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The counterions of polyoxometalates (POMs) impact properties and applications of this growing class of inorganic clusters. Here, we used density functional theory (DFT) to elucidate the impact of fully hydrated alkali metal cations on the geometry, electronic structure, and chemical properties of the polyoxotungstate anion [PW 12 O 40 ] 3- . The calculations show that the HOMO of the free anion [PW 12 O 40 ] 3- is a linear combination of the 2p AOs of the bridging oxygens, and the first few LUMOs are the 5d orbitals of the tungsten atoms. The S 0 → S 1 electron excitation, near 3 eV, is associated with the O(2p) → W(5d) transition. Anion/cation complexation leads to formation of [PW 12 O 40 ] 3- [M + (H 2 O) 16 ] 3 ion-pair complexes, where with the increase of atomic number of M, the M + (H 2 O) 16 cluster releases several water molecules and interacts strongly with the polyoxometalate anion. For M = Li, Na and K, [PW 12 O 40 ] 3- [M + (H 2 O) 16 ] 3 is characterized as a "hydrated" ion-pair complex. However, for M = Rb and Cs, it is a "contact" ion-pair complex, where the strong anion-cation interaction makes it a better electron acceptor than the "hydrated" ion-pair complexes. Remarkably, the electronic excitations in the visible part of the absorption spectrum of these complexes are predominantly solvent-to-POM charge transfer transitions (i.e. intermolecular CT). The ratio of the number of intermolecular charge transfer transitions to the number of O(2p)-to-W(5d) valence (i.e. intramolecular) transitions increases with the increasing atomic number of the alkali metals.

Several polyoxometalate-supported metal carbonyl complexes, K7Na3P2W23O80{Re(CO)3}2·38H2O (1), (C3H10N)8Na2P2W23O80{Re(CO)3}2·10H2O (1a) and (C3H10N)6KNa3P2W23O80{Mn(CO)3}2·7H2O (2), have been prepared from the dimerization of the monovacant Keggin [α-PW11O39](7-) with metal carbonyl complexes [M(CO)3](+) (M = Re, Mn) in acidic aqueous solutions. The resulting "twisted-sandwich" architectures are chiral, but their crystalline solids are racemic. A detailed investigation involving syntheses, crystal structures, and electrochemistry is presented.

A new pentanickel silicotungstate complex, K(10)H(2)[Ni(5)(OH)(6)(OH(2))(3)(Si(2)W(18)O(66))]·34H(2)O (KH-), has been synthesized and characterized by X-ray crystallography and several other methods. Dynamic light scattering, kinetics and other experiments confirm that in the presence of [Ru(bpy)(3)](2+) (the photosensitizer for light-driven water oxidations) and [Ru(bpy)(3)](3+) (the oxidant in the dark water oxidations) exists in an equilibrium between solution (soluble) and a [Ru(bpy)(3)](n+)- complex (minimally soluble) form. This new pentanickel polyoxometalate catalyzes efficient water oxidation in both the dark and on irradiation with 455 nm LED light with 1.0 mM [Ru(bpy)(3)](2+) photosensitizer and 5.0 mM Na(2)S(2)O(8), sacrificial electron acceptor. Four lines of evidence indicate that in this solution [symbol:see text] Ru(bpy)(3)](n+)- complex equilibrium remains molecular and does not decompose to nickel hydroxide particles.

A dodecazinc silicotungstate K(20)Na(2)[Zn(6)(OH)(7)(H(2)O)(Si(2)W(18)O(66))](2)·34H(2)O (1) has been synthesized and characterized by X-ray crystallography, elemental analysis, infrared, UV-vis spectroscopy, cyclic voltammetry, acid-base titration, and DFT calculations. The twelve zinc atoms between the two [Si(2)W(18)O(66)](16-) frameworks make this complex more stable hydrolytically than the heteropolytungstate ligands, [Si(2)W(18)O(66)](16-), themselves. The structurally unique central Zn(12) core is formed by the fusion of two [Zn(6)(OH)(7)(H(2)O)](5+) units through two edge-sharing Zn6 atoms. DFT B3LYP calculations give HOMO-LUMO and (HOMO - 1)-LUMO energy gaps of ∼3.65 and 3.91 eV, respectively, as compared to the band gap in ZnO of 3.35 eV.

A new polyoxometalate of earth adundant elements [{Co(4)(μ-OH)(H(2)O)(3)}(Si(2)W(19)O(70))](11-) has been synthesized, characterized and shown to be a water oxidation catalyst. The initial catalytic complex is unstable and slowly undergoes hydrolysis. The hydrolysis products have been isolated and characterized, and their catalytic water oxidation activity is assessed.

A new structurally distinct dimeric silicotungstate K9Na,Cu0.5[gamma-Cu2(H2O)SiWO31]2.38H2O (1) has been synthesized and characterized by infrared spectroscopy, elemental analysis, and variable-temperature magnetic measurements. Blue needle-like crystals of 1 were obtained by reaction of K8[gamma-SiW10O36] with 2 equivalents of Cu(n) in a 0.5 M sodium acetate solution (pH 4.2) and subsequent addition of an equal volume of ethylene glycol. The structure of 1 was determined by single-crystal X-ray diffraction: final R1 = 3.41% based on 9709 independent reflections. The structure consists of two [gamma-Cu2(H2O)SiWsO3,1]6- Keggin-like units with the [Cu2O(H2O)(micro3-O)(micro2-O)] moiety of one unit bonded to the top of a [W2O6] moiety of the other unit. Magnetic susceptibility measurements indicate competing ferro- and antiferromagnetic intramolecular coupling between the four S = 1/2 Cu(II) centers in the cluster anion.

A new monomeric diiron beta-Keggin derivative, [(CH3)2NH2]5[beta-SiFe2W10O36(OH)2(H2O)Cl].7H2O (1) with non-adjacent substituted sites has been obtained in good yield by reaction of Fe(III) cations with [gamma-SiW10O36]8- in aqueous solution. The use of hydrogen bonding counter-cations produced a disorder-free polyanion. This situation facilitates unequivocal identification of the terminal ligands on Fe atoms and interpretation of the magnetic properties, which are also addressed by DFT calculations. Electrochemical studies establish that 1 can be step-wise reduced by two electrons and then reversibly oxidized.

The tri-tungsten-vacant polyoxometalate, [alpha-AsW15(OH)4O52]13-, derived from the semi-vacant Wells-Dawson complex [alpha-AsW18(OH)4O58]7-, reacts with the late-transition metal cations, Cu(II) or Zn(II), to form sandwich-type species; the X-ray crystal structure of [alphabetabetaalpha]-(Cu(II)OH2)2(Cu(II))2(AsW15(OH2)3(OH)O52)2]12-, prepared by the acidification of [alphabetabetaalpha]-(Cu(II)OH2)2(Cu(II))2(AsW15(OH)4O52)2]18-, reveals that the missing heteroatoms are distal to the central Cu4 unit and the vertices of the vacant tetrahedron are occupied by one OH- and three OH2 groups.