Bivalent Ions

Bivalent Ions.— The complexation kinetics of Mg with COg and HCOs have been studied in methanol and water by means of the stopped-flow and temperature-jump methods. The rate constants for CO3 (6.8x10 and 1.5 x 10 1 mol s, respectively) and for HCOs (5.0 x 10 1 mol s in both solvents) are analysed in terms of a dissociative mechanism. 

The spin-lattice relaxation time Tx has been used to measure the exchange rate of water molecules between the inner hydration sphere of Ni + ions and the bulk in concentrated solutions of Ni(N03)2. (Previous n.m.r. methods have involved the determination of the spin-spin relaxation time, Tg.) An activation energy of 10.5 1.0 kcalmol is reported and the results indicate the existence of an inner-sphere nitrate complex with a stability constant of about 0.21 mol .  

In the first of three reports on the kinetics of the Ni +-acetate reaction to appear during the review period, Puentes, Morgan, and Matwiyoff have analysed temperature-dependent [l- C]acetate line-broadening and chemical shift data. The results were considered in the framework of a two-term rate expression, = k x -f k mAc, where wac is the acetate concentration and k-x and k, are defined by 

The rate constants at 300 K, A i = (3.9 0.3)x 10 s and 3 = (2.1 0.4) x 10 1 mol- s- , and the activation enthalpies, AH x = 13.4 0.6 kcal mol and AiT = 12.5 0.5 kcal mol , are consistent with acetate exchange controlled by loss of acetate and water, respectively, from [Ni(H20)BAc]+. In the latter case, loss of water is followed by acetate entry and subsequent rapid dissociation of the diacetate species. As was to be expected, no dependence of either dissociation rate constant upon ionic strength was observed (in the range 1—4 mol 1 and from 0 to 14 °C). 

ATI is ca. 4 kcal mol , a reasonable value for the rupture of a hydrogen bond. As 

Bivalent ions.—The pressure-jump technique has been used to study the formation of Be formate in aqueous solution. The rate constant (200 50 s at 25 °C) and activation energy (46 6 kJ mol ) for the rate-limiting substitution into the inner hydration sphere are similar to the other values which have been obtained for this metal. The rate law for the exchange of the fluoro-ligands of [BeF4] is  

The kinetics of formation and/or dissociation of the mono-complex of Ni with isoquinoline have been measured in a variety of solvents (Table 1). Neither the dissociation rate constant nor the overall equilibrium constant for complex formation correlates with Gutmann s donicity scale for solvents, but the activation enthalpy for dissociation does show a linear relationship with solvent donicity. This, conclude the authors, can be rationalized from a mechanistic point of view by assuming that the solvent molecules co-ordinated to the metal ion help to stabilize the transition state. A similar correlation between A/f J donicity (Table 1) is found  

The rate constants and activation parameters for the formation of mono-complexes of NP with ligands of varying denticity in DMF (Table 3) can be accommodated  

They are = Ats = 3.6 x 10 1 mol S k-i=37 s, k-z=760 s (at 25 °Q, and the fact that the pyridine molecules co-ordinated to the metal ion enormously enhance their own lability but have little effect on that of the co-ordinating water molecules suggests that steric crowding is an important feature with these bulky ligands. The volume of activation for methanol exchange at Ni + is -1-10.9 0.6 cm mol and this is consistent with an h mechanism. 

Ultrasonic studies have been reported on solutions of copper(ii) nitrate and perchlorate in ethylene glycol. In each case a single, concentration-independent relaxation effect is observed which is probably due to cation desolvation coupled with the diffusive approach of the two solvated ions. Tanaka has proposed a modification to the Bennetto-Caldin scheme for solvent exchange at bivalent metal ions. 

The possibility of an octahedral-tetrahedral equilibrium existing for the hydrated Zn(II) ion has been the subject of recent discussion, but a spec-trophotometric TJ study of the aqueous Zn(II)-chlorophenol red system failed to 

0 mol dm (HC104). These data are interpreted in terms of an a activated 

In contrast, no such invariability of mechanism apparently holds for ligand exchange at beryllium(II). Lincoln and Tkaczuk have used the variable-temperature proton nmr technique to investigate the exchange of 

A further high-pressure nmr investigation of acetonitrile exchange at cobalt(II) in pure acetonitrile has been reported, yielding the parameters )k(298 K) = (2.56 0.06) X 10 s  

The single relaxation observed in the ultrasonic absorption of copper(II) formate, acetate, and propionate has been used to evaluate a rate constant (20°C) for water substitution by carboxylate at Cu of (3.4 1.2) X 10 s The enthalpy of activation is 10 1 kcalmoF for the formate, and it was confirmed that the ligand derived from the strongest acid forms the weakest copper complex. 

