Infrared and Raman spectroscopic and theoretical studies of hexaaqua metal ions in aqueous solution

Abstract

Far-infrared spectra of the hexaaqua complexes, [M(OH2)6]n+, of divalent (MII=Mg, Zn, Cd and Hg) and trivalent (MIII=Al, Ga, In and Tl) metal ions in aqueous solution have been recorded. Together with existing or remeasured Raman spectra this has enabled a complete assignment for all six MO6 fundamentals for the first time. Force constant calculations have been performed for the hexaaquametal ions in Th symmetry. The normal coordinate treatment has been extended to the hydrated trivalent cations (MIII=Ti, V, Cr, Fe, Co, Rh and Ir) using experimental frequencies in the literature, obtained for solid or glassy state samples. A satisfactory linear correlation was found between the averaged metal–oxygen stretching frequencies and the parameter Z1/2/R02, which characterises the ion–dipole moment interaction, where Z is the charge of the metal ion and R0 the M–O bond length. The averaged stretching frequencies seem to be more directly related to the electrostatic contributions to the bond strength than the widely used ν1(Ag) symmetric stretching mode. The obtained M–O stretching force constants were also fairly well correlated to the Z1/2/R02 parameter.