Synthesis, characterization and X-ray crystal structures of seven-coordinate pentagonal-bipyramidal zinc(II), cadmium(II) and tin(IV) complexes of a pentadentate N3S2 thiosemicarbazone

New complexes of the general formula, [M(Hdap NMetsc)(HO)](NO) ·HO (M = Zn, Cd; H dapNMetsc = 2,6-diacetylpyridinebis(N- methylthiosemicarbazone) and [Sn((dapNMetsc)X] (X = Ph, Cl and I) (dapNMetsc = the doubly deprotonated form of 2,6-diacetylpyridine bis(N-methylthiosemicarbazone) have been synthesized and structurally characterized by a variety of physico-chemical techniques. X-ray crystallographic structure determination shows that in the zinc and cadmium complexes, the bis(thiosemicarbazone) ligand coordinates as a neutral NS pentadentate chelating agent through the two azomethine nitrogen atoms, the pyridine nitrogen atom and the two thione sulfur atoms. The NS donors of the ligand occupy the equatorial plane and the two aqua ligands occupy the sixth and seventh axial positions of the seven-coordinated cadmium(II) and zinc(II) ions. In the tin(IV) complexes, however, the thiosemicarbazone is coordinated to the tin(IV) ion as a dinegatively charged pentadentate chelating agent via the pyridine nitrogen atom, the two azomethine nitrogen atoms and the two thiolate sulfur atoms. The two apical positions of the seven-coordinate tin(IV) ion are occupied by either phenyl, chlorido or iodido ligands. In each of the complexes, the overall geometry adopted by the metal ion may be considered as a distorted pentagonal-bipyramid