Thermodynamic data complexes

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

Studies on metal-pyrazole complexes in solution are few. The enthalpy and entropy of association of Co(II), Ni(II), Cu(II) and Zn(II) with pyrazole in aqueous solution have been determined by direct calorimetry (81MI40406). The nature of the nitrogen atom, pyridinic or pyrrolic, involved in the coordination with the metal cannot be determined from the available thermodynamic data. However, other experiments in solution (Section 4.04.1.3.3(i)) prove conclusively that only the N-2 atom has coordinating capabilities. 

Thermodynamic data show that the stabilities of the caesium chloride-metal chloride complexes are greater than the conesponding sodium and potassium compounds, and tire fluorides form complexes more readily tlrair the chlorides, in the solid state. It would seem that tire stabilities of these compounds would transfer into tire liquid state. In fact, it has been possible to account for the heats of formation of molten salt mixtures by the assumption that molten complex salts contain complex as well as simple anions, so tlrat tire heat of formation of the liquid mixtures is tire mole fraction weighted product of the pure components and the complex. For example, in the CsCl-ZrCU system the heat of formation is given on each side of tire complex compound composition, the mole fraction of the compound... 

The Af-HjO diagrams present the equilibria at various pHs and potentials between the metal, metal ions and solid oxides and hydroxides for systems in which the only reactants are metal, water, and hydrogen and hydroxyl ions a situation that is extremely unlikely to prevail in real solutions that usually contain a variety of electrolytes and non-electrolytes. Thus a solution of pH 1 may be prepared from either hydrochloric, sulphuric, nitric or perchloric acids, and in each case a different anion will be introduced into the solution with the consequent possibility of the formation of species other than those predicted in the Af-HjO system. In general, anions that form soluble complexes will tend to extend the zones of corrosion, whereas anions that form insoluble compounds will tend to extend the zone of passivity. However, provided the relevant thermodynamic data are aveiil-able, the effect of these anions can be incorporated into the diagram, and diagrams of the type Af-HjO-A" are available in Cebelcor reports and in the published literature. 

Consider Ni exposed to Oj/HjO vapour mixtures. Possible oxidation products are NiO and Ni (OH)2, but the large molar volume of Ni (OH)2, (24 cm compared with that of Ni, 6.6 cm ) means that the hydroxide is not likely to form as a continuous film. From thermodynamic data, Ni (OH)2 is the stable species in pure water vapour, and in all Oj/HjO vapour mixtures in which O2 is present in measurable quantities, and certainly if the partial pressure of O2 is greater than the dissociation pressure of NiO. But the actual reaction product is determined by kinetics, not by thermodynamics, and because the mechanism of hydroxide formation is more complex than oxide formation, Ni (OH)2 is only expected to form in the later stages of the oxidation at the NiO/gas interface. As it does so, cation vacancies are formed in the oxide according to... 

For complexes like PtL2X2 (X = halogen L = NH3, PR3, etc.) where cis-and trans-isomers exist, the trans-isomer is usually thermodynamically more stable. The c/s-isomer may be formed first in a reaction and, in the case of platinum, may be relatively inert to substitution. (Thermodynamic data are relatively scarce trans-Pt(NH3)2Cl2 is some 13kJmol-1 more stable than the cis-isomer.)... 

It is worth noting that although PuFg(c) has not been characterized pentavalent fluorocomplexes are well known. In the hexava-lent state only oxyfluorocomplexes are known. However, thermodynamic data on these species are almost completely lacking. The thermal decomposition study (42J of the quadrivalent complex (NH4)4PuF8 according to scheme (10)... 

Complex chlorides of plutonium (34, 41) do not present such a wide range of formulae as the complex fTuorides but we have at hand thermodynamic information on two important species which have also been characterized with other actinides. In table II we have disregarded the complex halides for which no thermodynamic data are available. The enthalpy of formation of Cs2NaPuClg(c) (55) and Cs2PuClg(c) (56) have been obtained from enthalpy of solution measurements."The selected (8) values are AHf(Cs2NaPuCl6,c) =... 

In spite of well identified deficiencies, the status of the existing thermodynamic data on solid halides and related compounds is rather satisfactory. By contrast, the data on aqueous halogeno-complexes are fragmentary, sometimes contradictory and in most cases limited to stability constants in such a context, recommendation of best values can be made for a few species only. 

Hydrogen bonding and complexing properties TABLE 4. Thermodynamic data for formation of phenol adducts... 

Both the AH° and -A.S ° values are quite large for the Ni11 complex (633) bearing a pendent hydroxyethyl arm.1595 If this complex is treated with a bidentate ligand that binds exclusively in a cis fashion, coordination forces the macrocycle to fold out of the plane to give a so-called cis folded product. Thermodynamic data for the binding of ethylenediamine to give (635)... 

Classical methods for the investigation of complex formation equilibria in solution (UV/Vis spectrometry, thermochemical and electrochemical techniques) are still in use (for an appraisal of these and other methods see, e.g., ref. 22). Examples for the determination of the ratio of metal to ligand in an Hg-protein complex by UV spectrometry are given in ref. 23, for the study of distributions of complex species of Cd in equilibria by combined UV spectrometry and potentio-metry in ref. 24 and by potentiometry alone in ref. 25, and for the combination of calorimetry and potentiometry to obtain thermodynamic data in ref. 26. 

Christensen, J. J., and Izatt, R. M. (1970). Handbook of Metal Ligand Heats. Marcel Dekker, New York. A compendium of thermodynamic data for the formation of an enormous number of complexes. 

Table 6.2. Thermodynamic data for complexation of alkali metal ions by cryptands in water (Lehn Sauvage, 1975 Kauffmann, Lehn Sauvage, 1976).

In general, the cryptands (213) show a stronger correlation between thermodynamic stability and match of the metal ion for the cavity. Thermodynamic data for complexation of the alkali metal ions with a number of cryptands is summarized in Table 6.2. The data for the smaller (less flexible) cryptands 2.1.1, 2.2.1, and 2.2.2 illustrate well the occurrence of peak selectivity. 

Although concerted C=C and C-H insertion into benzene by methylene is possible, several experiments on the effect of various solvents on the product ratio suggest the intermediacy of a complex between methylene and benzene.67-69 Based on the obtained kinetic and thermodynamic data, the transient is believed to be a complex formed between singlet methylene and benzene. 

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