Caesium nickel(II) trichloride, CsNiCl3, and tricaesium nickel(II) pentachloride, Cs3NiCl5

There are two congruently melting compounds in the CsCl-NiCl2 system: orange CsNiCl3 (absorption maximum 466 nm) and turquoise Cs3NiCl5 (654 nm). The different colours are caused by the different coordination polyhedra of the nickel(II) ions. In Cs3NiCl5, which crystallizes in the tetragonal space group I4/mcm, there are isolated NiCl42- tetrahedra, whereas in CsNiCl3, which crystallizes in the hexagonal space group P6(3)/mmc, the Ni2+ ions are octahedrally coordinated. These octahedra run parallel to [001] and share triangular faces.