K2[Hg(SO3)2]·2.25H2O.

The characteristic feature of the structure of the title compound, dipotassium bis(sulfito-κS)mercurate(II) 2.25-hydrate, is a layered arrangement parallel to (001) where each of the two independent Hg(SO(3))(2) anions are grouped into centrosymmetric pairs and are surrounded by two K(+) cations to give the overall layer composition {K(2)Hg(SO(3))(2)}(2-). The remaining cations and the uncoordinated water molecules are situated between these layers. Within the Hg(SO(3))(2) anions, the central Hg atoms are twofold coordinated by S atoms, with a mean Hg-S bond length of 2.384 (2) Å. The anions are slightly bent [174.26 (3) and 176.99 (3)°] due to intermolecular O...Hg interactions greater than 2.8 Å. All coordination polyhedra around the K(+) cations are considerably distorted, with coordination numbers ranging from six to nine. Although the H atoms of the five water molecules (one with symmetry 2) could not be located, O...O separations between 2.80 and 2.95 Å suggest a system of medium to weak O-H...O hydrogen bonds which help to consolidate the structural set-up. Differences and similarities between the bis(sulfito-κS)mercurate(II) anions in the title compound and those in the related salts (NH(4))(2)[Hg(SO(3))(2)] and Na(2)[Hg(SO(3))(2)]·H(2)O are discussed.