Cadmium coordination spheres

Varying ratios of the ligands 7V-(2-thiophenyl)-2,5-dimethylpyrrole and V-methylimidazole were used to form tetrahedral zinc complexes with S4, S3N, and S2N2 coordination spheres. X-ray structural analyses and IR spectra were recorded for all compounds and the relevance to zinc finger proteins was discussed. The comparison to cobalt and cadmium structures showed only minor differences, supporting the theory that changes on substituting these metals into zinc proteins would be minor.538... 

A novel cadmate complex has been formed by the reaction of Cd(NH2>2 with I C=CH in the presence of acetylene in liquid ammonia.250 The potassium salt, K2Gd(CCH)4-2NH3 191, has been structurally characterized. The cadmium center is tetrahedrally coordinated to four acetylide units with which it forms Cd-C bonds of 2.23-2.25 A (Figure 29). The acetylide ligands are 7r-coordinated to two crystallographically distinct potassium ions whose coordination sphere is completed by two ammonia molecules. 

The difference in behaviour has been attributed to the preferred formation, by cadmium(II) and mercury(II), of [MX2]oo chains in which pairs of chloride anions link metal centres to give a square planar MX/ arrangement, the octahedral coordination sphere then being completed by two monodentate bu molecules [MX2]oo chain formation does not occur for copper(II) and zinc(II) and so the anions only occupy two of the six available co-ordination sites allowing bu to act as a bidentate chelating ligand. 

The very reactive Rieke cadmium metal was allowed to react under vacuum with benzoquinone in THF. This resulted in the formation of several different paramagnetic species, [Cd(C6H402 )(THF)3]+,C6H402 or [Cd(C6H402 )(THF)3]+ + C6H402 [215]. The tetrahedral coordination sphere for each species consists of three THF molecules and an anion radical that is asymmetrically or symmetrically coordinated to the metal dication. The original apparatus for the generation of the benzoquinone anion radical via electron transfer from cadmium metal was reported... 

The ability of cyclam derivatives to bind heavy metal ions [131] has been exploited with (60) [75] which shows good fluorescence enhancements with zinc and cadmium ions at pH 10. In such polyamine receptors, it is necessary that all the nitrogen lone pairs occupy the first coordination sphere of the metal ion in order that the fluorescence is efficiently revived. The unusual spectral shape of... 

Using a different metal salt, Cd(NOj)2, instead of CdClj oxidation reaction results in a slightly higher yield of the cadmium texaphyrin complex [60, 65]. However, upon purification, the product is obtained as a mixture of crystalline and non-crystalline solids. A single crystal X-ray diffraction study of the crystalline portion of the sample gave an unexpected result. The structure obtained (Fig. 20) revealed a six-coordinate pentagonal pyramidal cadmium (II) complex 156 (c.f. Scheme 20) where one of the two possible axial ligation sites is occupied by a benzimidazole [65]. The five donor atoms of the pentadentate texaphyrin macrocycle complete the coordination sphere about the cadmium with the cadmium... 

The first EXAFS studies of a cadmium environment in a protein were achieved for CdsZna- and Cdy-Metallothionein from rat liver (45). The two samples manifest identical EXAFS and the data are consistent with a shell of four sulfur atoms at —2.51 A. These results demonstrate that the inequivalence of cadmium atoms observed by ii Cd NMR studies of metallothioneins (46) does not arise from marked variations in atom type, coordination number, or metal-ligand distances within the metals first coordination sphere. 

A further complication in the identification of target sites and chemical forms of metals is the kinetic lability of coordinate covalent bonds. Metal ligands exchange rapidly in and out of the coordination sphere, in particular for first-row transition metals. This kinetic lability varies between metals, and, as indicated above, is influenced by the nature of the ligand, whether mono- or multidentate, and by the pH and ionic strength of its immediate environment. Copper, for example, forms relatively low affinity complexes with albumin or amino acids, but is tightly bound to ceruloplasmin. Similarly, mercury and cadmium form kinetically labile complexes with amino acids, glutathione, or albumin, but more stable chelates with metallothionein. 

The Pd-Cd-bonded complex [CdPd(CH3COO).j-CH3COOH]2] (123) was obtained from the reaction between palladium acetate and cadmium acetate in glacial acetic acid [89], The crystal structure shows that the coordination sphere of cadmium adopts a trigonal-prismatic geometry. Ozerov et al. prepared the Pd-Zn-bonded complex 124 through different reaction pathways (Scheme 12.37) [90]. Complex 124 represents the first Pd-Zn-bonded compound with Pd-Zn distances of2.3793(10) and 2.3723(10) A. 

Because of our interest in utilizing Cd NMR spectroscopy as a probe for calcium sites in biological systems (42,43,49,51), we have been interested in the isotropic Cd chemical shifts in these compounds. In the systems studied (Concanavalin A (42), Parvalbumin (43), Troponin C (50),. Calmodulin (49), and Insulin (51)), replacing calcium with cadmium places cadmium in an environment in which all of the atoms in the first coordination sphere are oxygens. The cadmium is assumed to be six coordinate. The isotropic chemical shifts for these compounds (82) fall into a characteristic range of chemical shifts from -85 to -130 ppm with respect to 0.1 M Cd(C104)2 To date, there are no model compounds that can be studied in aqueous solutions which have isotropic shifts within this range. 

Second sphere coordination has also been proposed on the basis of Cd NMR of binary and ternary complexes of cadmium-substituted The binding of coenzyme to the... 

The magnitude of scalar coupling constants provides valuable information concerning rates of exchange and the existence of inner- or outer-sphere coordination of ligands. For this reason, representative cadmium scalar coupling constants have been collected in Tables 10-14. The footnote references of these Tables contain additional values. 

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