~ 4MeCN 3 were prepared. Compound 2 contains the two-electron reduced [ H PMO,,O,]~-ion having the a-Keggin structure, 3 the four-electron reduced [H,S,MO,,O,]~-ion having the CC-Dawson structure. Compounds 1-3 were characterised by means of IR and NMR spectroscopy and X-ray diffraction. The structures of 1 and 2 are disordered in space group P1. The heteropolyanions show apparent T,, symmetry; their actual symmetry is T. Upon reduction the M o Mo distances increase slightly. The strongly alternating 'short' (mean 1.814 A) and 'long' (1.990 A) Mo-0-Mo bonds in the unreduced compound 1 become more equal in 2. The structural changes upon reduction observed in 3 were analysed in detail. The most significant alterations are an increase of the M o M o distances between corner-sharing MOO, octahedra in the equatorial belt by 0.066 A, and a decrease of the Mo-0-Mo bond lengths connecting the two halves of the anion. The alternation of the Mo-O(bridge) bonds is nearly evened out after reduction. All structural changes observed in 2 and 3 are consistent with the description that the additional electrons occupy delocalised molecular orbitals extending over the Mo-0, framework.Reduced heteropolyanions ('heteropoly blues') are of continuing interest in important areas of chemistry like analytical and medical applications, catalysis, electron transfer and delocalisation of electrons in mixed-valence compounds. Despite significant evidence obtained by electrochemical studies, and by UV, ESR and multinuclear NMR spectroscopy, structural data are still scarce. Pope and co-workers5'6 reported the structure of four-electron reduced P-[H,PMo, 2040]4-, Miiller et al. ' the structures of four-electron reduced P-[H,AsMo,,-

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Die Oxo‐sulfato‐vanadate(V)V2O3(SO4)4, K[VO(SO4)2] und NH4[VO(SO4)2] wurden dargestellt und ihre Strukturen röntgenographisch ermittelt. Die Struktur von V2O3(SO4)2 besteht aus einem dreidimensionalen Netzwerk von SO4‐Tetraedern und Paaren eckenverknüpfter VO6‐Oktaeder. Die Sauerstoffatome der Sulfationen sind an vier verschiedene Vanadinatome koordiniert. K[VO(SO4)2] und NH4[VO(SO4)2] sind isostrukturell. VO6‐Oktaeder, mit einem endständigen Sauerstoffatom, und Paare von SO4‐Tetraedern sind über Ecken zu Ketten verknüpft. Diese bilden über weitere gemeinsame Ecken Schichten. Die Sulfationen sind in Richtung planarer SO3‐ Moleküle und eines Sauerstoffatoms, das an Vanadium gebunden ist, verzerrt. Dies gibt eine Erklärung für den Mechanismus der bei 400°C reversiblen Reaktion K[VO(SO4)2] ⇄ K[VO2(SO4)] + SO3. Die Ramanspektren der genannten Verbindungen wurden gemessen und zugeordnet. Kristalldaten: V2O3(SO4)2, monoklin, Raumgruppe P21/a, a = 947,2(4), b = 891,3(3), c = 989,1(4)pm, β = 104,56(3)°, Z = 4, 878 unabhängige Reflexe, R(Rw) = 0,039(0,033); K[VO(SO4)2], orthorhombisch, Raumgruppe P212121, a = 495,3(2), b = 869,6(9), c = 1627(1) pm, Z = 4, 642 unabhängige Reflexe, R(RW) = 0,11(0,10); NH4[VO(SO4)2], orthorhombisch, Raumgruppe P212121, a = 495,3(1), b = 870,0(2), c = 1676,7(4) pm, Z = 4, 768 unabhängige Reflexe, R(Rw) = 0, 088(0,083).