Cation coordination number measurements of AlCl3, AlBr3, Al(ClO4)3, Al(NO3)3, BeCl2, GaCl3, GA(ClO4), InCl3, In(Cl)4)3, and Mg(ClO4)2, in water, and in aqueous acetone mixtures have been carried out by a proton magnetic resonance method. At low temperatures, solutions of these electrolytes exhibit separate proton resonance signals for bulk water, and water molecules in the first solvation shell of the cation. Integration of these signals provides a direct measure of the coordination number of the cation. The Al(III) and Ga(III) are solvated by six water molecules in all solutions, regardless of the anion present. The Be(II) is coordinated by four water molecules in water and in aqueous acetone mixtures. The In(III) and Mg(II) could only be studied in aqueous acetone mixtures, wherein they are each coordinated by six water molecules. With the possible exception of Be(II) no evidence for contact ion pairing was found.

The coordination number of Al(III) in aqueous mixtures of acetone, dimethylformamide, dimethylsulfoxide, dioxane, tetrahydrofuran, and tetramethylurea was measured over a range of solvent concentrations by a proton magnetic resonance technique. At low temperatures, proton exchange is slowed to such an extent in these mixtures that separate resonance signals are observed for bulk water and water molecules in the Al(III) solvation shell. Coordination numbers were measured by direct integration of the two water signals. The measurements show that in aqueous mixtures of acetone and tetramethylurea, and over a small solvent concentration range, dioxane and tetrahydrofuran, only water solvates Al(III). This was indicated by the value of six molecules obtained for the contribution of water to the Al(III) solvation shell in these mixtures. In aqueous mixtures of dimethylformamide and dimethylsulfoxide, wherein separate signals were also observed for the nonaqueous components, both solvents contribute to the Al(III) solvation shell. No evidence for ion pairing was found in any of the mixtures.

C104)3 69 lated to the solvating properties of these nonassociated solvents compared to those of water and methanol.It should be noted that the simple factors which seem to explain the activation enthalpies for exchange on vanadyl ion do not work for nickel(II). For instance the activation energy for water exchange is 5 kcal mol-1 less than that for methanol exchange on nickel(II). This difference is in the opposite direction to that predicted from basicity, steric, or crystal field effects. This fact seems to point to some as yet unconsidered factor, such as transition-state solvation, as being an important kinetic factor. It can only be hoped that study of a wider range of solvents and metal ions will lead to a better understanding of the factors controlling ligand exchange.23 (23) Note Added in Proof.-Slightly revised kinetic parameters have been obtained by least-square computer analysis of the data in acetonitrile, dimethylformamide, and methanol.

Nonideality in diffusion of ionic and hydrophobic solutes and pair dynamics in water-acetone mixtures of varying compositionCation coordination measurements of Sc(NOa)a, Y(NO a )8, and Th(NO a )4 in water-acetone mixtures have been made by a proton magnetic resonance method. At low temperatures, solutions of these salts exhibit separate proton resonance signals for bulk water and water molecules in the primary solvation shell of the cation. Integration of these signals provides a direct measure of the number of water molecules coordinated to the cation. Values of 3.9, 2.4, and 2.9 were obtained for Sc(In), Y(In), and Th(IV), respectively, over a range of salt and solvent concentrations. These values may reflect extensive ion pairing in these solutions, particularly in the Y(nI) and Th(IV) cases. The PMR spectra of Th(IV) solutions revealed multiple signals for the protons of coordinated water molecules, lending evidence to the postulate of ion pairing in these systems.

Angeles, Cali fornaa 90032) BY CooLixci aqueous solutions of Alm to -30 to -50°, it is possible to observe separate 1H n.m.r. signals corresponding to bulk water and water molecules in the first Alm solvation shell.l Integration of these signals provides a direct, unambiguous value of the cation co-ordination number. This method has been applied to the study of A P solvation in aqueous solvent mixturesB and to the determination of co-ordination numbers of several di-and tri-valent ions, including Mgn, in water when possible, and in aqueous solvent mixtures. Because of the recent controversy