Periodic Table, The: Past, Present, And Future

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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