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

beta-[SiW12O40](4-) (C-3nu symmetry) is sufficiently higher in energy than its alpha-isomer analogue that effectively complete conversion to alpha-[SiW12O40](4-) (T-d) is observed. By contrast, beta- and alpha-[AlW12O40](5-) (beta- and alpha-1; C-3nu and T-d, respectively) are sufficiently close in energy that both isomers are readily seen in Al-27 NMR spectra of equilibrated (alpha-beta) mixtures, Recently published DFT calculations ascribe the stability of beta-1 to an electronic effect of the large, electron-donating [AlO4](5-) (T-d) moiety encapsulated within the polarizable, fixed-diameter beta-W12O36 (C-3nu) shell. Hence, no unique structural distortion of beta-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 beta-1 (beta-Cs4.5K0.5[(AlW12O40)-W-III].7.5H(2)O; orthorhombic, space group Pmc2(1), a = 16.0441(10) Angstrom, b = 13.2270(8) Angstrom, c = 20.5919(13) Angstrom, Z = 4 (T = 100(2) K)) with previously reported structures of alpha-1, alpha- and beta-[SiW12O40](4-), and beta(1)-[SiMoW11O40](4-).