Stability constant mixed chelate

During the early sixties Thompson and Loraas (77) reported the formation of mixed complexes of reasonable stability (log K 3.0—5.3) between lanthanide—HEDTA and ligands such as EDDA (N,N -ethylenediaminediacetic acid), HIMDA (N-hydroxyethyliminodiacetic acid) and IMDA (iminodiacetic acid). This observation together with the remarkably large formation constants (72) for the bis-EDDA complexes [log A2 =4.73 (La) 8.48 (Lu)] suggested a coordination number larger than six for the tripositive lanthanide ions in aqueous solution, in view of the fact that mixed chelates of the t5q>e M (HEDTA) (IMDA) axe not formed when M =Co(II), Ni(II) or Cd(II). 

Table 46 Stability Constants /SVOAL of Some Mixed-Ligand Chelates of Oxovanadium(IV)a...

Table 35. Logarithms of stability constants of mixed chelate species 455] M(HEDTA)(Ly (p = 0.1 at 25°)...

Sutton (666) and Kul ba (458-460) have prepared a number of bipyridyl and phenanthroline complexes of various thallium(III) salts those with nitrate and perchlorate are generally bis-chelate compounds, whereas the halides give compounds of stoichiometry TIX3L. Stability constants have been reported for some of the complexes (457). Other mixed ligand species containing ethylenediamine (461) and oxalate (456) and salts containing both bipyridyl and phenanthroline coordinated to the same thallium(III) ion (456) are also claimed. 

Amine Donors. Complexes with l,3-bis[2(S)-aminomethyl-l-pyrrolidinyl]propane and other optically active tetra-amines containing pyrrolidinyl groups with six-membered chelate rings have been studied, and the stereochemistry of the complex depends on the position of the two pyrrolidinyl groups in the ligand. Studies of ability of similar complexes to hydrolyse esters have been reported. Stability constant measurements for nickel(ii) monoamine complexes have been made in a number of hydroxylic solvents. Magnetic and optical spectral measurements of some nickel(ii) succinimide and mixed amine-succinimide complexes indicate that... 

Most widely used methods are based on (i) the competition of protons with the bound metal ions for thiolate ligands, i.e., the pH stability of the metal-thiolate clusters or (ii) the competition with a metal chelator. In the former case the pH stability of the Cd-thiolate clusters is followed by absorption spectroscopy of the CysS-Cd(II) LMCT band at 250 nm. From the apparent p/Ca values the apparent stability constants of the clusters can be derived [103]. The apparent p/sTa values are determined either by taking the pH values of half-maximum absorbance or using a non-linear curve fit of the pH plot [104]. The other method is based on the competition for a single metal ion between the chelator 5F-BAPTA (1,2-bis-(2-amino-5-fluorophenoxy)ethane-A,W,A ,A -tetraacetic acid) and the protein followed by NMR spectroscopy [71]. Although this method was established for the zinc metalloforms of MTs, its applicability to cadmium metalloforms has also been demonstrated [105]. In this case, by analogy with zinc finger proteins a lower affinity for mixed Cys/His coordination of Cd(II) in MTs compared to sole Cys coordination has been shown. 

N-acyl ED3A chelating surfactants are capable of very dramatic, highly synergistic lather enhancement on mixed surfactant systems particularly in the presence of divalent connterions. The lather stability on mixtnres of SLES (3 moles EO) and Na LEDS A at a constant concentration of 1% in the presence and absence of water hardness is presented in Figure 5. Synergistic lather enhancement is evident when the system composition passes 30% LED3A. 

Another study aimed at uncovering unusual properties that chelation sites impart to polymers involved differential thermal analysis of the copper(II) chelates of a series of bis(/3-diketones) (IX-15) (iJ). A linear decrease in thermal stability with an increase in the methylene chain length was found. This is similar to purely organic chain compounds. If the chelate site alone were the decomposition mode, the decomposition temperature might be expected to be constant. It was concluded that the bis(]3-diketonate) site did not confer any unusual thermal properties. The interpretations drawn from these results could be clouded because aluminum oxide was mixed with the chelate as a standard during the DTA analysis. Its effect on the results was not discussed. 

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