Aqueous metal ions play an important role in many areas of chemistry. The acidities of [Be(H2O)(4)](2+), [M(H2O)(6)](2+), M = Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+, and [M(H2O)n](2+), M = Ca2+ and Sr2+, n = 7 and 8, complexes have been predicted using density functional theory, second-order Moller-Plesset perturbation theory (MP2), and coupled cluster CCSD(T) theory in the gas phase. pK(a)'s in aqueous solution were predicted by using self-consistent reaction field (SCRF) calculations with different solvation models. The most common binding motif of the majority of the metal +2 complexes is coordination number (CN) 6, with each hexaaquo cluster having reasonably high symmetry for the best arrangement of the water molecules in the first solvation shell. Be2+ is tetracoordinated, but a second solvation shell of 8 waters is needed to predict the pK(a). The Ca2+ and Sr2+ aquo clusters have a coordination number of 7 or 8 as found in terms of the energy of the reaction M(H2O)(7)(2+) + (HO)-O-2 -> M(H2O)(8)(2+) and the pK(a) values. The calculated geometries are in reasonable agreement with experiment. The SCRF calculations with the conductor-like screening model (COSMO), and the conductor polarized continuum model (CPCM) using COSMO-RS radii, consistently agree best with experiment at the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels of theory. The CCSD(T) level provides the most accurate pK(a)'s, and the MP2 level also provides reliable predictions. Our predictions were used to elucidate the properties of metal +2 ion complexes. The pK(a) predictions provide confirmation of the size of the first solvation shell sizes. The calculations show that it is still difficult to predict pK(a)'s using this cluster/implicit solvent approach to better than 1 pK(a) unit.