Crystal structure and optical spectroscopy of Er2[Pt(CN)4]3.21H2O and Er2[Pt(CN)4]2.SO4.11.5H2O.

We have synthesized two forms of erbium tetracyanoplatinates, Er2[Pt(CN)4]3·21H2O (red form) and Er2[Pt(CN)4]2·SO4·11.5H2O (yellow form), and determined their crystal structures by X-ray diffraction. While the red form crystallizes in the orthorhombic space group Pbcn, with a = 15.4848(3) A, b = 13.8186(2) A, c = 19.07820(10) A, α = β = γ = 90°, and Z = 4, the yellow form precipitates in the tetragonal space group I4cm, with a = b = 14.321(2) A, c = 13.338(3) A, α = β = γ = 90°, and Z = 4. Both forms show [Pt(CN)4]2- chains but differ markedly in color and morphology. This is due to the incorporation of sulfate ions in the latter modification, leading to an increased Pt−Pt distance. The observed optical absorption and emission behavior of the title compounds is correlated with the Pt−Pt distances.