The theoretical results show in agreement with the experiment that the gold(II) complexes are better stabilized with heavier halogen ligands and that the tridentate ligand stabilizes the complex better than the bidentate ligand. The formation of the gold(II) complexes from [(terpy)Au III X2] + is about 60 kJ mol -1 less energy-demanding than that from [(bipy)Au III X2] + . In theoretical calculations, this value is even higher and reaches 80 -90 kJ mol - 1 . The discrepancy is caused by the fact that we measure Ea in the experiment, whereas we calculate H in the theory (see Figure ). Typically, the simple bond cleavages are quazibarrierless endothermic processes and therefore the measured activation energy corresponds to the reaction enthalpy of the bond cleavage process (BDE). In the case of [(terpy)Au III X2] + , the starting complex has a pseudo square planar geometry around the gold centre formed by two pyridine units and the two halogen atoms (see the initial structure in Figure ). The third pyridine unit interacts only weakly with the gold ion. The X elimination proceeds via a transition structure in which the initially loosely bound pyridine unit coordinates to the gold centre. The measured activation energy thus corresponds to the barrier height of this process.