Crystal Structure of [Cu(en)₂][Hg(SCN)₄]

Abstract

CRYSTALS of the compound [Cu(en)₂][Hg(SCN)₄] were kindly supplied by the Chemistry Department of Birkbeck College and an X-ray investigation was carried out to determine its complete crystal structure. The complex Co[Hg(CNS)₄] has already been subject to X-ray investigation¹, showing that as these crystals were tetragonal with space group I&4bar; there was good evidence that the thiocyanate groups would be tetrahedrally situated about the mercury atom. A study of the copper di-ethylenediamine complex has revealed that although the sulphurs of the thiocyanates were still tetrahedrally co-ordinated to the mercury atom, the presence of the Cu(en)₂ was now sufficient to destroy the tetrahedral arrangement of the remainder of the thiocyanate groups with respect to the mercury atom, thus indicating that there is a flexibility of the —SCN group arising out of a rotation of the group about the Hg—S bond; this flexibility depends on the arrangement of the neighbouring groups. In cases where the surroundings of the Hg(SCN)₄ are symmetrically situated about it, as in the Co[Hg(SCN)₄] complex, the thiocyanate groups can take positions compatible with the external symmetry of the crystal. On the other hand, if there is no symmetry around the Hg(SCN)₄ complex, this being the case of the [Cu(en)₂][Hg(SCN)₄], the thiocyanates take up unsymmetrical positions satisfying the structural features of geometrical packing and charge neutralization. This may explain why these crystals occur in a system of lower symmetry.