Hydroxoaqua Metal Ions in Solution

The dimerization of cis-[Cr(bpy)2(H2O)(OH)] in solution was studied by titration of the corresponding diaqua ion cis-[Cr(bpy)2(H2O)2] with OH . This titration converts the diaqua ion, first into the hydroxoaqua ion and then into the dihydroxo ion ds-[Cr(bpy)2(OH)2]. If dimerization of the hydroxoaqua ion occurs, these reactions should be formulated as follows 

Evidence for dimerization of the hydroxoaqua ion is based on the decrease of the number of chromium particles in reaction 10 and its increase in reaction 11. The number, v, of discrete chromium particles per diaqua ion should decrease from v = 1 to v = as the diaqua ion is titrated with 1 mol of OH and then rise again to v = 1 as a second mole of OH is added, v may be determined by measuring a colligative property of the solution. A most suitable method for ionic solutes is Three-Phase Vapor Tensiometry, TPVT i The three-phase solvent system consists of a saturated solution of an electrolyte in water, in equilibrium with the crystalline phase of that electrolyte and with water vapor. An isobaric temperature difference (AT)p is established when the pure solvent is equilibrated with a solution of a foreign solute in the same solvent, at constant pressure. The apparent number, Vm of free particles per formula of solute depends on the molality of the solute (m), the three-phase ebulioscopic constant Kg, and (AT)p 

the true number of free particles per formula of solute, is determined by linear extrapolation to m = 0. The validity of this relationship was confirmed experimentally in over 40 solute-solvent systems using solutes with known v. The precision of v determinations is 0.05. 

The solute used was [Cr(bpy)2(H2O)2](NO3)3 i H2O5. A saturated solution of barium nitrate was used as solvent. A weighed sample of [Cr(bpy)2(H2O)2](NO3)3 I H2O was dissolved in the saturated barium nitrate solution and neutralized stepwise by the addition of weighed portions of Ba(OH)2 8 H2O. v was determined after the addition of each portion of barium hydroxide. Since both the nitrate counterion of the chromic species and the barium ion added in the form of barium hydroxide were common to solute and solvent, they did not affect v, which was exclusively dependent on the chromic species. If dimerization of [Cr(dpy)2H2O(OH)] occurred, the initial v of the diaqua salt solution should be reduced by the addition of barium hydroxide and reach its minimum value after 1 equiv of barium hydroxide was added. The results, presented in Fig. 6 confirm this expectation. Addition of more than 1 equiv of OH converts the dinuclear hydroxoaqua ion to the mononuclear dihydroxo ion and raises v as expected. However, dimerization is not complete at a molality of 0.24 m (formula weights of the 

TPVT titrations of cis-[Co(en)2(H2O)2] and [Co(tren)(H20)2] gave similar results . The equilibrium constant of (12) was approximately 1 in these three ions, so that in dilute solutions ( 10 M) the tetrapositive, binuclear ions will be almost completely dissociated into dipositive hydroxoaqua ions. The last conclusion was confirmed by Feltham-Onsager plots of the equivalent conductance (at 0°C) versus the square-root of the concentration of four salts [Co(NH3)4(H20)(OH)](N03)2 Cr(NH3)4(H2O)(OH)]-(NO,), [Co(NH3)4(H2O)(OH)]Br2 = and [Cr(H2O)d(NO3)3 -F NaOH . The conductivity data were for solutions of concentrations between 10 and 10 M. At these concentrations the Feltham-Onsager plots are linear. All four plots had the typical slope of 1 2 electrolytes which was less than half of the slope of 1 4 electrolytes. 

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