Superatoms. Группа авторов
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Название: Superatoms
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119619567
isbn:
Figure 2.25 Ground state geometries of neutral and anionic C6H6 − x (BO2) x . The gray, white, pink, and red spheres correspond to carbon, hydrogen, boron, and oxygen, respectively.
Source: Driver [73].
Figure 2.26 Ground state geometries of neutral and anionic C5BH6 − x (BO2) x . The gray, white, pink, and red spheres correspond to carbon, hydrogen, boron, and oxygen atoms, respectively.
Another example where the use of both octet rule and aromaticity rule can simultaneously give rise to stable monoanions is BC5H6 − x (CN) x , x = 1–6. Here, the H atoms in C6H6 are sequentially replaced by superhalogen CN. The electron affinities of these clusters calculated using density functional theory and B3LYP hybrid exchange‐correlation potential with 6‐31 + G* basis set implemented in the Gaussian 16 code are given in Figure 2.27. Note that electron affinity of BC5(CN)6 is close to 6 eV, making it a hyperhaolgen. It is interesting to compare the electron affinities of these clusters with isoelectronic C5H5 − x (CN) x , x = 1–5. The results are compared with the electron affinities BC5H6 − x (CN) x in Figure 2.27. As the number of CN ligands increases, the electron affinities of both these two classes of clusters become identical, indicating that the ligands as well as total number of “valence” electrons play a role in stabilizing the monoanions.
Figure 2.27 Electron affinities of BC5H6 − x (CN) x , x = 1–6 and their isoelectronic C5H5 − x (CN) x , x = 1–5.
Unusually stable clusters can also be created by satisfying three electron‐counting rules, simultaneously. Consider, for example, Mn[BC5(CN)6]2 − that satisfies the aromaticity rule, the octet rule, and the 18‐electron rule, simultaneously. The geometry of neutral and anionic Mn[BC5(CN)6]2 − cluster computed by Giri et al. [94] is shown in Figure 2.28. Here, Mn is sandwiched between two BC5(CN)6 molecules. Another consequence of the electron shell closure is that the magnetic moment of the Mn atom is quenched. The octet shell closure enables CN to have an electron affinity of 3.86 eV. The electron affinity of BC5(CN)6, which satisfies both the octet rule and aromaticity rule is 5.87 eV. Mn[BC5(CN)6]2, satisfying three electron‐counting rules simultaneously, has an electron affinity of 6.40 eV. This provides a recipe for designing highly stable negative ions.
Figure 2.28 Equilibrium geometries of (a) neutral and (b) anionic Mn[BC5(CN)6]2 cluster.
Source: Adapted with permission from Giri et al. [94]. © Royal Society of Chemistry.
2.3.2 Dianions
Unusual stability of Mn[BC5(CN)6]2 − suggests that if Mn is replaced by Cr, one extra electron will be needed to satisfy the octet, aromaticity, and 18‐electron rule, simultaneously. Thus, Cr[BC5(CN)6]2 2− should be a stable cluster. Giri et al. [94] calculated the second electron affinity of Cr[BC5(CN)6]2 2− to be 2.58 eV, which is substantially larger than the 0.9 eV value of B12H12 2−. Molecular dynamics simulation of Cr[BC5(CN)6]2 2− at 300 K confirmed that it is thermally stable. From the snapshots of the geometries taken at different time intervals in Figure 2.29, we see that there is very little change in its geometry.
Another example of an ultra‐stable dianion designed by satisfying multiple electron‐counting rules is B12(CN)12 2− where the H atoms of B12H12 2− are replaced by CN molecules. Recall that CN needs an extra electron to satisfy the octet rule. Thus, B12(CN)12 2− satisfies both the octet and Wade‐Mingos rules. Zhao et al. [99] found the optimized geometry of B12(CN)12 2− to be a perfect icosahedron (see Figure 2.30) with the second electron bound by 5.3 eV. This colossal stability of B12(CN)12 2− has been recently verified experimentally by Mayer et al. [100] and the measured electron affinity of 5.55 eV agrees well with predicted value. To date, B12(CN)12 2− is the most stable dianion known in the gas phase.
Figure 2.29 Molecular dynamics simulation of Cr[BC5(CN)6]22− cluster.
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