The class of polyoxometalates (POMs) is composed of edgeand corner-shared {MO 6 } octahedra with early-transitionmetal ions in high oxidation states (e.g. W VI , V V ). [1] POMs are discrete molecular species which have gained increasing attention over the last 30 years, largely owing to a unique combination of their properties. In addition to the enormous structural and compositional variety, which is unmatched in inorganic chemistry, POMs can be tuned in solubility, redox activity, color, thermal stability, charge density, and so on. As a result, POMs exhibit potential applications in many different and diverse areas such as catalysis, magnetism, bio-and nanotechnology, and medical and materials sciences. [2] Polyoxovanadates (POVs) are a subclass of POMs, and they also exhibit a rich structural variety, [1,2] largely because the vanadium center can adopt variable coordination geometries, including octahedral, square-pyramidal, and tetrahedral. Furthermore, the reduction of vanadium from the + V to the + IV oxidation state is facile, and as a result, mixedvalence POVs can be formed. [3] For this reason, POVs are very interesting for redox applications in catalysis and materials science. [4] Although POVs cover a large range of size, shape, and composition, they have not been investigated as much as polyoxotungstates and -molybdates. [2] In particular, Müllers group has been quite active in this area, reporting several reduced and mixed-valence heteropolyoxovanadates. [5] These authors isolated some interesting clathrate-type POVs containing a variety of heteroatomic groups such as Cl À , N 3 À , HCOO À , and NO 2 À , thus demonstrating that the shape of the anionic guest predetermines the overall shape and size of the resulting POV.
The area of isopolyvanadates is dominated by the famous decavanadate ion [H x V 10 O 28 ] (6Àx)À (V 10 ). [6] It is well established that this POM can also be isolated in mono-, di-, tri-, and tetraprotonated forms. [7] However, incorporation of other transition metals besides vanadium into this structural type has not been reported yet. In particular, substitution of one or more vanadium atoms by late 4d or 5d transition metals in high oxidation states (e.g. Pt IV , Pd IV ) would result in mixedmetal derivatives with highly promising catalytic properties.
To date, only a few Pt-containing POMs are known, and interestingly all of them are polyoxotungstates and -molybdates with the Anderson-Evans or Keggin structure. In 1983 Lee et al. reported on [H 3 Pt IV W 6 O 24 ] 5À , [8] and in 1984 Lee and Sasaki described the a and b isomers of the molybdenum analogue [Pt IV Mo 6 O 24 ] 8À . [9] During the last two decades the Lee group has investigated both polyoxomolybdate and -tungstate [H x Pt IV M 6 O 24 ] nÀ (M = Mo, W) systems in much detail, reporting numerous derivatives with different degrees of protonation. [10,11] In 2003 the Lee group also reported on the Pt IV -containing Keggin ion [a-SiPt IV 2 W 10 O 40 ] 8À , which represents the first late-transition-metal oxo complex. [12] In 2004 Hill and coworkers reported the Pt IV -containing Knoth-type tungstophosphate dimer [O = Pt IV (H 2 O)(PW 9 O 34 ) 2 ] 16À , but no 183 W or 195 Pt NMR spectra were shown. [13] Already in 1997 Liu et al. reported on a Pt IV -containing Wells-Dawson ion with the formula [a 2 -P 2 W 17 Pt(OH 2 )O 61 ] 6À , but this formulation is not supported by the shown 31 P and 183 W NMR spectra. [14]