Aggregation of perchlorates in aprotic donor solvents .2. Alkaline-earth metal perchlorates

Non-aqueous solutions of alkaline-earth metal perchlorates M(ClO4)(2) (M = Ba, Sr, Ca, Mg) have been investigated by IR and Raman spectroscopy. The association of M(2+) and ClO4- into mononuclear species M(ClO4)(x) (x = 1 or 2) depends on the cation and on the solvent. In a given solvent the association extent decreases from Ba to Mg. In all cases it has been possible to find at least one associating solvent (ester, cyclic ether) where the association into M(ClO4)(2) is complete. In the associated species the coordination of ClO4 can be monodentate (C-3v) or bidentate (C-2v). The monodentate coordination is the only mode in Mg(ClO4)(2) while bidentate coordination is predominent with the other cations. In that case there is a minor proportion of monodentate ClO4 groups which decreases from Ca to Ba. The two kinds of coordination are distinguished by the shift Delta v(1) of v(1) relative to free ClO4- which is either positive (monodentate) or negative (bidentate). This criterion has been established by the investigation of the other vibrational modes of ClO4. Wavenumber shifts and splittings have been correlated to the cation polarizing power. Force constant variations calculated on BaClO4+ and LiClO4 are correctly interpreted by Lewis structures.