Crystal and molecular structure of hexan-2,5-dione bis(4-phenyl-thiosemicarbazonato) nickel(II), (C20H22N6S2Ni): a model study of the enhancement of the antibacterial activity of a tetradentate N,S donor ligand on metal complexation

The crystal structure of the nickel(II) complex (C₂₀H₂₂N₆S₂Ni) of the N₂S₂ ligand hexan-2,5-dionebis(4-phenyl-thiosemicarbazone) has been solved using diffractometric data. The complex, exhibiting greater antibacterial activity than the free ligand, crystallizes in the space group C2 with a = 17.414(1) Å, b = 8.485(1) Å, c = 15.129(3) Å, β = 104.09(3)°, Z = 4, d(obsd) = 1.425 g cm^?3, d(calc) = 1.438 g cm^?3 and μ(Mo-K_ga) = 10.978 cm^?1. The structure has been refined by full-matrix least squares to a final R = 0.033 and R_w = 0.041 using 1743 reflections with I ≥ 3σ(I) out of 2049 unique reflections measured (2° ≤ gq ≤ 27°). The hydrogens were either located or placed in their calculated positions. The nickel(II) ion lies in the tetrahedrally distorted square planar ligand field of the tetradentate ligand forming two five membered and one seven membered chelate rings. It is observed that the lack of conjugation in the seven membered chelate rings of the present complex and of similar complexes leads to dissymmetry in the ring geometry. The metal ion is coordinatively unsaturated and available for additional coordination in its axial directions.