Hydration mechanisms of Cu(2+): tetra-, penta- or hexa-coordinated?

To shed light on the hydration mechanisms of Cu(2+), we carried out simulations in both gas and aqueous phases by using the ab initio molecular dynamics technique equipped with the method of constraint. The simulations provide relatively complete free-energy information, from which the coexisting coordination pictures are clearly revealed. In both phases, the 5-fold complex is the most thermodynamically favorable state whereas the classically-accepted 6-fold occurs as a very weak stable state. In the gas phase, the 4-fold complex is a more reachable state than the 6-fold, but it cannot hold stably in the aqueous phase. The extracted thermodynamic values illustrate that in the gas phase the entropy term dominates the evolution processes to the 5-fold whereas in the aqueous case the energy term is dominant.