Zinc chloride and (under some conditions) zinc nitrate solutions in aqueous DMSO, on the other hand, yield two ultrasonic absorption maxima with the chloride this is taken as evidence for an octahedral-tetrahedral coordination change accompanying the addition of the third bound Cr, while with the nitrate the high-frequency relaxation (observable only at low-water-mole fractions) is attributed to outer-sphere complex formation. The rate constants (25 C) for solvent loss with the nitrate (Xh20 = 0.59) and the chloride (Xhjo = 0.039 and 0.904) are, respectively, 2.2 X 10 , 4.1 X 10 , and 3.3 x lO s.  

The pressure-jump technique has been used to study the kinetics of BeS04 formation in water/DMF mixtures. The rate of substitution depends on the composition of the solvated metal ion [Be(H20),(DMF)4, f and also on the solvent molecule which is being replaced. 

It has been pointed out that, in the exchange of solvent water on (M = Mn, Fe, Co, Ni) [or, incidentally, on M(NH3)5(OH2) - (M = Cr, Co, Rh, Ir)], the partial molar volumes of the transition states display less than one-third of the variability of the partial molar volumes of the initial states as M is varied in other words, trends in A V are largely determined by initial state, rather than transition state, properties. 

5 wt% methanol solution finds that the kinetic parameters are 

Evidence has been presented for an Id mechanism for complex formation at Ni(II) in acetic acid, as well as for the reaction of this metal with isoquinoiine in water, DMF, acetonitrile, methanol, and ethanol in the latter case, activation volumes of, respectively, 7.4, 9.3, 12.8, and 

6 cm mol are reported. The rate of dissociation of the Ni(II) isoquinoiine complex in THF is strongly affected by the free-water concentration in the medium, and comparison of the activation enthalpy for this reaction in pure THF with previous results in other solvents confirms the correlation with the Gutmann donor number. 

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Lead (qv) is a member of Group 14 (IVA) of the Periodic Table because it has four electrons in its outer, or valence, shell. However, the usual valence of lead is +2, rather than +4. The two s electrons have higher ionisation energies. As a result, tetravalent lead exists as a free, positive ion only in minimal concentrations. Furthermore, the bivalent or plumbous ion differs from the other Group 14 bivalent ions, such as the starmous ion of tin, because Pb " does not have reducing properties. 

Table 2. Thermal behaviour of the gels. Temperatures [°C], where minimal storage moduli were observed on reheating. Concentrations of ions in mmol / lOOg gel, [V " ] = added bivalent ion, [Z ] = added monovalent ion, [C1 ] = 2.5, 2[V+ ]+[Z+]=2.5 or [Na ]+[K ]=2.5.

Apart from the two classifications described above, electrolytes may also be classified according to the number and valence of the ions produced. Thus, sodium chloride and copper sulfate may both be termed binary electrolytes since one molecule of each of these chemical substances is capable of producing two ions. In the case of sodium chloride, both the ions produced are univalent so that this substance may also be called a uni-univalent electrolyte. Copper sulfate, however, yields two bivalent ions and so may be called a bibivalent electrolyte. The valences of the ions are quoted in the positive-negative sequence. Calcium chloride and potassium sulfate are both ternary electrolytes since one molecule of each yields three ions the former is bi-univalent, whilst the latter is uni-bivalent. 

If one assumes to a first approximation that the contraction of the cell is a linear function of the covalence of the metal-ligand bond, one can derive for the bivalent ions the following equations ... 

As is well known (Morse and Berner, 1972) even very small concentrations of HPO (and some organic solutes) inhibit nucleation of CaC03, most likely because these adsorbates block essential surface sites on the substrate or on the mineral clusters. Mg2+ is known to inhibit many nucleation processes, especially also the nucleation of Mg bearing minerals. The water exchange rate of Mg2+ is slower than that of many cations, such as Pb2+, Cu2+, Zn2+, Cd2+, Ca2+. The inhibition effect of Mg2+ may be due to its sluggishness to (partial) dehydration. (Mg2+ has among the bivalent ions a very large enthalpy (-AHj,) of hydration.)... 

The bivalent ions of cadmium, cobalt, and copper, in concentrations of the order of 10-100 fiM, accelerate inactivation in experiments of the type shown in Figs. 4 and 5. An excess of Zn2+ prevents, or reverses, inactivation by these ions. 

