Several literature data which refer to the formation of weak complexes of either alkali and alkaline-earth metal ions or cationic amino compounds with inorganic and organic anions, have been reviewed and compared to each other. In general, the stability of these weak species strongly depends on the charges of both reactants, while is often independent of the nature of the ligand considered. Negatively charged and oxygen donor containing ligands are favoured in complexation of alkali and alkaline-earth metal ions, while negligible or small effects can be assigned to the presence of nitrogen donors. The regularities observed in the trend of the values of formation constants, as a function of the charges involved, allow us to estimate reliable values also for weak species not directly studied. Although these complexes are not very stable, their rote in the speciation of biological and natural fluids may be even relevant, because of the high concentration of some ionic substances taken in to account.