Crystal structure at 100 K of bis-[1,2-bis-(di-phenyl-phosphan-yl)ethane]-nickel(II) bis-(tri-fluoro-methane-sulfonate): a possible negative thermal expansion mol-ecular material

In the title salt, [Ni(C(26)H(24)P(2))(2)](CF(3)SO(3))(2) or [Ni(dppe)(2)](2+)·(OTf(-))(2) [dppe = 1,2-bis-(di-phenyl-phosphan-yl)ethane and OTf(-) = tri-fluoro-methane-sulfonate], the Ni atom (site symmetry ) has a square-planar geometry with the bidentate ligands chelating the metal. As a result of the steric hindrance of the phenyl rings, the counter-ions are blocked from the metal coordination sphere. The dynamic disorder of the anion existing at 296 K is reduced at 100 K and based on these two temperatures, negative thermal expansion behaviour is observed.