Stability of transition metal complexes involving three isomeric quinolyl hydrazones

► The stability constants of the three isomeric quinolyl hydrazones towards M2+ is as follows: (para ≥ meta > ortho). ► The insertion of the Me-group in the 8-position decreases the stability of the complexes. ► The complexation of the ortho-derivative is endothermic, whereas that of the para-derivative is exothermic. ► The interaction of the hydrazones with CoCl2·6H2O afforded an octahedral complex (ortho) and square planar complexes (meta and para).