X-ray absorption studies of metal complexes in aqueous solution at elevated temperatures

Experimental methods for performing X-ray absorption spectroscopy (XAS) on metal complexes in dilute aqueous solutions at temperatures up to 573 K are presented. The solutions are contained in silica tubes for study by X-rays of energies greater than 16 keV. For experiments using X-rays of energies in the range 6.5–16 keV, we have developed a titanium cell with Kapton windows. These cells can be heated in an aluminium oven up to 573 K. Using these methods we have studied the tetrahedral: octahedral equilibrium for cobalt(II) in aqueous chloride solutions, Na2MoO4 · 2H2O in aqueous solution and cadmium(II) in aqueous chloride solutions. For cobalt(II) in chloride concentrations between 5 and 7 m we have found evidence that [CoCl4]2− is not the only important tetrahedral species. In molybdate(VI) solutions, [MoO4]2− is the predominant species over the 1 temperature range 298–573 K. Cadmium(II) in aqueous chloride solutions shows two major cadmium complexes [Cd(OH2)6]2+ and [CdCl4]2−. Bond distances for the principal species in these solutions are presented and are found to be in general agreement with those obtained from X-ray crystallographic studies on related solids.