Complex clusters the size of small proteins can be generated in reduced molybdate solutions by linking defined fragments. An example of this is the giant cluster (NH4)25±5[Mo154(NO)14O420(OH)28(H2O)70]·ca. 350 H2O with a molar mass of more than 30000 gmol−1, the anion of which is shaped like a tire with a nanodimensional cavity more than 20 Å in diameter and has an extremely large inner and outer surface as well as a large number of mobile electrons.

The compounds Cs12[VIV 18O42(H2O)]·14H2O (1a), K12[VIV 18O42(H2O)]·16H2O (1b), Rb12[VIV 18O42(H2O)]·19H2O (1c), K9[H3VIV 18O42(H2O)]·14H2O·4N2H4 (1d), K11[H2VIV 18O42(Cl)]·13H2O·2N2H4 (2a), K9[H4VIV 18O42(Br)]·14H2O·4N2H4 (2b), K9[H4VIV 18O42(I)]·14H2O·4N2H4 (2c), K10[H3VIV 18O42(Br)]·13H2O·0.5N2H4 (2d), K9[H4VIV 18O42(NO2)]·14 H2O·4N2H4 (3), Cs11[H2VIV 18O42(SH)]·12H2O (4), K10[HVIV 16VV 2O42(Cl)]·16H2O (5a), Cs9[H2VIV 16VV 2O42(Br)]·12H2O (5b), K10[HVIV 16VV 2O42(Br)]·16H2O (5c), Cs9[H2VIV 16VV 2O42(I)]·12H2O (5d), K10[HVIV 16VV 2O42(I)]·16H2O (5e), K10[HVIV 16VV 2O42(HCOO)]·15H2O (6), (NEt4)5[VIV 10VV 8O42(I)] (7), and Na6[H7VIV 16VV 2O42(VO4)]·21H2O (8) have been characterized by elemental analyses, redox titrations, IR and EPR spectroscopy, and magnetic susceptibility measurements, as well as by single crystal X-ray structure analyses. The anions containas a common structural featurean approximately spherical {V18O42} shell, which can act as a unique host system for neutral and anionic guests. Remarkably, the electron population of these clusters candepending on the specific reaction conditionsbe varied in a wide range between 10 and 18 3d electrons without changing the overall structure. The related magnetic properties in connection with the phenomenon of electron-dependent spin organization on a spherical cluster shell have been discussed in detail.

“This is a substance or group of substances about which there has been much discussion” is a statement we read in an older standard textbook of inorganic chemistry with reference to a (usually) amorphous blue material that was first mentioned by C. W. Scheele in 1778 and discussed by J. J. Berzelius in 1826, and that today is known as “molybdenum blue”. Now based on the knowledge of compound 1 with an archetypal structure, the more than 200 year old mystery regarding the nature of this multifunctional material with its interesting properties has been partly unveiled.

The compounds (NMe4)5[As2Mo8V4AsO40] · 3 H2O 2a, (NH4)21[H3Mo57V6(NO)6O183(H2O)18] · 65 H2O 3a, (NH2Me2)18(NH4)6[Mo57V6(NO)6O183(H2O)18] · 14 H2O 3b and (NH4)12[Mo36(NO)4O108(H2O)16] · 33 H2O 4a (3a and 4a were not correctly reported in the literature regarding to their composition, structures and the oxidation states of the metal centres) which contain large isolated anionic species, have been prepared (among them 3a, 3b, and 4a in rather high yield) and characterized by complete crystal structure analysis as well as IR/Raman, UV/VIS/NIR, ESR spectroscopy and magnetic susceptibility measurements, redox titrations, bond valence sum calculations, elemental analyses and thermogravimetric studies. Perspectives for polyoxometalate chemistry referring to the synthesis of “extremely” large nanoscaled species are discussed, together with the occurrence of a large transferable {Mo17} building block in the compounds 3a, 3b and 4a which also exists in the corresponding iron compound Na3(NH4)12[H15Mo57Fe6(NO)6O183(H2O)18] · 76 H2O 7a.

processes of the cluster anions. In this context it is remarkable that the trinuclear units can even occur in a quasiisolated state, as in the anions of 1 and 4. The compounds were characterized by IR, UV/Vis/NIR and EPR spectroscopy as well as by magnetochemical investigations and single-crystal X-ray structure determinations. Susceptibility data are especially useful for identifying the trinuclear units as well as the nature of their magnetic coupling with the rest of the structure.