Stepwise formation of complexes

On the whole, the thermod3mamic functions found for the stepwise formation of complexes in aqueous solution agree very well with the models proposed for complexes of different character. Thus for interactions between hard acceptors and hard donors, postulated to be mainly electrovalent, the expected stepwise decrease of ASn generally occnrs, often accompanied by a similarly expected decrease of AHn- With interactions between soft acceptors and soft donors, postulated to be mainly covalent, virtually constant values of AHn from step to step are often found, while in other cases values of AHn becomes step by step less exothermic. Both modes of behaviour are compatible with the current model. 

These examples and some others that are given in Table III show that for selected ions, which form strong complexes, it is possible to make unambiguous structure determinations from solution diffraction data and to obtain direct information on coordination changes that take place during the stepwise formation of complexes. Thermodynamic data provide only indirect information on these structural changes, indicated, for example, by abnormal changes in enthalpy and entropy values or in stability constants for the formation of the complexes. 

M taken from Crabtree et ul. 178). Data obtained at other ionic strengths 23, 154, 189, 584, 615), in mixed solvents 46, 154, 404, 579), for 5-Me-phen 494), and for otlier metal ions 17,154, 181, 454, 455) arfe also available. Data derived kinetically may be found in Section IV, A. The changes in enthalpy (kcal/mole) and entropy (eu) occurring in the stepwise formation of complexes are tabulated in Tables IV and V for bipyridyl and phenanthroline complexes. Some of the data from Atkinson and Bauman 23) have been criticized 189, 493). As given, values are overall and not stepwise. For the ferrous bipyridyl... 

The stepwise formation of complexes may be described by the following set of equilibrium constants, which characterize the stability of the complexes (charges are omitted) ... 

Thermodynamics of the stepwise formation of metal ion complexes in aqueous solution. S. Ahrland, Struct. Bonding (Berlin), 1973,15,167-188 (86). 

Ahrland, S. Thermodynamics of the Stepwise Formation of Metal-Ion Complexes in Aqueous Solution. Vol. 15, pp. 167-188. 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

The stepwise formation of fluoro complexes in aqueous solution has been extensively studied in recent years by standard techniques (192). The introduction of the LaF3 electrode has enabled fluoride activities to be measured directly, even with the "insoluble fluorides such as CaF2 or PbF2 (34, 81). Values obtained depend on the salts added to maintain constant ionic strength, and care must be taken in making comparisons since not all results are extrapolated to zero strength (15). Key references to work in this field can be gleaned from standard compilations of stability constants. 

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