The structure and absolute configuration of( - )₅₄₆-cis,trans,cis-bis(diaminopropionato)cobalt(III) bromide, [Co(C₃H₇N₂O₂)₂]Br

One of the two cis isomers of bis(L-diaminopropionato)cobalt(III) bromide, [Co(C₃H₇N₂O₂)₂]Br, was examined by X-ray methods. The red crystals belong to space group D₂³-P2¹2¹2 of the orthorhombic system (a = 11.76 ± 0.02, b = 7.49 ± 0.02, c = 5.91 ± 0.02 Å). The density of 2.21 g/cm³ calculated for two formula weights per unit cell agrees well with the measured value of 2.19 ± 0.01 g/cm³. X-Ray data were collected by the equiinclination Weissenberg method, and intensities of 450 independent reflections were estimated visually. The structure was refined by least-squares methods to a conventional R factor of 0.070. The absolute configuration of the complex was determined by least-squares refinements of the two mirror image structures. The complex is monomeric and contains two ligands per complex. Molecular symmetry 2 is imposed on the cation by crystallographic requirements. Coordination around the central Co atom is that of a distorted octahedron. The carboxyl groups are cis to each other as are the terminal amine groups. The α-amino groups occupy trans positions. The C₂ axis helicity of the ethylenediamine part of the molecule is while the helicity of the aamino acid ring is Δ.