Superatoms. Группа авторов
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Название: Superatoms
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119619567
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Figure 2.23 PES spectrum of Al4H6 anion (left panel) and mass spectra of Al4Hm−. The inset shows the geometry of Al4H6−.
Source: X. Li et al. [106]. © American Chemical Society.
Wade‐Mingos rule can also be used to rationalize the bonding in Zintl ions, which are multiply charged clusters belonging to the post‐transition metal elements in groups 13, 14, and 15. Consider, for example, Sn12 2−. While studying the semiconductor to metal transition, Wang and coworkers [59] observed a very simple photoelectron spectrum (PES) of Sn12 −, which was different from that of its isoelectronic cousin, Ge12 −. The optimized geometry of Sn12 − was found to be a distorted cage with C5v symmetry. Adding an extra electron resulted in closed shell Sn12 2− cluster with icosahedral (Ih) symmetry (Figure 2.24). The PES of Sn12 2− was found to be similar to that KSn12 −, which can be viewed as K+Sn12 2−. Named stannaspherene, Sn12 2− has a diameter of 6.1 Å, which is large enough to accommodate a metal atom. Indeed, four years prior to the above experiment Kumar and Kawazoe [82] had predicted M@Sn12 (M = Zn, Cd) to be a perfect endohedral cluster with Ih symmetry and a large HOMO–LUMO gap. Cui et al. [83] have subsequently synthesized a series of endohedral clusters with the composition M@Sn12 (M = Ti, V, Cr, Fe, Co, Ni, Cu, Y, Nb, Gd, Hf, Ta, Pt, Au). Kandalam et al. [84] showed that Mn@Sn12 is a stable cluster with a magnetic moment of 5 μB and can be regarded as a magnetic superatom. The bonding pattern of Sn12 2− was found to be similar to that of B12H12 2− where the 12 5s2 localized electron pairs in Sn12 are equivalent to replacing 12 B‐H bonds [59].
Pb12 2−, which is isoelectronic with Sn12 2−, was also found [58] to be a cage cluster whose stability can similarly be justified in terms of the Wade‐Mingos rule. Named “plumbaspherene,” Pb12 2− cage has a diameter of 6.29 Å and can accommodate a metal atom inside it. This is consistent with an earlier experiment where an extremely stable AlPb12 + cluster was synthesized in the gas phase. AlPb12 + can be viewed as Al3 +: Pb12 2− just as KSn12 − was viewed as K+: Sn12 2−. Note that a series of endohedral cage compounds, M@Pb12 2− (M = Ni, Pd, Pt), stabilized by K+ counterions, have been synthesized in solution and in crystalline form. Both of their icosahedral symmetries have been confirmed [85, 86] by X‐ray diffraction and nuclear magnetic resonance (NMR) experiments.
Figure 2.24 Optimized structure of (a) Sn12−, (b) Sn122−, and (c) KSn12−. The bond distances and cage diameters are in angstroms.
Source: Cui et al. [59]. © American Chemical Society.
Zintl compounds composed of alkali and alkaline earth cations and Zintl anions such as Sn5 2−, Pb5 2−, Pb9 4−, Sb7 3−, and Bi4 2− have been known since 1930s [19, 87]. The entry of Zintl ions into cluster science dates back to the work of Recknagel [88], Duncan [89], Castleman [90], and their coworkers. For example, the unusual stability of [Na4Bi3]+ in the mass spectra of Na n Bi m + clusters was attributed to the presence of a Bi3 3− Zintl ion. However, the stability of Zintl ions cannot be always understood in terms of the Wade‐Mingos rule. The exceptions to this rule include Ni2Sn7Bi5 3− [91], [Ge9(η4‐Ni(CO))]3− [92], and [Ge9(η4‐Pd(PPh3))]3− [93].
2.3 Stabilizing Negative Ions Using Multiple Electron‐Counting Rules
In the preceding section, we discussed how stable clusters can be designed by satisfying any one of the electron‐counting rules such as the jellium rule for free‐electron systems; the octet rule for low atomic number elements; the 18‐ and 32‐electron rule for clusters containing transition and rare earth metal atoms, respectively; the aromatic rule for organic molecules; and the Wade‐Mingos rule for boron‐based clusters and Zintl ions. These rules can also be used to design reactive clusters by requiring that they either contain less or more electrons than needed for shell closure. In particular, we discussed the design of superalkalis and superhalogens. Over the past five years, Jena and his group [94, 95] have been studying systematically how unusually stable or reactive clusters can be designed by using multiple electron‐counting rules simultaneously and how such clusters can be used to promote reactions and properties otherwise unthinkable. Consider, for example, multiply charged negative ions. We know that most atoms in the periodic table have positive electron affinity, i.e., they gain energy as an extra electron is attached. However, an atom with two extra electrons will spontaneously eject the second electron due to Coulomb repulsion. On the other hand, a cluster or a molecule in the dianionic form can be stable if they satisfy one of two criteria; either they are large enough so that the electron–electron repulsion is smaller than the binding energy or addition of electrons satisfies electronic shell closure. As we have pointed out before, B12H12 2− is a classic example of a stable dianion in the gas phase. Its stability stems from the fact that the second electron satisfies the Wade‐Mingos rule. A lot of work has been done to understand the stability of multiply charged ions. While there are examples of stable dianions, clusters carrying three or more extra electrons are rare. In the following we discuss how the use of multiple electron‐counting rules can be used to design clusters with large electron affinities as well as clusters capable of carrying three or more extra electrons.
2.3.1 Monoanions
In previous sections we have discussed the design of stable monoanions by using a single electron‐counting rule. Here, we discuss how the stability of monoanions can be further enhanced by using multiple electron‐counting rules. Recall that C6H6 has a negative electron affinity, which can be systematically increased by replacing H by F (see Table СКАЧАТЬ