Stability constants coordination complexes

Electrochemical experiments allow the determination of complex stability constants for Fe2+ by measuring complex redox potentials over a range of pH values. The Fe34YFe2+ redox potential of the siderophore complex, as with the spectral characteristics of the complex, is dependent on the inner coordination environment of the iron. These considerations will be addressed later (Section III.D). 

Martell, A. E., and R. J. Motekaitis, Determination and Use of Stability Constants, VCH Publishers, New York, 1992. A very useful description of how complex stability constants can be measured and applied, as well as advice on choosing values form published data from one of the modem masters of coordination chemistry. 

The polyesters have then been transformed into their respective poly a-aminocarboxylic acids. These multidentate ligands poly-A propyleneglycine L- and DL- as well as poly-l-isopropyl-2-carboxyl-ethylenimine and poly-7V-ethylenealanine have then been examined as to their properties as metal complexating agents. The metal complexation stability constants of these polyampholytes are interpreted in the light of their structural differences, they show a four fold coordination with respect to the examined bivalent metal ions. Poly-A-piopyleneglycine has also been incorporated in cross-linked resin structures and the complexation behavior towards Zn, Cu, Ca and Mg has been determined. 

Exciting developments have occurred in the coordination chemistry of the alkali metals during the last few years that have completely rejuvenated what appeared to be a largely predictable and worked-out area of chemistry. Conventional beliefs had reinforced the predominant impression of very weak coordinating ability, and had rationalized this in terms of the relatively large size and low charge of the cations M+. On this view, stability of coordination complexes should diminish in the sequence Li>Na>K>Rb> Cs, and this is frequently observed, though the reverse sequence is also known for the formation constants of, for example, the weak complexes with sulfate, peroxosulfate, thiosulfate and the hexacyanoferrates in aqueous solutions. 

Compared to later elements in their respective transition series, scandium, yttrium and lanthanum have rather poorly developed coordination chemistries and form weaker coordinate bonds, lanthanum generally being even less inclined to form strong coordinate bonds than scandium. This is reflected in the stability constants of a number of relevant 1 1 metal-edta complexes ... 

Treichel, Knebel, and Hess provided further data on these systems by studying reactions of [Pt(PRj)2(CNCH3)2] with various halide ions and with pseudohalides. A series of five-coordinate complexes were obtained from reactions with iodide ion (PRj = PPhj, PPh2Me, PPhMe2, PEtj), and a study was carried out to measure the stability of these complexes with respect to ligand loss 155). Stability constants for several of these complexes were obtained from spectroscopic data. Other reactants (Cl, Br, CN, SCN) generally yielded the appropriate [Pt(PRj)2(CNCH3)X] species, as expected. 

Regular pyrimidines are less effective ligands for Ni11 ions. They may use, inter alia, their C=0 donor to yield monodentate coordination.1835 Insertion of a sulfur atom into a pyrimidine moiety increases considerably its binding ability.1836 Thiolation of uridine at C(2) or C(4) results in formation of a quite effective S,N3 four-membered chelate in the complexes with Ni11. Thiolation of purine at C(6) increases the stability constant by 3.5 orders of magnitude. 

In the case of macrocyclic tacn side arms, stability constants of type (820) dinickel complexes are considerably higher and six-coordination can be achieved due to the binding of additional coligands, either at both Ni11 (in (828))2031 or at only one of them (in (829)).2082... 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

The stability constants of zinc complexes of 4,6-dimethyl-2-thiopyrimidine have been determined by potentiometric measurements. The crystal structure shows infinite zigzag chains of ZnL2 units with each zinc coordinated by an N3S2 donor set in a trigonal-bipyramidal geometry.853... 

Mapsi et al. [16] reported the use of a potentiometric method for the determination of the stability constants of miconazole complexes with iron(II), iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The interaction of miconazole with the ions was determined potentiometrically in methanol-water (90 10) at an ionic force of 0.16 and at 20 °C. The coordination number of iron, cobalt, and nickel was 6 copper and zinc show a coordination number of 4. The values of the respected log jSn of these complexes were calculated by an improved Scatchard (1949) method and they are in agreement with the Irving-Williams (1953) series of Fe2+ < Co2+ < Ni2 < Cu2+ < Zn2+. 

The above account of selectivity of inorganic plus organic chemistry in synthesis is given rather extensively to stress three points. All the four (Mg, Fe, Co and Ni) porphyrin products came from one source, the synthesis of uroporphyrin. The basis of selection is very different from that in primitive centres which use thermodynamic stability constant selectivity based on different donor atoms for different metal ions. Here, all ion complexes have the same donor atoms, nitrogen, the most constrained being the coordination of Mg2+ by five nitrogens exactly as is seen for Fe in haemoglobin. Hence, there also has to be a new control feedback to ensure that the appropriate quantities of each metal cofactor is produced in a balanced way, that is synthesis from uroporphyrin has to be divided based upon... 

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