Stability and Structure in α- and β-Keggin Heteropolytungstates, [X<i>ⁿ</i>⁺W₁₂O₄₀]^(8-<i>ⁿ</i>^)-, X = p-Block Cation

β-[SiW12O40]4- (C3v symmetry) is sufficiently higher in energy than its α-isomer analogue that effectively complete conversion to α-[SiW12O40]4- (Td) is observed. By contrast, β- and α-[AlW12O40]5- (β- and α-1; C3v and Td, 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- (Td) moiety encapsulated within the polarizable, fixed-diameter β-W12O36 (C3v) 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-Cs4.5K0.5[AlIIIW12O40]·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-.