Periodic Table, The: Past, Present, And Future

Group 1 elements tend to be anhydrous. Many Group 2 metal salts are hydrated.

•Three lithium salts — carbonate, phosphate, and fluoride — have very low solubility unlike the salts of the other Group 1 metals. These anions form insoluble salts with the Group 2 metals.

•Lithium is the only Group 1 metal to form a nitride, Li₃N. The Group 2 metals all form nitrides.

All four of these properties can be attributed to the significant difference in ionic radius between the large “typical” Group 1 metals and the significantly smaller lithium ion (Table 11.1). For example, it is only the larger low-charge-density cations that can stabilize the large low-charge anions, such as hydrogen carbonate, in a crystal lattice. Lithium ion, being more the size of a Group 2 metal, cannot. Instead, formation of the higher lattice energy carbonate compound will be energetically preferred. The opposite argument can be used with the formation of ionic nitrides (and carbides, see in the following): that only a small higher charge-density cation can stabilize the high-charge anion.

Specific Resemblance of Lithium to Magnesium

The best examples of resemblance specifically between the chemistry of lithium and that of magnesium are

•The only tricarbides(4–), C₃⁴⁻, of the Group 1 and 2 elements are Li₄C₃ and Mg₂C₃.

•Many lithium salts exhibit a high degree of covalency in their bonding as do those of magnesium.

•Lithium forms organometallic compounds similar to those of magnesium.

•Lithium is believed to occupy the same receptor site as magnesium in the treatment of bipolar disorder [20].

As with the nitrides, the formation of tricarbides can be interpreted in terms of stabilization by higher charge-density cations. The covalent behavior can also be explained as a result of lithium and magnesium being higher charge-density cations than the other members of their respective group.

Mackinnon [21] has pointed out that some other claimed evidence for the diagonality of these two elements is fallacious. For example, heating lithium nitrate gives lithium nitrite and oxygen gas (like the other Group 1 elements), not lithium oxide, nitrogen dioxide, and oxygen (like the Group 2 elements), contrary to some sources.

Does Li–Mg Isodiagonality Extend to Scandium? . . . and Beyond?

Though diagonality has traditionally been considered a unique property of the early elements of the 2nd Period and 3rd Period, there have been suggestions that diagonality extends into subsequent periods (as will be discussed in the following). The first complete diagonal series is shown in Figure 11.4.

Figure 11.4 The first diagonal series.

As an example of isodiagonality extension, scandium, too, forms an insoluble fluoride, carbonate, and phosphate. Uniquely, scandium is the only other metal to form a carbide containing the tricarbide(4−) ion, though the compound, Sc₃C₄, also contains carbide(4−) and dicarbide(2−) ions within the same lattice structure. Also, in one of the few ores of scandium, jervisite, the same lattice site is occupied by scandium and magnesium: (Na,Ca,Fe(II)) (Sc,Mg,Fe(II))Si₂O₆.

Scandium, in turn, has a resemblance to zirconium and thence to tantalum. For example, scandium forms [Sc₆Cl₁₂]^3– clusters, while zirconium and tantalum (and also niobium) form related [M₆Cl₁₂] cluster species.

Isodiagonality of Beryllium and Aluminum

The second pair to be examined here, that of beryllium and aluminum, has more common features unique to the diagonality. It is of relevance that, in the topological study of the chemical elements by Restrepo et al. [22], beryllium and aluminum were the only pair for which isodiagonal similarities exceed Group resemblances. This pair is the only diagonality example mentioned by House [23]. In addition to the other evidence that is widely cited, he refers to the two ions being particularly toxic — perhaps indicating a common biochemical bonding site.

Beryllium has a specific similarity to aluminum (and, to gallium) in terms of its aqueous (ionic) chemistry. Feinstein commented upon the similarities between the two elements in the context of analysis procedures. One example he gave was [24]:

… the spectrophotometric method for beryllium or aluminum using the ammonium salt of aurin tricarboxylic acid.

The similarity is particularly apparent when the Pourbaix (Eh–pH) diagrams [25] of the respective elements are compared (Table 11.2). The lower coordination number of the beryllium cation in acid solution may be explained as Be²⁺ being physically too small to accommodate six surrounding water molecules at a bonding distance.

In terms of compounds, there are several similarities:

•Beryllium and aluminum form carbides containing the carbide(4–), C^4– ion, both Be₂C and Al₄C₃ reacting with water to producing methane.

Table 11.2 A comparison of aqueous beryllium and aluminum species

•They form dimeric chlorides containing pairs of chlorine bridging atoms: ClBeCl₂BeCl and Cl₂AlCl₂AlCl₂.

•The two elements form methyl organometallics, Be(CH₃)₂ and Al(CH₃)₃, with bridging CH₃ groups. Both compounds are spontaneously flammable in air and are explosively hydrolyzed by water.

Does Be–Al Isodiagonality Extend to Germanium? . . . or to Titanium?

Roesky [26] has suggested that the diagonal relationship in this series continues to germanium. This proposal came as a result of his work on organometallic compounds of aluminum and attempts to synthesize germanium analogs.

However, Habashi [27] has pointed out that the chemistry of the aluminum ion more resembles that of the Group 3 elements rather than that of the lower members of Group 13 (see Chapter 9). Following from this proposal, the next member of the diagonal series should be considered as titanium. Titanium, like aluminum, is a low-density metal that reacts with the oxygen in air to form a tenacious protective layer of oxide to prevent further corrosion. One example of chemical similarities is that, for both aluminum and titanium, the fluorides are hexacoordinate species while the other halides are low-melting tetrahedrally coordinated species, such as Al₂Cl₆ and TiCl₄, which are hydrolyzed by water. The second complete diagonal series is shown in Figure 11.5.

Up to this point, similarities have been considered among valence-isoelectronic series, such as Be²⁺, Al³⁺, and Ti⁴⁺. However, titanium also readily forms an ion Ti³⁺. Thus, in this (and the later example of vanadium and molybdenum), matching compounds in the same oxidation can be considered. As an example here, titanium, like aluminum, forms alums such as CsTi(SO₄)₂∙12H₂O, analogous to CsAl(SO₄)₂∙12H₂O.