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

Abstract The nickel arsenatotungstate K 10 [As 2 W 19 (H 2 O){Ni(H 2 O)} 2 O 67 ]·18H 2 O ( 1 ) has been synthesized. Due to its instability in water, attempts to obtain crystals of 1 suitable for X-ray diffraction have failed. The stabilization of the [As 2 W 19 (H 2 O){Ni(H 2 O)} 2 O 67 ] 10− core has been reached by synthesizing the analogue mixed {CsK} salt. The crystal structure of Cs 6 K 2 [Ni(H 2 O) 6 ][As 2 W 19 (H 2 O){Ni(H 2 O)} 2 O 67 ]·17H 2 O ( 2 ) has been resolved. It consists of two [α-AsW 9 O 33 ] 9− sub-units linked via a belt containing a tungsten and two nickel cations. Comparison of infrared and electronic absorption spectroscopic data for 1 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(H 2 O) 6 ] 1.5 [As 2 W 19 (H 2 O){K(H 2 O)}{Ni(H 2 O) 4 }O 67 ]·21H 2 O ( 3 ), has been isolated and characterized by elemental analysis, UV–Vis and infrared spectroscopies, and X-ray diffraction. In 3 , the two vacant sites of the [As 2 W 19 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.