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      2.2.2.1 Superalkalis and Superhalogens

      Alkali atoms, with outer electronic configuration of ns ¹, have one excess electron while halogen atoms, with outer electronic configuration of ns ² np ⁵, need one extra electron to achieve the octet shell closure of their respective ionic cores. As a result, both the atoms are reactive. Gutsev and Boldyrev [44, 45] were the first ones to use the octet rule to design superalkali and superhalogen clusters that not only mimic the chemistry of alkali and halogen atoms, respectively, but also surpass their properties. The ionization potentials of superalkalis are lower than those of the alkali atoms while the electron affinities of superhalogens are higher than those of the halogen atoms. The composition of superalkali is M _{k + 1}X, where M is an alkali atom and X is an atom with valence k. An example of a superalkali is Li₃O whose ionization potential of 3.54 eV [46] is smaller than that of the Li atom, namely, 5.39 eV. The composition of a superhalogen, on the other hand, is MY _{k + 1}, where Y is a halogen atom and M is a metal atom with valence k. A typical example of a superhalogen is LiF₂, which has an electron affinity of 5.45 eV [47]. This is much larger than the electron affinity of F, namely 3.4 eV. The reason for these superior properties is inherent in the nature of the distribution of electrons. Note that the phase space occupied by the outer electrons increases with cluster size. In superalkalis, this makes it easier to remove an electron, hence leading to a lower ionization potential. In a superhalogen, on the other hand, the increased phase space for the electron distribution causes a reduction in electron–electron repulsion; hence, leading to a higher electron affinity. That superhalogens can promote unusual reactions was already realized by Bartlett in 1962, long before Gutsev and Boldyrev coined the word. Bartlett and coworkers showed that O₂ and noble gas atoms such as Xe can be ionized by using PtF₆ and estimated its electron affinity as 6.8 eV [48, 49]. The fact that clusters can mimic the chemistry of alkali and halogen atoms with superior properties provides new opportunities to design supersalts with superalkalis and superhalogens as building blocks [50].

Numerous studies of superalkalis and superhalogens have been carried out over the past 20 years, and Chapter 3 of this book covers the advancements in superhalogen design and synthesis. For the purpose of illustration, we show in Figure 2.12 the electron affinities of fluorinated coinage metal clusters [51]. Note that the electron affinities are higher than that of F once the number of F atoms is greater than 2 and reach a value as high as 8.6 eV in AuF₆.

The octet rule can also be applied to nonmetallic elements to design superalkalis and superhalogens. Consider, for example molecules such as BH₄, BO₂, CN, and NO₃. Each of these molecules needs one extra electron for electronic shell closure. Indeed, with electron affinities of 4.42, 4.32, 3.86, and 4.03 eV, respectively, these molecules are superhalogens. Following the guidelines of Gutsev and Boldyrev, one could imagine that a new class of clusters with electron affinities even higher than those of superhalogens can be designed. Consider, for example, a cluster with composition MZ _{k + 1}, where Z is a superhalogen. This new class of clusters, named hyperhalogens, was discovered by Willis et al. [52] during the study of the interaction of Au with BO₂ molecules. The electron affinities of Au(BO₂) _n (n = 1–6) is compared with that of BO₂ in Figure 2.13. Note that for n ≥ 2, the electron affinities are larger than that of BO₂. This provides a way to design species with ever increasing electron affinities. We will discuss later how the synthesis of such highly electronegative species can be used to promote unusual reactions.

Figure 2.12 Electron affinity of coinage metal atoms decorated with F.

      Source: Koirala et al. [51]. © American Chemical Society.

Figure 2.13 Electron affinity (EA) of Au(BO₂)_n as a function of n (black line).

      Source: Adapted with permission from Ref. [52]. © John Wiley & Sons.

Superhalogens and hyperhalogens can be used in the design of novel salts for applications in solar cells, batteries, and hydrogen storage materials. While we discuss these applications in detail in Chapter 10, in the following chapter we show how hyperhalogen concept led to the synthesis of a hypersalt, KAl(BH₄)₄. Note that Al(BH₄)₃ is a volatile pyrophoric liquid. Although it contains 16.8 wt % hydrogen, it cannot be used as a hydrogen storage material because of safety concerns. However, by adding one more BH₄ unit to Al(BH₄)₃, an Al(BH₄)₄ hyperhalogen can be formed. By combining it with a K cation, Knight et al. [53] synthesized KAl(BH₄)₄ hypersalt, which is solid and nonvolatile under ambient conditions (Figure 2.14).

Figure 2.14 Al(BH₄)₃ (left panel) and KAl(BH₄)₄ (right panel).

      Source: Adapted with permission from Knight et al. [53]. © American Chemical Society (courtesy of D. Knight and R. Zidan, private communication).

      2.2.2.2 Superchalcogens

Atoms in the Group 16 of the periodic table require two extra electrons to satisfy the octet rule. When isolated, these atoms cannot retain both the electrons due to electron–electron repulsion. However, an atomic cluster could be stable as a dianion if it is large enough to reduce electron–electron repulsion. The question is: how small a cluster has to be so that it can retain two extra electrons without fragmenting or ejecting the second electron spontaneously? Such a cluster could be viewed as a superchalcogen that is stable, yet mimics the chemistry of Group 16 elements. Chen et al. [54] studied this possibility by focusing on M(CN)₄ clusters where M is a divalent alkaline earth metal atom (Be, Mg, Ca, Zn, Cd), which contributes two electrons while each CN molecule would need one electron to satisfy the octet rule. The authors calculated the equilibrium geometries and total energies of neutral, monoanionic, and dianionic M(CN)₄ clusters using density functional theory. The results are presented in Figure 2.15. The energy gains in adding the first (second) electron to M(CN)₄ clusters are 3.13, 2.94, 2.89, 2.78, and 2.59 eV (0.32, 0.97, 1.21, 0.83, and 0.56 eV), respectively, for M = Be, Mg, Ca, Zn, and Cd. The stability of the M(CN)₄ ²⁻ indicates that the octet rule can be effectively used to rationally design doubly charged species that are stable in the gas phase.

It is interesting to compare the relative robustness of clusters obeying the jellium and octet shell closure rules in the rational design of cluster‐assembled materials. It was discussed earlier that Al₁₃ ⁻ cluster, in spite of its being stable and chemically “inert,” coalesces when crystals of KAl₁₃ and [(CH₃)₄N⁺][Al₁₃ ⁻] are formed.

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