutowski showed from computational studies that the ammonium radical should resemble sodium in terms of its electronegativity [9]. The most convincing synthetic evidence of a pseudo-alkali metal radical has been that of tetramethylammonium with mercury. This species has been shown to have the formula [N(CH3)4]Hg8 [10].
The explanation of the ammonium amalgam was reported in 1986 by Garcia et al. [11]. They provided convincing evidence that for the tetrabutylammonium amalgam, the structure was actually: ([N(t-Butyl)4]+[Hg4]−. By comparison, the structure of the tetramethylammonium amalgam should be: [N(CH3)4]+[Hg8]−. Thus, the claimed ammonium radical does not exist but instead, the mercury cluster is another case of a metal with a significant electron affinity (see Chapter 2). In fact, the stability of mononegative mercury clusters has been investigated and their stability confirmed [12]. In fact, the clustering up to eight atoms results in a considerably increased (more negative) electron affinity, explaining why these anionic metal clusters so readily form [13].
Pseudo-Halogens
As ammonium and its relatives are to alkali metal cations, so pseudo-halogens (a term devised in 1925) are to halide anions. The following definition is appropriate:
A pseudo-halogen is a polyatomic analogue of the Group 17 elements whose chemistry resembles that of one or more of the halogens. The polyatomic pseudo-halide ion may substitute for a halide ion and the resulting compounds should resemble in their chemistry those of the equivalent halide compound.
Denoting a pseudo-halide ion as (PseudoX)− and a halide ion as X−, corresponding pseudo-halogen molecules of the forms (PseudoX)−(PseudoX) and (PseudoX)−X can form. As an example, the pseudo-halide ion, C≡N−, can be oxidized to form its own diatomic “parent” molecule, N≡C−C≡N, or “partner” with a halogen, such as chlorine, to form N≡C−Cl.
Among the more common pseudo-halide ions are the valence-isoelectronic series of cyanate, OCN−; thiocyanate, SCN−, and selenocyanate, SeCN−. The azide ion, is also considered a pseudo-halide ion even though its parent pseudo-halogen does not exist.
Those listed earlier are five of the “traditional” pseudo-halide ions. There is continuing interest in this category of ions/compounds [14]. One of the newer additions is the (CS2N3)− ion. This ion satisfies the full criteria in that the parent pseudo-halogen, (CS2N3)2 has been synthesized as has an inter-pseudo-halogen, (CS2N3−CN) [15].
Cyanide Ion as a Pseudo-Halide Ion
Earlier, it was described how the ammonium ion, despite being a polyatomic ion, behaved much like an alkali metal ion. However, the best example of a pseudo-element ion is cyanide. Not only does it behave very much like a halide ion but also the parent pseudo-halogen, cyanogen, (CN)2, exists.
The cyanide ion resembles a halide ion in a remarkable number of ways:
•Compounds of cyanide ion with silver, lead(II), and mercury(I) ions are insoluble, as are those of chloride, bromide, and iodide ions.
•Just as solid silver chloride reacts with ammonia to give the diamminesilver(I) cation, so does silver cyanide.
•The cyanide ion is the conjugate base of the weak acid hydrocyanic acid, HCN, parallel to fluoride ion being the conjugate base of the weak acid, hydrofluoric acid.
The existence of cyanogen, C2N2, the “parent element” makes a stronger case for the concept of pseudo-elements.
•Cyanide ion can be oxidized to cyanogen in a similar manner to the oxidation of halides to halogens. The parallel is particularly close with iodide ion since they can both be oxidized by very weak oxidizing agents such as the copper(II) ion.
•Cyanogen reacts with base to give the cyanide ion and cyanate ion (CNO−), in a parallel manner to the reaction of dichlorine with base to give chloride and hypochlorite ion (ClO−).
Cyanogen forms pseudo-interhalogen compounds such as iodine monocyanide, ICN, in the same way that halogens form interhalogen compounds such as iodine monochloride, ICl.
The Tetracarbonylcobaltate(−I) Ion
This pseudo-halide ion was first synthesized in the 1930s. It was shown that the compound tetracarbonylhydrocobalt(−I), HCo(CO)4, is highly acidic, with a pKa of 8.5 [16]. In fact, its synthesis is by acidification of sodium tetracarbonylcobaltate(−I). As a result of its acidity, it was the first catalyst employed for the hydroformylation of alkenes (the oxo reaction) [17].
A Cautionary Note
To end with a cautionary note: terminology. Before claiming a pseudo-element is a “pseudo-alkali metal” or a “pseudo-halogen,” it should meet specific criteria. Does it indeed have many matching chemical properties of one of those elemental groups; or is it simply a large polyatomic monopositive cation or mononegative anion?
Combo Elements
The compound carbon monoxide has several similarities to dinitrogen, N2. For example, they are both triply bonded molecules with similar boiling points: −196°C (N2) and −190°C (CO). A major reason for the parallel behavior is that the dinitrogen molecule and the carbon monoxide molecule are isoelectronic. This similarity extends to the chemistry of the two molecules. In particular, there are several transition metal compounds where dinitrogen can substitute for a carbon monoxide entity. For example, it is possible to replace one or two carbon monoxides bonded to chromium in Cr(CO)6 to give isoelectronic Cr(CO)5(N2) and Cr(CO)4(N2)2.
