The cis to trans isomerization reaction of bis(glycinato)copper(II) was investigated by quantum chemical methods. An electronic reaction barrier (cis --> trans) of about 7.9 kcal/mol was determined for a ring-twisting mechanism, where both glycine residues stay bidentally attached to the central copper ion throughout the reaction. Ring-opening mechanisms could be ruled out for being energetically too demanding. Isomerization rates suggest the kinetic stability of the cis isomer in the gas phase only at very low temperatures (100 K). At ambient temperatures the isomerization to trans occurs spontaneously. Therefore the experimentally proved stability of the cis isomer in the solid state may be exclusively attributed to crystal packing effects.