Group 13 (Triels)
Is Group 13 really a group? At the top is boron whose chemistry is dominated by unique cluster species. Then comes aluminum, which would be happier in Group 3 (see Chapter 9). Next is gallium with its near room temperature melting point. And at the bottom, there is thallium that likes to masquerade as a Group 11 element or as a lower Group 1 element (see Chapter 10).
Boron Is Not Boring, It’s Unique
As soon as a chemist sees an icosohedron, boron comes immediately to mind. This beautiful and symmetrical molecule is the centerpiece of what makes this element unique. Of course, now a plethora of open- and closed cluster molecules and ions are known. Initially, these other species were believed to be simply fragments of an icosahedron. It was in 1971 that Wade showed that this was not true: instead, they were arranged into three families (closo-, nido-, and arachno-). Subsequently refined by Mingos, the criteria for these skeleta are now known as the Wade–Mingos rules [26]. The rules provide a straightforward and elegant rationalization of the shapes of “electron-deficient” cluster compounds in terms of the number of skeletal electron pairs (SEPs) these molecules.
Group 14 (Tetrels)
Just as Groups 1, 17, and 18 are regarded as epitomizing the smooth change in properties descending the respective group, Groups 14 to 16 represent the “discontinuity” groups. These are the groups that span the range of element behavior from nonmetal, through metalloid, to metal. In these cases, though there are sometimes similarities in chemical formula of compounds, there is little that can be chosen to select for group trend. In fact, group individuality is more interesting.
Graphite: The Forgotten Allotrope
Though the diamond and fullerene allotropes of carbon have taken the limelight in recent years, here the focus will be on oft-forgotten graphite. Graphite, with its layer structure of conjugated aromatically bonded atoms, has the ability to trap other atoms and molecules between the carbon sheets [27]. These are known as intercalation compounds:
Graphite intercalation compounds (GICs) are complex materials having a formula CXm where the ion Xn+ or Xn− is inserted between the oppositely charged carbon layers. Typically, m is much less than 1.
GICs are of interest in providing the electrode framework in battery systems. One specific example, of the many known species, is that between graphite and potassium. Molten potassium is absorbed into the black graphite layers to give a bronze-colored ionic solid with limiting composition of [K]+[C8]− [28].
Cubane: A Whole New Field of Inorganic Chemistry
Over the history of organic chemistry, cubane, C8H8, was considered simply a hypothetical molecule. With 90° bond angles, no one thought it could actually be synthesized, that is, until it was in 1964 [29]. Not only was it synthesizable but, when produced, it was a stable molecule. Why mention this in a book that is essentially inorganic chemistry? The pseudo-cubane structure is one that permeates cluster inorganic chemistry, and by its name, recognizes the simplest structure from which they are all derived. For example, there are the thallium–oxygen pseudo-cubanes, such as Tl4(OCH3)4 [30]. Silicon forms pseudo-cubanes, Si8(SitBuMe2)8. Phosphorus forms pseudo-cubanes where it alternates with boron, or aluminum, or nitrogen, or carbon, such as P4(CtBu)4. But of all the pseudo-cubanes, one must take top billing: that of the iron–sulfur pseudo-cubanes that are such crucial redox systems in so many biochemical processes (Figure 7.3) [31].
Figure 7.3 The common iron–sulfur pseudo-cubane core of many biological redox systems.
Group 15 (Pnictogens)
As for Group 14, the elements of Group 15 span a wide range of behaviors. And there are always surprises awaiting discovery. As an example, nitrogen is cited as having a single allotrope, N2. However, we see things from the perspective of our own SATP world. Under the conditions of the very low pressure at the edge of the Earth’s atmosphere, tetranitrogen, N4, is to be found [32].
Tetrahedral Ions
No, not those containing a tetrahedral bonding arrangement, but those species containing a tetrahedron of atoms. The yellow (not white [33]) allotrope of phosphorus, P4, provides the prototypical example. But just as cubane spawned pseudo-cubanes, so there are other molecules and ions adopting this very bond-strained tetrahedral shape. Some examples of these valence-isoelectronic ions are [Bi2Sn2]2−, [Sb2Pb2]2−, [Si4]4−, [Ge4]4−, [Sn4]4−, [Pb4]4−, and [Tl4]8−. Such species are examples of Zintl ions [34]. Zintl compounds are brittle, high-melting, intermetallic compounds, which contain polyatomic anions. First investigated in the 1930s, Zintl phases are formed by reacting a Group 1 or Group 2 metal with an element in any of Groups 13, 14, 15, or 16.
Arsenic in Biological Systems
One aspect often overlooked by inorganic chemists is that of substitution of one element for another in a biological organism or process. In this chapter, the substitution of one element by another in the same group will be the focus. For example, one bacterium can utilize arsenates instead of phosphates [35]. There has even been a computational study of whether such a substitution may be more widespread among bacteria [36].
Group 16 (Chalcogens)
With Group 16, there is the progression from nonmetals, oxygen and sulfur (not “sulphur” [37]); to metalloid, selenium; then to the two weak metals, tellurium and (radioactive) polonium. The allotropes of oxygen used to be dismissed as simply dioxygen and trioxygen (ozone). But no more. Tetraoxygen, O4, exists at very low pressures in the upper atmosphere [38]; while octaoxygen, O8, is formed as a dark red solid cubic structure under high pressure, low temperature [39].
Sulfur on Io
Sulfur has more allotropes than any other element [40] and even when it melts upon heating the chemistry continues to be complex [41]. Yet the most interesting sulfur chemistry is not on Earth, but on Jupiter’s moon, Io. Most people have seen NASA photos of the startingly multicolored moon, unique in the Solar System, which even possesses a sulfur lake [42]. One molecule identified in
