Fabrizio Marsicano

THE EXISTENCE AND STABILITY OF MIXEDLIGAND COMPLEXES IN AQUEOUS SOLUTIONS CONTAINING ZINC AND CYANIDE IONS AT ELEVATED pH VALUES

Abstract The formation and stability of binary Zn2+[sbnd]CN− and ternary Zn2+[sbnd]CN−[sbnd]OH− complexes were studied by glass electrode potentiometry in aqueous solution at 25°C and in a medium of ionic strength 0.1 mol dm−3. The solution pH was varied to cover the range 4→10. The potentiometric data were interpreted with the aid of the ESTA suite of computer programs, plots of formation functions, and a novel algorithm for model selection. It is demonstrated that the ternary complex Zn(CN)3(OH)2- is formed in significant amounts in solutions of pH > 8.5. Some evidence was also obtained for the existence of the five-coordinate species Zn(CN)3(OH)2 3- and Zn(CN)5 3- in solutions at sub-millimolar concentration levels. No polynuclear complexes were detected under these conditions. Formation constants are reported for both binary Zn2+[sbnd]CN− and ternary Zn2+[sbnd]CN−[sbnd]OH− species.

A FORCE FIELD FOR MOLECULAR MECHANICS STUDIES OF IRON PORPHYRINS

The development of parameters for the MM2(87) force field to model the iron porphyrins has been described. The mean values of the bond lengths and bond angles of iron porphyrins in the Cambridge Structural Database have been used as a starting point in developing the force field, which was then refined using two representative structures, each of penta- and hexa-coordinated high-and low-spin FeII and FeIII porphyrins, intermediate-spin penta- and hexa-coordinate FeIII, and intermediate-spin tetra-coordinate FeII porphyrins. The bond lengths were usually reproduced to within 0.015 A of the observed values, bond angles to within 1.5° and torsional angles to within 4°. The use of the force field has been exemplified in an investigation of the conformation of intermediate spin 5,10,15,20-tetraphenylporphyriniron(II)[Fe(TPP)] using quenched dynamics techniques and the generic structure algorithm of Gerber, Gubernator and Muller (Helv. Chim. Acta, 1988, 71, 1429) for ring systems. Both approaches have shown that the two lowest-energy conformations available to the molecule have, respectively, a planar and an S4-ruffled porphyrin core.

A POTENTIOMETRIC AND CALORIMETRIC STUDY OF THE THERMODYNAMICS OF FORMATION OF SOME OF THE COMPLEXES OF THE d10 METAL IONS SILVER(I), MERCURY(II), AND CADMIUM(II) WITH THIODIGLYCOL, THIOUREA, AND THE SULPHITE ION

Abstract The standard free energy, enthalpy, and entropy changes of complex formation of the d 10 metal ions Ag(I), Hg(H), and Cd(II) with the S-donor-atom ligands thiodiglycol, thiourea, and sulphite were investigated at 298.15 ± 0.05 K. The free-energy changes δGi ° were calculated from stability constants βi, which were determined by silver- and mercury-electrode potentiometry, if suitable literature data were not available. Enthalpy changes δHi°, were measured with a titration calorimeter, and entropy changes δSi ° were calculated from the relation δGi °=δHi ° TδSi °. The standard state, denoted by the superscript ° in δG δH, and TδS°, refers, in most cases, to a hypothetical 1 M solution in which the ionic strength, μ, is 0.5 M. It is demonstrated that a large proportion of the extra stabilization of the HgL n complexes compared with the corresponding AgLn complexes is a result of a more favourable entropy contribution. The entropies of complexation are interpreted in terms of the Powell, Latimer, an...

Surface water pH measurements — theory and practice

Abstract Reliable measurement of pH in dilute and poorly buffered aqueous solutions is problematic. Of particular concern are the errors introduced as a result of differences in ionic strengths between the calibration solutions and the test sample. Residual junction potentials, stream potentials, and the mode of construction of the liquid junction have been implicated. Errors of 0.2 pH units and higher are commonplace. The relative merits of the calibration of various electrode systems with standard buffer solutions, dilute buffers, and dilute hydrochloric acid solutions on the precision and accuracy of electrode responses were investigated. A simplified free diffusion junction reference electrode was developed and tested. The dilute acid calibration technique appears to be significantly more accurate than the calibration with buffers. The free diffusion junction in its present form does not lead to significant improvements in accuracy or reproducibility.

The linear free-energy relation in the thermodynamics of complex formation. Part 2. Analysis of the formation constants of complexes of the large metal ions silver(I), mercury(II), and cadmium(II) with ligands having ‘soft’ and nitrogen-donor atoms

A potentiometric and calorimetric study has been made of the complex formation of AgI, HgII, and CdII with ligands containing ‘soft’ and N-donor atoms. It has been found that linear enthalpy relations exist, and that they mirror quite closely the division of the free-energy data into separate groups for the ‘soft’ and N-donor atom ligands, for the pair of metal ions AgI–HgII and for AgI–CdII. It seems that a linear entropy relation also exists, to within experimental error, for the pair AgI–HgII which has a gradient and intercept that can be rationalised in terms of the Powell–Latimer–Cobble semi-empirical theory of ionic entropy. The entropy contributions are small compared with those of the enthalpy in these predominantly enthalpy-stabilised complexes. The significance of the relatively simple behaviour of the free energies compared with their enthalpy and entropy components is discussed.

