Hexadentate complex

The dissociation of iron from a tetradentate siderophore complex is more rapid than from the analogous hexadentate system (3). This may be a reason for some organisms to produce tetradentate siderophores instead of hexadentate siderophores despite the concentration effect noted in Section III. A. As was illustrated in Fig. 19, it is also thermodynamically easier to reduce iron(III) in tetradentate complexes than hexadentate complexes, making it easier to induce release of iron from the complex by a redox mechanism. 

The spin state lifetimes in solution of the complexes II and III have been measured directly with the laser Raman temperature-jump technique189). Changes in the absorbance at 560 nm (CT band maximum) following the T-jump perturbation indicate that the relaxation back to equilibrium occurs by a first-order process. The spin-state lifetimes are r(LS) = 2.5 10 6 s and r(HS) =1.3 10 7 s. The enthalpy change is AH < 5 kcal mol-1, in good agreement with that derived from x(T) data in Ref. 188. The dynamics of intersystem crossing processes in solution for these hexadentate complexes and other six-coordinate ds, d6, and d7 spin-equilibrium complexes of iron(III), iron(II), and cobalt(II) has been discussed by Sutin and Wilson et al.u°). 

ESCA studies on these hexadentate complexes by Lazarus, Hoselton, and Chou191) were not successful. It has been found that the broad satellite structure in the observed 2 p X-ray photoelectron spectra were due to a radiation-induced decomposition of the HS isomer in the spectrometer rather than the result of the multiplet splitting. 

Condensation of the symmetric dialdehyde XL with diamines affords quadridentate complexes (XLI) when M(II) (M = Co, Ni, Cu) acetates are used as template agents, and hexadentate complexes (XLII) in the... 

Fig. 23.2 Structure of EDTA. (a) EDTA contains two donor N atoms and four donor O atoms. It can therefore form a hexadentate complex (b) with a metal Ion, e.g. Pb +.

For the present heterogeneous system (Mn-support-GP-tacn + PO), the structure and the solvent effects in the catalytic experiments resemble most those of the latter, hexadentate complexes. For such hexadentate complexes, a temporary removal of one of the pendant arms is necessary to create a coordinative vacancy on the metal. The particular role of methanol might be to assist in the temporary deligation via hydrogen bond formation with 2-OH-alkyl groups. The system is unique in that the covalent fink to the surface can participate in the metal coordination via the 2-hydroxy group, as indicated by the arrow in Scheme 1. 

Solution. The hexadentate complex [Ni(edta)] would have the largest overall formation constant because of the chelate effect. 

The structure of Cr(III)- and Fe(III)-EDTA complexes in aqueous solutions was investigated by Kanamori and co-workers [116,117]. The simultaneous existence of complexes on which the central trivalent atom is quinque-, hexa-, or heptadentate can be observed through the study of the Raman spectra in the region 300-600 cm where the skeletal vibrations of the coordination polyhedra appear. In Fe -EDTA complexes, the presence in the equilibrium of hexa- and heptadentate complexes was proposed. Furthermore, a dimeric complex formed in alkaline solutions which consists of two quinquedentate hexa-coordinate Fe(III) moieties with a Fe—O—Fe bridging unit was detected. Its main Raman band is located at 325 cm and assigned to Fe—O—Fe stretching [117], The Raman spectra of Cr(III)-EDTA complexes were studied at different pH s in the range 0.6-10.3. The forms, which exist in the both pH extremes, are well characterized at low pH (below pH 1.8), [Cr(HEDTA)(H20)l is the stable form inversely, above pH 7.4, only the hydroxo complex [Cd(EDTA)(OH)] is present. Between these forms, several possible pathways are discussed. The Raman results support the presence of the hexadentate complex [Cr(EDTA)] between pH 3.4 and 6.3. 

Fig. 2.2 Principle of using His-Tag proteins and IMAC chromatography columns to purify proteins. The matrix of the HiTrap Chelating Column forms a tetradentate complex with Ni +. His-Tagged proteins can then provide the final two bonds to form the stable hexadentate complex, emd immobilising the protein on the column...

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