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, β=103.92 (5)deg, V=1176 (1) A3, Z=4, Dx=1.875 g cm-3, λ(Mo Kα)=0.71073 A, μ=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: μ-(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, β=97.81 (5)deg, V=1060 (1) A3, Z=4, Dx=1.969 g cm-3, λ(Cu Kα)=1.5418 A, μ=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. An eight-membered ring is formed between the two standard six-membered metal(III)-pyrophosphate chelate rings. The ability to distinguish facial from meridional isomers of triamminechromium pyrophosphate provides a key step in the determination of the absolute configurations of triamminechromium complexes of ADP and ATP. (orig./BHO)