Stepwise complex formation

Complexes formed by tetradentate siderophores involve stepwise complex formation and therefore, have somewhat different equilibria from their hexadentate analogs. Initial chelation will occur with a tetracoordinate FeL complex forming. A subsequent equilibrium then occurs, where the FeL complexes will react in a 2 1 stoichiometry with free ligands in solution to form a single Fe2L3 complex (coordinated water and charges not shown for clarity). 

Thiocyanate ions react with Fe(III) in a moderately acidic medium to yield a red colour which, for a long time, has been the basis for the determination of iron(III), or total Fe after oxidation of Fe(II) to Fe(IIl) [21-23]. Owing to stepwise complex formation in solution, FeCSCN) ", Fe(SCN)2, and further complexes can be formed. The concentrations of the reagents and the pFl of the medium determine which complexes are prevalent. In general, only FelSCN) " is formed at microgram concentrations of Fe(III). The higher complexes are more intensely coloured. 

The complexes 27 are solid crystalline substances, which are more stable than the individual constituent compounds. The intermediacy of compound 115 was confirmed by stepwise complex formation and a mechanism for formation of 27 has been proposed. While dioxide 62 is easily hydrolyzed by cold water, attempts to separate the complexes into the individual components by heating at 90 °C for 15 h in moist benzene were unsuccessful. Complexes 27 did not form when tetrahydrofuran (THF) or chloroform was used as solvent. Attempts to prepare complexes 27 from the initial dioxides and triazines failed. Complexes formed using diethylcyanamide contained no solvent molecules apparently the ethyl groups, rather than solvent molecules, fill voids present in the structure <2005RJ0289>. 

With these restrictions in mind, we shall consider the thermod5mamic data for stepwise complex formation of iron(II) with several diimine ligands and with ethylenediamine (en), listed in Table 6 together with some data on the respective cobalt (II) and nickel(II) complexes. One... 

The reactions of sodium tellurite and potassium tellurite with monosulphide sulphur in alkaline solution have been investigated. Stepwise complex formation was observed in the TeO -S -OH and TeO -S -OH systems. Thio-salts of Te with less than 3 ligands were found to be unstable and to disproportionate into tellurite and trithiotellurite ions. The most... 

Solution dynamic study of mononuclear metal-ligand complexes is well established. The description of self-assembly reaction dynamics presents new challenges in coordination chemis-try.249 xjjg synthesis of assemblies from inert components produces a picture of stepwise complex formation. However, it is unclear if self-assembly of labile components proceeds through a clean stepwise mechanism. 

Similarly to the aquo complexes, the dimethyl sulphoxide solvates of most metal ions, including zinc and cadmium (studied by Ahrland), have hexacoordinated octahedral structures, and the halide complexes, in both solvents, a tetrahedral structure. However, the octahedral tetrahedral transformation associated with the stepwise complex formation with halide ions occurs at different complex... 

The stereochemical transformation of the mercury halide complexes in the course of the stepwise complex formation could also be followed by means of X-ray examinations [Ga 68]. The results at times cast doubt on the correctness of the conclusions derived from the equilibrium data. In the case of mercury halides, the stability of the third stepwise-formed complex is much higher than that of the fourth, and the value of X3/X4 is therefore large. This would indicate that the former complex already has tetrahedral symmetry. According to the X-ray examinations, however, the reason for the anomalous stability sequence is that the second complex is linear, the third is pyramidal and only the fourth has tetrahedral symmetry [Ga 68]. 

Once such functions are obtained, they can be used for the calculation of equilibrium constants of Eqs. (2.1), (2.4), and (2.5). As one can see, Eqs. (2.9) and (2.10) are quite similar to the functions widely used in Bjerrum s theory of stepwise complex formation in solutions (Leden s and Bjerrum s functions accordingly) [5,6]. Thus, the mathematical analysis and calculation of the constants become well-elaborated routine. Some examples of such calculations are given below. 

Stepwise complex formation in non-aqueous solvents is a much less frequent phenomena than in aqueous solutions, where the stepwise substitution of water molecules by mono-dentate ligands is a common process with hydrated metal ions. Experience from work with aqueous solutions demonstrates two essential factors for a study of stepwise complex formation a) effective solvation, whichdissociates the metal salt MXj into and mX , b) the possibility... 

The Concepts of Stepwise Complex Formation, Its Internationalized Notation, and Its Influence on Contemporary General Chemistry... 

Leden I (1964) On the Thermochemistry of Stepwise Complex Formation. Pure Appl Chem 8 171-178. doi 10.1351/pacl96408020171... 

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