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
