The in situformed hydrazone Schiff base ligand (E)-N′-(2-hydroxy-3-methoxybenzylidene)benzoylhydrazone (H2L1) reacts with copper(ii) acetate in ethanol in the presence of pyridine-4-carboxylic acid (isonicotinic acid, HL2) to green-[Cu(HL1)(L2)]·H2O·C2H5OH (1) and brown-[Cu(L1)(HL2)] (2) complexes which crystallize as concomitant tautomers where either the mono-anion (HL1)−or di-anion (L1)2−of the Schiff base and simultaneously the pyridine-carboxylate (L2)−or the acid (HL2) (both through the pyridine nitrogen atom) function as ligands. The square-planar molecular copper(ii) complexes differ in only a localized proton position either on the amide nitrogen of the hydrazone Schiff base in 1or on the carboxyl group of the isonicotin ligand in 2. The proportion of the tautomeric forms in the crystalline solid-state can be controlled over a wide range from 1:2= 95 : 5 to ∼2 : 98 by increasing the solution concentration. UV/Vis spectral studies show both tautomers to be kinetically stable (inert), that is, with no apparent tautomerization, in acetonitrile solution. The UB3LYP/6-31+G* level optimized structures of the two complexes are in close agreement with experimental findings. The solid-state structures feature 1D hydrogen-bonded chain from charge-assisted O(−)H–N and O–H(−)N hydrogen bonding in 1and 2, respectively. In 1pyridine-4-carboxylate also assumes a metal-bridging action by coordinating a weakly bound carboxylate group as a fifth ligand to a Cu axial site. Neighboring chains in 1and 2are connected by strong π-stacking interactions involving also the five- and six-membered, presumably metalloaromatic Cu-chelate rings. [ABSTRACT FROM AUTHOR]