The combo elements are a subset of isoelectronic behavior in which the sum of the valence electrons of a pair of atoms of one element (above being nitrogen) matches the sum of the valence electrons of two horizontal neighboring elements (carbon and oxygen):
A combo element can be defined as the combination of an (n – x) group element with an (n + x) group element to form compounds that parallel those of the (n) group element.
Boron–Nitrogen Analogs of Carbon Compounds
The best example of a combo element is that of the boron and nitrogen combination matching with a pair of carbon atoms, boron having one less valence electron than carbon, and nitrogen one more.
The simplest analogue is that of boron nitride, BN, for the graphite allotrope of carbon. Unlike graphite, boron nitride is a white solid that does not conduct electricity. This distinction is probably a result of the different way the layers in the two crystals are stacked.
The layers in the graphite-like form of boron nitride are an almost identical distance apart as those in graphite. However, the boron nitride layers are organized so that the nitrogen atoms in one layer are situated directly over boron atoms in the layers above and below, and vice versa. This arrangement is logical, because the partially positive boron atoms and partially negative nitrogen atoms are likely to be electrostatically attracted to each other. By contrast, the carbon atoms in one layer of graphite are directly over the center of the carbon rings in the layers above and below.
In a further analogy to carbon, application of high pressures and high temperatures converts the graphite-like allotrope of boron nitride to diamond-like forms. The poly-morph [18] with a zinc-blende type structure was first synthesized in 1957 [19] while the wurtzite form was not prepared until 2009 [20]. Thus, there are two allotropes, one with two polymorphs of boron nitride to match with those of carbon.
There has been an increasing interest in hybrid boron–nitrogen/carbon materials to benefit from the properties of both. An example is the coating of carbon nanotubes with a layer of boron nitride [21].
Figure 14.3 A comparison of benzene and borazine.
One of the other similarities between boron–nitrogen and carbon compounds is the compound borazine [22]. Borazine, B3N3H6, is a cyclic molecule analogous to benzene, C6H6.
In fact, borazine is sometimes called “inorganic benzene” (Figure 14.3). This compound is a useful reagent for synthesizing other boron–nitrogen analogs of carbon compounds, but currently it has no commercial applications.
The polarity of the boron–nitrogen bond means that borazine exhibits localized π-bonding between pairs of atoms rather than the complete delocalized aromatic ring π-system of benzene [23]. Hence, borazine is much more prone to chemical attack than is the homogeneous ring of carbon atoms in benzene. Though the liquids have similar densities, there are significant differences in melting and boiling points (Table 14.1).
There has been interest in preparing other compounds in which the boron–nitrogen pair have been substituted for a carbon–carbon pair. For example, B–N substitutions have been made in diphenylacetylene [24]. These combo element replacements show that the polarity of the B–N bond influences the molecular structure. There is now increasing interest in inorganic polymers employing a boron–nitrogen backbone [25].
Table 14.1 A comparison of some physical properties of benzene and borazine
Superatoms
Certain clusters of atoms can behave as if they were single entities. That is, free electrons in the cluster occupy a unique set of molecular orbitals. These bonding orbitals are all occupied, resulting in a “closed shell” system. Loss of one electron provides alkali metal ion behavior while addition of one electron results in halide ion behavior. The most comprehensive definition is:
A superatom is any cluster of atoms that seems to exhibit some of the properties of elemental atoms.
The best documented superatoms are clusters of aluminum atoms generated with a negative charge. For example, it is claimed that the Al13 cluster behaves as a super-halogen [26]. Correspondingly, the [Al13I]− cluster behaves like an inter-polyhalide ion, such as BrI−. By contrast, the Al142+ cluster behaves as an alkaline earth metal.
Computational studies on the iron–oxygen cluster, FeO4, have suggested that it, too, has a closed-shell structure. According to the computations, despite its closed-shell structure, this cluster has an electron affinity that is larger than that of any known halogen atom [27].
Synthetic Metals
Finally, a term, “synthetic metals” needs to be included here in this compilation for completeness, which, despite its name, does not really fit. The definition commonly accepted is [28]:
A synthetic metal possesses metallic conduction but is formed entirely of such nonmetallic atoms as carbon, nitrogen, hydrogen, the halogens and oxygen.
The first use of this term was to describe ammonium amalgam (see earlier). However, the prototypical example of a synthetic metal is polysulfur nitride [29].
Commentary
Is this THE END? No, our perspectives on the patterns and trends in the Periodic Table will never become fixed. Until 1999, there was no knowledge of a knight’s move relationship. Topological studies continue to find new unexpected linkages [30]. And the superatom clusters that have been synthesized, such as [EuSn6Bi8]4− [31], continue to push the boundaries beyond what a classical inorganic chemist would ever have believed possible. Maybe long-lived isotopes of ephemeral elements will be synthesized, and their actual chemistry studied. Perhaps, this book marks the END OF THE BEGINNING and that some of the thoughts therein will lead to the opening of new vistas for the future.
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Index
Actinoids, Actinide Hypothesis, 256
Actinoids, Definition, 256
Actinoids, Early-Actinoids relationships with Transition Metals, 262
Actinoids, Later-Actinoids and Lanthanoid similarities, 265
Actinoids, Nobelium(II) and Group 2 element similarities, 266
Actinoids, Oxidation States, 260
Actinoids, Thorium(IV) and Cerium (IV) and Hafnium(IV) similarities, 264
Periodic Table, The: Past, Present, And Future Page 21