Coordination chemistry of thiosemicarbazones

Thiosemicarbazones are compounds of the form:
in which conjugation the C=S bond is stabilized by conjugation with the lone pair of N4:
The CNNCN backbone of these molecules is kept planar by their thione-thiol tautomerism, which gives rise to delocalization of charge throughout the chain of the molecule:
A number of thiosemicarbazones, and many of their complexes with metals, have significant biological and pharmacological activity.

Thiosemicarbazones usually coordinate to metal ions as monodentate ligands through the thiocarbonyl sulphur atom (I) or as didentate chelating ligands through the sulphur and the azomethine nitrogen N1 (II):
However, their versatility as ligands is increased by their easy deprotonation (V), which in certain cases allows them to be N2-mono-N1,S-didentate (III), bis(S-monodentate) (IV) or S‑mono-N1,S-didentate (VI):
Certain metal ions are also capable of mediating the cyclization of thiosemicarbazones to 1,3,4-thiadiazoles, 1,2,4-triazoline-5-thiones or 1,2,4-triazines, or their decomposition to cyclooctasulphur, sulphate or thiocyanate.

We are currently investigating the coordination chemistry and other properties of α-heterocyclic thiosemicarbazones derived from 2-cyanopyridine (VII), 2‑cyanopyrazine (VIII) or 5-acetylbarbituric acid (IX), or from appropriate derivatives of these compounds.
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