This paper is a review of the preparation and structures of the polymercury cations and , the mercury chain compounds, Hg 3-δ AsF 6 , Hg 3-δ SbF 6 , Hg 3-δ NbF 6 and Hg 3_δ TaF 6 , and the mercury layer compounds Hg 3 NbF 6 and Hg 3 TaF 6 . The chain compounds Hg 3-δ MF 6 are hard, golden yellow insoluble crystals with a shiny metallic appearance that have linear chains of mercury atoms parallel to the
aand baxes of a tetragonal cell. They are non-stoichiometric because the Hg-Hg distance is not com mensurate with the tetragonal lattice of octahedra. The layer compounds have a silver colour with a distinct metallic lustre, but they are quite soft. They have sheets of hexagonally close-packed mercury atoms separated by a layer of or octahedra. In contrast to the chain compounds the layer compounds are stoichiometric. The interconversion of the chain and layer compounds is described and discussed.