Stability and structure in α- and β-Keggin heteropolytungstates, [Xⁿ⁺W₁₂O₄₀]^(8-n)-, X = p-block cation

β-[SiW₁₂O₄₀]^4- (C_3v symmetry) is sufficiently higher in energy than its α-isomer analogue that effectively complete conversion to α-[SiW₁₂O₄₀]^4- (T_d) is observed. By contrast, β- and α-[AIW₁₂O₄₀]^5- (β- and α-1; C_3v and T_d1 respectively) are sufficiently close in energy that both isomers are readily seen in ²⁷Al NMR spectra of equilibrated (α-β) mixtures. Recently published DFT calculations ascribe the stability of β-1 to an electronic effect of the large, electron-donating [AlO₄]^5- (T_d) moiety encapsulated within the polarizable, fixed-diameter β-W₁₂O₃₆ (C_3v) shell. Hence, no unique structural distortion of β-1 is needed or invoked to explain its unprecedented stability. The results of these DFT calculations are confirmed by detailed comparison of the x-ray crystal structure of β-1 (β-Cs_4.5K_0.5[Al^IIIW₁₂O ₄₀]·7.5H₂O; orthorhombic, space group Pmc2₁, a = 16.0441(10) Å, b = 13.2270(8) Å, c = 20.5919(13) Å, Z = 4 (T = 100(2) K)) with previously reported structures of α-1, α- and β-[SiW₁₂O₄₀]^4-, and β₁-[SiMoW₁₁O₄₀]^4-.