These calculations indicated that each palladium(II) center in 4a retains a geometry close to square planar, and that the central bromide is slightly outside of the plane of the four palladium atoms (27°) (Figure ). The Gibbs free energy was also calculated to assess the stability of the species, and we found that there is a significant driving force for each step, from monomer to tetramer, at standard state (Figure ). Since all intermediates (4a-7a) are negatively charged, and the reactions are run in aqueous media, [ArPdBr2(OH2)] - (7a) and three neutral [ArPdBr(OH2)2] species were chosen as the references. As Pd-OH2 bonds will be broken in every step and replaced by Pd-Br bridges, which are essentially half bonds, it appears that the driving force for the formation of the higher oligomers (4a, 5a) cannot be the bonding enthalpy. We therefore split the Gibbs free-energy change into the entropic contribution (-TDS) and the enthalpic contribution (DH). We found that for every step there is an unfavorable enthalpic change, except from trimer 5a to tetramer 4a, which is minimally exothermic. On the other hand, the entropic contribution favors every step significantly due to the release of bound water molecules. The stability of the complexes therefore arises from entropy (released molecules) rather than from enthalpy (released heat). This may have a significant impact on the reactivity, as described below. The calculations showed that the most stable oxidative-addition adduct was tetramer 4a, and this same adduct was also the one found with the highest relative abundance by ESI-MS.