X-ray and neutron diffraction studies of the hydration of SeO^2-₄ and CrO^2-₄ anions by isomorphic substitution

Abstract

The structure in aqueous solutions of the hydrated selenate and chromated ions SeO^2- ₄ and CrO^2- ₄ have been derived from X-ray and neutron diffraction measurements using CrO^2- ₄ as an isomorphous substitute for SeO^2- ₄. The Se(Cr)-O bond lengths are 1·63 ± 0·02 Å and do not differ from those found in crystal structure.

A well defined hydration shell of about 12 water molecules surrounds the XO^2- ₄ ions at a Se(Cr)-H₂O distance of 3·96 Å. The diffraction results indicate a geometrical orientation of the water molecule around the tetrahedral anions very similar to that one found in the case of simple anions such as Cl^-, with Se(Cr)-D distances of 3·25 and 4·4 Å and with a Se(Cr)-O distance of 3·96 Å.