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Unlike the studies of dianions, work on trianions is rather scarce. One of the early studies of the trianions was due to Compton and coworkers who reported the mass spectra of (C₆₀)₂(CN)₅ ³⁻ and (C₆₀)₂(CN)₇ ³⁻ [102]. Note that observation of species in mass spectra does not necessarily mean that the trianions are stable, but simply that they exist only within experimental conditions. Many metastable multiply charged ions are observed due to repulsive Coulomb barrier. Indeed, (C₆₀)₂(CN)₅ ³⁻ is metastable against autodetachment of the third electron. Cederbaum and coworkers [103] examined the stability of a number of trianions and found that the best candidate, B(C₂CO₂)₃ ³⁻, is thermodynamically unstable against electron emission by −0.4 eV.

Following the discovery of colossal stability of the B₁₂(CN)₁₂ ²⁻, Zhao et al. [104] calculated the optimized geometry and total energy of BeB₁₁(CN)₁₂ in neutral, monoanion, dianion, and trianion form. Note that with Be replacing a B atom, an additional electron will be required to satisfy the Wade‐Mingos rule and the octet rule, simultaneously. In Figure 2.32 we show the geometry, thermal stability at 800K, and electronic structure of BeB₁₁(CN)₁₂ ³⁻, which is stable against auto‐ejection of the third electron by 2.65 eV. When CN is replaced by BO or SCN, the third electron affinities of the resulting trianions BeB₁₁(BO)₁₂ ³⁻ and BeB₁₁(SCN)₁₂ ³⁻ are, respectively, 1.30 and 0.59 eV.

Figure 2.32 BeB₁₁(CN)₁₂³⁻ (a) geometry, (b) AIMD simulation as a function of temperature and total energy fluctuation, (c) Raman and infra‐red (IR) simulation spectra, (d) natural bond orbital (NBO) charge distribution, and (e) energy diagram and frontier orbitals.

Source: Zaho et al. [104]. © John Wiley & Sons.