From molecular to two-dimensional Anderson polyoxomolybdate:: Synthesis, crystal structure, and thermal behavior of [{Ni(H2O)4}2{Ni(OH)6Mo6O18}]•4H2O and [Ni(H2O)6][Ag2{Ni(OH)6Mo6O18}]•8H2O

Two 2D Anderson polyoxomolybdate-based compounds, [{Ni(H2O)(4)}(2){Ni(OH)(6)Mo6O18}]center dot 4H(2)O (1) and [Ni(H2O)(6)][Ag-2{Ni(OH)(6)Mo6O18}]center dot 8H(2)O (2), were synthesized and characterized by elemental analysis, IR spectroscopy, SEM, and single-crystal XRD. Their thermal behavior was studied by TGA, DSC, XRTD, and IR spectroscopy. Both 1 and 2 are arranged in 2D networks. In 1, [Ni(OH)(6)Mo6O18](4-) Anderson units are covalently linked to each other by two different types of Ni2+, whereas in 2, the corresponding interactions are ensured exclusively by Ag+ ions. In addition, all the Ni2+ cations in 2 are located in interplanar positions and act selectively as counterions. Upon the addition of silver, the thermal stability of the Anderson anion is increased from 170 degrees C for pure nickel compound 1 up to 220 degrees C for mixed silver-nickel 2. Below 220 degrees C, 2 exhibits reversible hydration-dehydration processes with preservation of the Anderson anion structure. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).