cis-Bis(bipyridyl)dichloroiron(III) tetrachloroferrate(III), [Fe(bpy)2Cl2][FeCl4]; structure at 4.2 and at 115 K by neutron diffraction

The crystal structure of deuterated cis-[Fesup(III)(bpy)2Cl2] [Fesup(III)Cl4] has been determined by single-crystal neutron diffraction methods at 4.2 K. Crystals are orthorhombic, Pccn. At 4.2 K a = 14.97(1), b = 12.21(1), c = 13.26(2) A. Refinement of all 2895 measured unique reflections with anisotropic thermal motion [220 parameters (nv)] converged to R(F2) = 0.050, chi = 2.6. In addition, 869 reflections were measured at 115 K on the deuterated crystal and 426 reflections at 4.2 K on a hydrogenous crystal. These sets have low resolution in b and the refinements were constrained accordingly. These converged to R(F2) = 0.060, chi = 1.4, nv = 195 and R(F2) = 0.062, chi = 3.8, nv = 145. From these results the geometry and the temperature and isotopic dependence of U11, U33 and U13 were obtained. The crystal structure closely resembles that determined by X-ray diffraction at 295 K, but with precision better by a factor of three. This improvement allows a small distortion in the bipyridyl ligand to be defined. The thermal motion of the H and D atoms clearly shows the effect of zero-point intramolecular motion at 4.2 K. With that correction, the thermal parameters increase fairly uniformly over the atoms by a factor of three between 4.2 and 115 K, corresponding to a crystal Debye temperature of ca 115 K. (Auth.)