1991 Volume 64 Issue 6 Pages 1859-1865
The crystal structures of the diastereomeric salt pair, Λ-lel3-[Ni(S,S-chxn)3](d-tart)·3H2O (1) and Δ-lel3-[Ni(R,R-chxn)3](d-tart)·5H2O (2) (chxn=trans-1,2-cyclohexanediamine and d-tart=(+)-(R,R)-tartrate dianion), have been determined by single-crystal X-ray diffraction techniques. Crystal 1 is orthorhombic with the space group P212121, a=10.093(2), b=13.589(4), c=22.011(4) Å, and Z=4. Crystal 2 is also orthorhombic with the same space group, a=11.197(2), b=13.102(2), c=22.402(2) Å, and Z=4. In 1, the d-tart ion makes a familiar face-to-face contact with the Λ complex, in which the two alcoholic and one carboxylic O atoms of the d-tart ion are involved in the multiple hydrogen bonds to the three H–N groups on the triangular face of the complex. This contact mode resembles the one found earlier in the chloride d-tart salt of the corresponding Λ Co(III) complex, Λ-lel3-[Co(S,S-chxn)3]Cl(d-tart)·2H2O (3). On the other hand, no such face-to-face contact is present in 2, though it has been found in the corresponding Δ Co(III) complex salt, Δ-lel3-[Co(R,R-chxn)3]Cl(d-tart)·2H2O (4) in which the d-tart ion is obliged to rotate the distal carboxylato group so as to avoid the steric repulsion otherwise imposed on it by one of the bulky chxn ligands. Detailed comparison of the above four crystal structures revealed that the packing modes of the respective complex cations and counterions are surprisingly similar to one another, indicating that the absence of such a face-to-face contact in 2 is attributed to the weaker affinity of the d-tart ion for the divalent Δ Ni(II) complex, rather than to the steric demands of the crystal packing in 2. In 2 are found two deformed contact modes similar to each other, in which the d-tart ion directs its three O atoms to the triangular face, but only one of them is hydrogen-bonded to one or two of the three H–N groups on the triangular face. The d-tart ion thereby avoids the steric repulsion that would be imposed on it if it should make a usual face-to-face contact with the Δ complex.
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