Synthesis and X-ray characterization of nickel(II) arsenatotungstate complexes:: isolation of an intermediate leading to the formation of a sandwich-type polyoxometalate

The nickel arsenatotungstate K-10[As2W19(H2O){Ni(H2O)}(2)O-67].18H(2)O (1) has been synthesized. Due to its instability in water, attempts to obtain crystals of I suitable for X-ray diffraction have failed. The stabilization of the [As2W19(H2O)Ni(H2O)(2)O-67](10-) core has been reached by synthesizing the analogue mixed {CsK} salt. The crystal structure of Cs6K2[Ni(H2O)(6)][As2W19(H2O) {Ni(H2O)O(67.)17H(2)O (2) has been resolved. It consists of two [alpha-AsW9O33](9-) sub-units linked via a belt containing a tungsten and two nickel cations. Comparison of infrared and electronic absorption spectroscopic data for I and 2 has confirmed the structure proposed for 1. The instability of 1 led us to investigate the behavior of 1 in water, UV-Vis spectroscopy revealed that the formation of this complex is a multi-step reaction. An intermediate, the complex K-8[Ni(H2O)(6)](1.5)[As2W19(H2O) {K(H2O) (Ni(H2O)(4})O-67].21H(2)O (3), has been isolated and characterized by elemental analysis, V-Vis and infrared spectroscopies, and X-ray diffraction. In 3, the two vacant sites of the [AS(2W19)O(67)](14-) anion are occupied by a nickel and a potassium, forming a {WNiK}belt. It follows that the stability of 2 in water is due to the large ionic radius of Cs+, which prevents the inclusion of the alkaline cation into the cavity of the [AS(2)W(19)O(67)](14-) anion. The complex 3 represents a unique example of a fully characterized intermediate leading to the formation of a sandwich-type polyoxometalate. (C) 2002 Elsevier Science B.V. All rights reserved.