Aécio P. Chagas

Some features of the thermochemistry of coordination compounds

The present review covres the thermochemical data for adducts of zinc halide group elements with mono- or bidentate nitrogen, oxygen or sulphur donor atom ligands and also some adducts of arsenium halide group elements.

Ion-exchange equilibria with aluminum pectinates

Abstract Pectins, which play an important role in the structure of the plant cell wall, is used in medical treatment and for the prevention of metal intoxication, and also as a gelling component in the food industry. The ions of various metals (iron, zinc, copper, manganese, calcium and aluminum) are involved in biological reactions and can bind to pectins for transport through the cell wall into the cytoplasm; Al 3+ ions, however, are toxic to plants. Despite the serious problems caused by such aluminum toxicity, little is known about the interaction of the Al 3+ ions and pectins, especially those demethylated by pectin methylesterase (PME). In the present paper, the ion-exchange equilibrium ( K e ) between solid aluminum pectinates (obtained from enzymatic hydrolysis) with differing degrees of demethylation (DM) and aqueous solutions of iron, zinc, copper, manganese and calcium nitrates was studied. The order of preference for PME demethylated pectins (Fe 3+ >Al 3+ >Cu 2+ ≅Mn 2+ >Zn 2+ ≅Ca) shows that aluminum has a greater affinity for the carboxyls of the pectins, an affinity that can be related to the Al toxicity in plants sensitive to the Al 3+ ion. In the ionic exchange with Fe, Cu and Mn small variations in K e with DM was observed whereas those with Zn and Ca remained constant. A cooperative effect for the ion exchange between the aluminum ions and those of Fe, Cu and Mn was observed, whereas a competitive one was found for the exchange with Zn and Ca. Possibly the cooperative effect is due to the greater affinities of Fe, Cu and Mn for the carboxyls, whereas the competitive effect was due to the lesser affinities of Ca and Zn. These results were compared with those of a prior study of the ion-exchange process of aluminum pectinates with differing DM obtained through alkaline hydrolysis.

Estimation of standard enthalpies of formation of crystalline inorganic and organometallic complexes

Abstract Good linear correlations have been found between standard enthalpies of formation of crystalline inorganic and organometallic complexes MXnLm and enthalpies of formation of ligands L or LH in their standard reference state. These correlations may be used to estimate enthalpies of formation of new complexes.

Enthalpy of sublimation of adducts—an evaluation involving MX2·nL (M = Zn, Cd, Hg; X = Cl, Br, I; n = 1, 2, 3; L = organic base containing CO or PO groups) adducts

Abstract The approximation that the molar enthalpy of sublimation of an adduct is equal to the molar enthalpy of sublimation or vaporization of the ligand, has been applied to the calculation of the mean enthalpy of dissociation of the metal-ligand bond, D (MO), for adducts of the type MX2·nL (M = Zn, Cd, Hg; X = Cl, Br, I; n = 1, 2, 3; L = ligand containing PO or CO groups) and for other similar compounds. Based on a thermochemical cycle, a linear equation D (MO) = AΔrH(a) + B was obtained, with A = 1/n and ΔrH(a) derived from the enthalpy of the reaction MX2·nL(s) = MX2(g)+nL(g), whose thermochemical value was calculated from experimental data. By using data from nearly 80 compounds and calculating D (MO) for various approximations, it was verified through linear regression that the cited approximation is the one which correlates the best with the stoichiometries of the compounds. This result is a good argument for the use of this approximation in D (MO) calculations.

Thermochemical study of adducts of tetramethylurea with zinc, cadmium, and mercury halides

The adducts [Zn(tmu)2X2](X = Cl, Br, or I), [Cd(tmu)2I2], and [M(tmu)X2](M = Cd or Hg and X = Cl or Br) have been characterized. The shift of the CO stretching vibration to low frequency indicated that tetramethylurea (tmu) is oxygen bonded to the metals. The standard enthalpy values (ΔHR⊖) for the reactions MX2(s)+n tmu(I)→[M(tmu)nX2](s) were determined by means of solution calorimetry. The values of standard enthalpy of formation (ΔHf⊖) of the above adducts in kJ mol–1 were: –1 006 (ZnCl2), –926 (ZnBr2), –844 (ZnI2), –690 (CdCl2), –602 (CdBr2), –775 (Cdl2), –515 (HgCl2), and –454 (HgBr2). The standard enthalpy of vaporization of tmu (ΔHv⊖= 51.12 ± 0.73 kJ mol–1) was obtained from vapour-pressure measurements. The values of the standard enthalpy of the reactions: MX2(s)+n tmu(g)→[M(tmu)nX2](s), MX2(g)+n tmu(g)→[M(tmu)nX2](s), and MX2(g)+n tmu(g)→[M(tmu)nX2](g) were also determined.

