omit to mention the charge when it is not relevant) and nitrate (N) anions may proceed up to the tetrahedral complexes [ZnIm 4 ] 2-and [ZnN 4 ] 2-. Table shows that the free energy of formation becomes increasingly negative for coordination numbers up to four in these complexes (five-and six-fold coordinations, with free energies of -4.11 eV and -3.39 eV respectively, are found to be unstable when compared with four-fold coordination (-4.85 eV), but of similar stabilities than two-and three-fold coordination (-3.74 eV and -4.41 eV respectively). This trend is, however, not reproduced by the calculations in the gas phase, in which the favored coordination is limited to 3 Im -ligands (the formation energies of [ZnIm 3 ] -solvated with ethanol or water is -27.9 eV, lower than that of the four-coordinated [ZnIm 4 ] 2-). Hence, implicit solvation turns out to be essential to account for the stable tetrahedral anionic coordination of the Zn 2+ cation. It seems timely to point out that the implicit solvent does not alter the geometry of the ZnIm n X m complexes appreciably with respect to that in vacuo: the Root-Mean-Square Deviations (RMSDs) between the superimposed atomic coordinates of the optimized clusters determined with both methods are negligible -e.g. < 0.01 Å in bond distances-. Changes in the energetics of these small clusters should therefore not be attributed to the inclusion of the implicit solvent, but rather to the unbalance of negative charge in the clusters and to the shielding of the long-range repulsion between the anionic ligands, which changes the energetics but does not affect the geometries, for small clusters. Differences in the structural features of the complexes predicted by the vacuo and solvated computations do become increasingly relevant with growing cluster size. For instance, the unshielded repulsions in vacuo between the Im -moieties restrict the "folding" (distortion of the structures to form more dense structures -see Figure for an example) of the extended metal-organic networks with several Zn atoms, discussed below in Sec. 3.2 Differences between in vacuo and solvated computations also emerge for the coordination of Zn 2+ with explicit solvent molecules. Whereas both approaches agree qualitatively in the stability of the tetrahedral [Znw 4 ] 2+ water (w) complex, for ethanol (E), the three-fold complex [ZnE 3 ] 2+ is favored over the [ZnE 4 ] 2+ complex under implicit solvation, but it is not so pronounced in vacuo. Destabilization of the tetrahedral ethanol complex results from steric repulsion between the bulky ethyl groups, which is apparently compensated for by the unshielded, hence stronger, Zn 2+ . . . O interactions in the computations performed in the gas phase.