Structures of the meridional and facial isomers of triamminechromium pyrophosphate dihydrate.

The meridional and facial isomers of triamminechromium pyrophosphate were separated by chromatography on Dowex-50-H+, and crystallized as isoionic species. The meridional isomer crystallized as a monomer [Cr(HP2O7)(NH3)3(H2O)].2H2O but the facial isomer crystallized as a centrosymmetric dimer of [Cr(HP2O7)(NH3)3]. 2H2O, in which the remaining water in the coordination sphere of each chromium is replaced by a phosphate oxygen from the other monomer unit. Meridional isomer: monoaquatriammine(pyrophosphato)chromium(III) dihydrate, Mr = 332.1, monoclinic, P21/c, a = 7.825 (2), b = 10.107 (3), c = 15.322 (5) A, beta = 103.92 (5) degrees, V = 1176 (1) A 3, Z = 4, Dx = 1.875 g cm-3 lambda(Mo K alpha) = 0.71073 A, mu = 12.6 cm-1, F(000) = 684, final R = 0.050 for 1828 reflections. The most notable difference between this compound and the corresponding tetraammine complex reported previously is a significant shortening of the metal-ligand bond lengths not only for the water ligand but also for the ammonia ligands. The dimer: mu-(pyrophosphato-O,O',O")-bis[triamminechromium(III)] dihydrate, Mr = 314.0, monoclinic, P21/c, a = 8.695 (2), b = 10.327 (3), c = 11.913 (4) A, beta = 97.81 (5) degrees, V = 1060 (1) A3, Z = 4, Dx = 1.969 g cm-3, lambda(Cu K alpha) = 1.5418 A, mu = 125.7 cm-1, F(000) = 644, final R = 0.047 for 1389 reflections. This structure, which sits on a center of inversion, forms a tricyclic complex involving two Cr atoms. The structure is characterized by reciprocal coordination between the metal ions and anionic O atoms of the pyrophosphate moieties.(ABSTRACT TRUNCATED AT 250 WORDS)