The hydrothermal in situ Cu(II)/2,3-pyrazine dicarboxylic acid reactions at 100 and 140 degrees C result in Cu(II) 3-hydroxy-2-pyrazinecarboxylate and oxalate complexes, respectively. This is the first example of a combination of experimental and theoretical study on in situ metal/ligand reactions under hydro- and solvothermal conditions.