A. Heydweiller 81 sought to measure the cohesion between a solute and solvent by means of the volume changes which occur on soln. If m be the mass of an ion, and u its mobility, the product kmu is called the Icnenmodulus, and 1c is a constant 0 00112 for univalent ions, and 006372 for bivalent ions. The modulus was calculated in various ways, and the result was considered to represent the cohesion... 

The ion-exchange reaction of the synthetic zeolites NaX and NaY with cobalt, zinc and nickel ions is shown to be non-stoichiometric at low bivalent-ion occupancy, the hydrolytic sodium loss being about twice as large for NaX ( 5 ions/unit cell) as for NaY. The effect is more pronounced at high temperatures and disappears at high occupancies. Reversibility tests in NaX toward zinc and cobalt ions, as studied by a temperature-variation method, show the temperature history to be an important factor in the irreversibility characteristics. The low-temperature partial irreversibility, induced by a high-temperature treatment (45°C) is interpreted in terms of a temperature-dependent occupancy of the small-cage sites by divalent cations, which become irreversibly blocked at low temperature (5°C). 

Stoichiometry. The effect of bivalent ion occupancy upon the stoichiometry is shown in Figure 1. The stoichiometry factor / is defined as the number of Na+ ions desorbed/M2+ ions adsorbed and the deviation from 2 is a measure of the hydrolytic sodium loss. As before, it appears that NaX is much more sensitive to excess sodium loss than NaY at low occupancy of M2+, the data are comparable with the results of Table II 6-8 ions/unit cell (NaX) and 2.5-3 (NaY), which again differ by a factor of about 2. 

Figure 1. Stoichiometry factor vs. bivalent ion occupancy in NaX (upper curve) and NaY (lower curve) at 25°C for cobalt (squares), nickel (circles), and zinc (triangles) (---------------) confidence interval at 95% level...

Reversibility. Apparent irreversibility phenomena of ion exchange in NaX were studied with zinc and cobalt ions using a temperature-variation method described in the experimental section. In view of the high selectivity of NaX for bivalent cations at low zeolite loading, the concentration of bivalent ions in the equilibrium solution is quite sensitive to small changes in the surface composition. In fact, the adsorption removal of bivalent cations at low loading, below 0.2, is quantitative or nearly so (99.5% or better). Consequently the value of the equilibrium concentration is an ideal criterion for assessing either reversibility or equilibrium conditions. 

The most important point is the effect of thermal history upon the equilibrium level of cobalt and zinc ions in solution. Within experimental error, the results obtained with the 45 °C—two day systems are identical to the 45°C systems which had received a prior one-day treatment at 5°C. The duration of the experiments has very little effect upon the equilibrium distribution, as evidenced by the fact that the results obtained by longterm equilibrations at both temperatures and for both ions were nearly identical to those shown in Table III. Most important however is the finding that the equilibrium levels of cobalt and zinc at 5°C are significantly higher than these which are obtained after a 45°C treatment. This indicates that the 5°C distribution over the various possible sites, as induced by a 45°C pretreatment, differs from the normal low-temperature distribution in that a significant portion of the adsorbed bivalent ions which participate in the 45°C equilibrium no longer do so at 5°C. In other words, when returned to 5°C, part of the solid-phase metal ions appear irreversibly sequestered in sites where they are out of reach at low temperature. 

The temperature also affects the process of coagulation.8 With the chlorides of potassium, sodium, lithium and ammonium the velocity of flocculation varies inversely as the temperature 9 with the chlorides of barium, strontium, calcium, magnesium and cadmium the velocity varies directly as the temperature with aluminium chloride it is independent of the temperature. Heating thus stabilises the sol towards univalent cations but diminishes the stability towards bivalent ions.10 The... 

It is true that a decreases monotonically in the series 0(—II), F(—I), Ne, Na(I), Mg(II) and Al(III) but far less at the beginning than between the infinite value for O-2 and a value for gaseous F not far from 2 A3. However, there is a shallow minimum close to the bivalent ion in the series Br(—I), Kr, Rb(I), Sr(II), Y(III) and Zr(IV) and close to the trivalent ion in the series I(—I), Xe, Cs(I), Ba(II), La(III) and Ce(IV). The important point is not the exact position of this flat minimum, but the completely different behaviour from the gaseous ions. One might have suspected, once more, that a is a decreasing function of increasing fractional atomic charge4 36,39) of the atom. However this cannot be the whole truth because a is connected with the positions... 

Plesset, M. S., Helfferich, F., and Franklin, J. N. (1958). Ion exchange kinetics. A nonlinear diffusion problem. II. Particle diffusion controlled exchange of univalent and bivalent ions. J. Chem. Phys. 29, 1064-1069. 

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