A proper selection of ligands is the key issue in the design and synthesis of transition metal complexes with predefined structure and properties. Polydentate chelating Schiff-base ligands are ideal for this purpose because they allow modifications of ligand systems, the structure of which can be fine-tuned by condensation reactions.  This work illustrates the conformational and configurational flexibility, and interesting donor properties of polydentate Schiff base systems of two types: a potentially tetradentate, rigid OONO ligand, o-vanillin semicarbazone (Scheme 1, top), and its symmetric analogue, bis(o-vanillin)carbohydrazone (Scheme 1, bottom). The relative positions of the donor atoms in the former incorporates two coordination pockets: one bidentate and one tridentate. In addition, due to the keto-enol tautomerism, several conformers are possible, that lead to the different bridging modes through phenolic or enolic oxygen. Consequently, the nuclearity of the resulting systems is difficult to predict. The latter ligand may be considered as an extension of o-vanillin semicarbazone, now with possible extra metal binding domains. Its coordination versatility also arises from the tautomeric equilibrium between keto and enol forms. The enol tautomer can exist as syn or anti isomers as a consequence of the double bond character of the central N-C linkage. The anti configuration provides two contiguous tridentate pockets, each of which can accommodate one ion, thus the leading structural pattern being an azine-bridged dinuclear unit.  Synthesis and structural characterization of various type of copper(II), vanadium(V), nickel(II), manganese(II) complexes will be presented. The counterion of the starting salts and the solvent mixture used for the synthesis of complexes had a significant impact on both their composition and packing. The coordination chemistry of both types of ligands was basically established, but there are still exciting synthetic challenges (such as construction of multinuclear systems with 4f metals or heteronuclear 3d-4f clusters) that remain to be explored.