The linear free-energy relation in the thermodynamics of complex formation. Part I. Correlations involving enthalpy changes on complex formation of silver(I) and mercury(II) with ‘soft’ donor ligands

The free energies of complex formation of various metal ions with ligands having ‘Soft’ donor atoms P, As, S, Se, and I–, Br–, Cl–, and CN– form linear free-energy relations (l.f.e.r.s) involving all these ligands. A potentiometric and calorimetric study has been made in which one l.f.e.r., for the pair of cations Ag+ and Hg2+, has been analyzed in terms of the separate enthalpy and entropy contributions. A linear enthalpy relation (l.e.r.) has been found which lies parallel to the l.f.e.r., but, unlike the l.f.e.r., does not pass through the origin. The intercept of the l.e.r. is rationalized in terms of the Powell, Latimer, and Cobble model for the partial molar entropies of complexes in aqueous solution.

THE FLUORIDE COMPLEXES OF THE TRIVALENT FIRST-ROW METAL IONS, WITH SPECIAL REFERENCE TO THE FLUORIDE COMPLEXES OF V(III) AND Sc(III)

Abstract A fluoride ion-selective electrode is used to study the ScIII and V111 fluoride systems log βn=6.18, 11.52, and 15.8 for n=1,2, and 3 respectively was obtained in 0.5 mol.dm−3 NaClO at 25°C for ScIII. A better fit to the potentiometric data was obtained by inclusion of the species Sc2 F3 3+, for which log β32=19.0 was indicated by the computer program (MINIQUAD) used for analysing the data. Inclusion of a variety of other polymers of similar composition also gave nearly as good an improvement in fit, so that this value must be regarded with some caution. The other constants determined were not very sensitive to the inclusion of these polymers, however. V3+ ions were generated in solution by electrochemical reduction of VO2+, and log K 1=5.00 determined for the V3+/F− system in 1.0 mol.dm−3 NaClO4 at 25°C. It is shown that log K 1 (F−) for the first row trivalent transition metal ions ScIII to GaIII show an inverse order of stability as compared with that usually observed, which is interpreted in ...

Patterns in Lewis acid-base interactions in aqueous solution

Abstract The observable patterns in free energies of complex-formation in aqueous solution have lead to many classifications (A and B, ‘hard’ and soft) of Lewis acids and bases. These can be illustrated with LFER (linear free energy relationship) diagrams [1]. Figure 1 is consistent with Pearsons ideas on hard and soft acids and bases (which are interpreted as representing ionic and covalent bonding) if Ag(I) is softer than Hg(II). figure 2 displays the same pattern as Fig. 1, but is inverted, indicating that Bi(III) is harder than Hg(II). These patterns, which are similar for all metal ions, may be reproduced using eqn. 1, which is essentially that due to Drago and coworkers [2]. E and C are the tendency of the Lewis acid A and base B to undergo ionic and covalent bonding. Eqn. 1 is not able to correlate formation constants for complexes of ligand having large donor atoms (S, P, As, Cl, Br, I)with smaller Lewis acids (Cu(II), Co(III), the proton), but works well for small ligands (F−, OH−, NH3) with all Lewis acids. The deviation from the prediction of eqn. 1 are interpreted as being due to (i) steric hindrance to solvation of the Lewis acid by large donor atoms, and (ii) specific solvation effects, which operate through the increased softness induced in the Lewis acid by attachment of very soft donor atoms. Mechanism (i) is supported by evidence from the correlations in the NMR and infra-red involving the structural trans effect, while mechanism (ii) is supported by results on proton-basicities of organic bases in the gas-phase. Eqn. 1 is extended to give eqn. 2, which correctly predicts the formation constants of 266 different complexes involving 31 different Lewis acids (including the proton) with 16 different unidentate bases. The D parameters are identified with desolvation effects arising form mechanism (i) and (ii) above.

The chelate effect: a simple quantitative approach

Equations are constructed which accurately predict the stability constants of metal complexes of polydentate ligands from the stability constants of the complexes of the unidentate analogues. The factors considered are the asymmetry of the standard state, and the reduction of steric hindrance and mutual electrostatic repulsion in the complexes of the polydentate as compared with the unidantate ligands. The equations have been applied succassfully to complexes of polyamine ligands and of ligands containing alkylamine and carboxylate functional groups. The same equations can be used to calculate formation constants of complexes containing unidentate ligands which are unstable in aqueous solution from the stability constants of stable complexes of polydentate ligands. Thus, logK1 4.3 has been calculated for the complex [Fe(NH3)]3+.