The crystal structures of the diastereoisomers formed between the hydrogen-D-tartrate anion and the cations (+)-(Λ-[Co(en)₂(NO₂)₂]⁺ (1) and (-)-(Δ)-[Co(en)₂(NO₂)₂]⁺ (2) have been determined by three-dimensional X-ray analysis. The crystal structures are comprised of octahedrally coordinated cobalt atoms, hydrogentartrate anions and water molecules interconnected by a complex hydrogen bonding network. In (1), columns of complex parallel to a 2₁ screw axis along a, are linked via hydrogen bonding contacts to a total of six chains of 'head-to-tail' hydrogentartrate strands. In contrast, in (2) the chains of hydrogentartrate anions associate with each other to form well defined 'walls' which sandwich hydrogen-bonded columns of complex cations such that the structure may be thought of as a layer structure of hydrogentartrate anions and complex cations.

Crystals of both compounds are orthorhombic, space group P2₁2₁2₁ with Z = 4, unit cell parameters for (1): a 7.670(1), b 12.160(1), c 18.028(1)Ǻ, V 1681.4 Ǻ³ and for (2): a 7.735(2), b 8.505(5), c 26.846(9) Ǻ, V 1766 1 Ǻ³. The structures were each refined by a full-matrix least-squares procedure to final R 0.026, Rw 0.027 for 1764 reflections with I ≥ 2.5σ(I) for (1) and R 0.065, Rw 0.073 for 1322 reflections for (2).