The crystal structures of potassium Tris(oxalato)-chromate(III) and -aluminate(III) trihydrate: A reinvestigation

Abstract

The crystal structures of the isomorphous complexes K₃ [M(oxalate)₃],3H₂O, M = Cr^III and Al^III, have been determined from three- dimensional diffractometer intensity data by conventional Patterson and difference Fourier techniques. The crystals are monoclinic; M = Cr, a 7.714(1), b 19.687(4), c 10.361(2) Ǻ, β 108.06(3)°; M = Al, a 7.712(1), b 19.518(3), c 10.286(2) Ǻ, β 108.21(3)°; space group P2₁/c with Z = 4. Block-diagonal least-squares refinement of all non-hydrogen atoms con- verged at R 0.040, R_w 0.051 (M = Cr); R 0.048, R_w 0.056 (M = Al). The structures are comprised of discrete M(ox)₃^3- anions, K⁺ cations and water molecules. One potassium ion and one water molecule appear to be cooperatively disordered over two possible pair sites (α, β) with occupancy ratios α/Ǻ c. 3 : 1. The geometries within the M(ox)₃^3- anions are as expected with <Cr-O> 1.969(13), <Al-O> 1.896(15) Ǻ. The oxalate ligands are non-planar with <C-O(inner)> 1.284(6), 1.279(12); <C-O(outer)> 1.223(5), 1.223(10) and <C-C> 1.538(16), 1.534(21) Ǻ for the two structures.