Ion pair association of complexes of La3+ and Nd3+ with Br− and NO3− in N,N-dimethylacetamide

Abstract The formation constants and the respective variations of enthalpy were obtained by means of calorimetric titration of the lanthanides La 3+ and Nd 3+ with bromide and nitrate in N,N-dimethylacetamide. These thermochemical data were calculated for the 1:1 and 1:2 species which are less stable than the corresponding species obtained with chloride. The order of stability Cl − > Br − >NO 3 − was established for both species of La 3+ , and the order Br − >Cl − >LNO 3 − for 1:1 species of Nd 3+ . NdCl 2 + and NdBr 2 + species were not detected. Our results support the view that the metal-anion interactions involve inner sphere species.

Complexes of lanthanide(III) ions with chloride in N,N-dimethylacetamide

By using the calorimetric titration technique, the following equilibra: Ln3+ (sol)+Cl− (sol)⇄LnCl2+ (sol) and LnCl2+ (sol)+Cl− (sol)⇄LnCl2+ (sol) in N,N-dimethylacetamide (DMA) solution, at 298°K, have been studied for the lanthanide series. The equilibria constants K1 and K2, and the molar enthalpy changes ΔH1 and ΔH2 were determined. The log K and ΔH values present a break at gadolinium, these values are correlated with the P(M) function. This break can be interpreted as due to electronic effects and the change in coordinating ability of the lanthanide.

Estimation of standard enthalpies of formation of inorganic and organometallic compounds. II

Correlations between the standard enthalpies of formation of organometallic and inorganic compounds MXnLm and of ligands LH or LHm, both in their standard reference states, are examined in detail and illustrated with examples. They provide a simple method of predicting new values and assessing experimental data.

Thermochemistry of dichlorobis[N-(2-pyridyl)acetamide]-zinc(II), -cadmium(II), and -mercury(II)

The standard enthalpy changes ΔHR⊖=–70.80 ± 0.60, –35.02 ± 0.72, and –22.90 ± 1.1 kJ mol–1 for M = Zn, Cd, and Hg respectively have been measured from the reaction MCl2(s)+ 2pya(s)→[M(pya)2]Cl2(s)[pya =N-(2-pyridyl)acetamide], by using solution calorimetry. From these values, the standard enthalpy of formation of the ligand (ΔHf⊖=–149 ± 21 kJ mol–1) obtained via combustion calorimetry, and literature standard enthalpies of formation of the metal chlorides, the ΔHf⊖ values have been determined as –785 ± 22, –724 ± 22, and 544 ± 23 kJ mol–1 for M = Zn, Cd, and Hg respectively. The standard enthalpy of sublimation of the ligand (103.8 kJ mol–1) has also been obtained. The standard enthalpies of the reactions MCl2(g)+ 2pya(g)→[M(pya)2]Cl2(c) and MCl2(c)+ 2pya(g)→[M(pya)2]Cl2(c) have been calculated.

MOLAR EXCESS GIBBS FREE ENERGIES OF MIXTURES OF ETHANOLAMINES AND WATER

Abstract Thermodynamic information about the vapor—liquid equilibria of systems containing water and ethanolamines is needed in the design and optimization of important industrial processes. This knowledge is also useful in the investigation of some phenomena such as the thermal conversion of ethanolamines or their complexation reactions with metals. Physico-chemical investigations involving aqueous solutions of ethanolamines are scarce, and sometimes contradictory, in the literature. It has been stated [1] that the system diethanolamine—water obeys Raoults law. However, there have been studies [2] indicating departure from ideal behavior for this system. The purpose of this work was to determine the molar excess Gibbs free energies of the systems formed by water and the ethanolamines.