Direct Observation of Triple Ions in Aqueous Solutions of Nickel(II) Sulfate: A Molecular Link Between the Gas Phase and Bulk Behavior

Electrospray ionization of an aqueous solution of nickel(II) sulfate provides direct experimental evidence for the formation of triple ions of the type [Ni-2(SO4)(H2O)(n)](2+) and [Ni(SO4)(2)](2-), whose existence in aqueous solution has previously been proposed based on relaxation spectroscopy [Chen et al. J. Sol. Chem. 2005, 34, 1045]. Formally, these triple ions are formed by aggregation of the solvated ions Ni2+ and SO42-, respectively, with the neutral ion pair NiSO4. In addition, also higher adducts are observed, e.g. the pentuple ions [Ni-3(SO4)(2)(H2O)(n)](2+) (n = 7-9) and [Ni-2(SO4)(3)](2-), of which the dicationic is extensively hydrated, whereas the anionic is not. The structures of the dinuclear nickel clusters are derived from ab initio calculations and their infrared spectra are compared with experimental data obtained for the gaseous ions [Ni2SO4(H2O)(5)](2+) and [Ni-2(SO4)(3)](2-), respectively. The calculations show that the structures are crucially controlled by the degree of solvation of nickel ion. Explicit consideration of solvating water molecules within the first coordination sphere suggest that the dicationic triple ion [Ni2SO4](aq)(2+) is bent and thus bears a permanent dipole moment, whereas the [Ni(SO4)(2)](aq)(2-) dianion tends to be quasi-linear. The experimental and theoretical data for the gaseous ions thus support the elegant, but indirect, deductions previously made based on solution-phase studies.