Other Cadmium Complexes

Given the state of the ion, one would expect the coordination chemistry of this metal ion to be dominated by examples of coordination to four two-electron donors. In this sense, the ligands containing thiolate or carboxylate groups are interesting to mimic zinc-thiolate or zinc-carboxylate active sites, which play a relevant role in bioinorganic chemistry and in the field of cadmium detoxification by chelation. 

Cadmium thiolates considered here include those compounds that, if mononuclear, only involve ligation by organothiolate anions (RS ) and, if polynuclear, only contain these groups as skeletal bridging ligands. A nice review on the structural chemistry of metal thiolate complexes was published in 1986 by Dance [80], 

The structural principles for [M (SPh)4] complexes were described in detail by Coucouvanis et al. [84]. The distortion of the tetrahedral MS4 core can be caused by intermolecular interaction between the thiolate sulfur atom and an ortho proton of the RS group. There are two possible conformations for the [M (SPh 4] unit  

Carballo, Castineiras, Donunguez-Martm, Garcia-Santos, and Niclos-Gutierrez 

The terminal positions on the adamantane-Uke cage can be substituted by other ligands such as halido ions, leading to complexes such as [Cd4(SC6H4Bu -4)7Cl3] [95] and [Cd4(SPh)6l4] [96]. 

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The reaction of dabco (l,4-diazobicyclo[2.2.2]octane) with Me2Cd yields a 1 1 adduct 197, which adopts a linear polymeric structure (Figure 34).255 The cadmium atom is coordinated by two dabco units and two methyl carbon atoms giving rise to a distorted tetrahedral environment. Finally, the organocadmium adduct 198 (Figure 35) has been isolated from the reaction of Me2Cd with Cd[(SeP-/-Pr2)2N]2.256 The solid-state structure consists of dimeric units where each methylcadmium unit is coordinated to three selenium atoms. The geometry about the cadmium center is tetrahedral with a Cd-C distance of 2.16 A, which is comparable to that observed in other cadmium alkyl complexes. 

Cadmium complexes have not been neglected in these and other physical studies. For example, an electron diffraction study of CdBr2 has been made, and it was concluded that the equilibrium structure corresponds to a linear Br—Cd—Br molecule, with a Cd—Br distance of 2.372 A and a Br—Br distance of 4.694 A.105 Raman and IR spectroscopic studies of CdCl2 have also been reported.106 The Cd2+—en system is of interest, and a detailed study of the vibrational spectra (IR and Raman) of [Cd(en)(N02)2] and its deuterated derivative has been reported.107... 

The preparation of some mixed ethylenediamine-oxalate or other ligand complexes of cadmium(Il) has been reported.190-192 The reaction of a mixture of cadmium oxalate and CdX2 (X = Cl, Br, I or SCN) with ethylenediamine gives the complexes Cd(en)X(C204)o.s. The... 

Cadmium complexes with glycine, 3,49 histidine,263 alanine,264,266 DL-norleudne,263 valine269 and numerous other amino acids267,268 have been reported. A study of the interaction of dipeptides with cadmium ions has also been described. 69... 

A number of zinc and cadmium complexes of adenine (82),554,555 adenine N-oxide,556 guanine,557 inosine,558 cytidine559 and other nucleosides560 have been studied. The structure of (9-methyladenine)ZnCl2 is polymeric each zinc ion is tetrahedrally coordinated to two chlorine atoms (Zn—Cl = 2.22 A), and to N-l and N-7 of neighbouring adenine moieties (Zn— N = 2.05A).561 A structural study of the related cadmium complex, CdCl2(DMSO)L (L = 9-methyladenine), has shown the complex to form a one-dimensional polymer. 2... 

Solution studies on the compounds [BU4N] [M(R2NCS2)3] (M = Zn or Cd R = Me or Et) show the zinc complex (unlike the cadmium complex) to be some 90% dissociated into the neutral bis complex and the free ligand anion. This is in accord with the structure of the solid complex,907 which shows that only one dithiocarbamato group is bidentate, the other two being formally unidentate. 

Because of the ease with which dimercaptopropanol can be broken down in the body there is a danger that chelation, followed by breakdown, will simply result in the translocation of the metal ions to other tissues such as brain or liver. High doses of dimercaptopropanol can adversely affect a number of essential metal-activated enzymes, such as catalase, carbonic anhydrase and peroxidase, and also produce dangerous systemic effects. Dimercaptopropanol cannot be used to remove cadmium because its cadmium complex is toxic to kidney tissue54). 

The first isolated and characterised species that could be envisioned as intermediates in the initiation step for the coordination polymerisation of epoxides when using metal carboxylate catalysts were complexes formed between cadmium carboxylates, solubilised in organic solvents by the tris-3-phenylpyrazole hydroborate ligand, and epoxides such as propylene oxide and cyclohexene oxide [68]. Other epoxide complexes with various metal derivatives have also been reported in the literature [69-72],... 

N3CI plane (207). The other halids and the cadmium complexes are isomor-phous with the appropriate forms, and undoubtedly possess related five-coordinate structures. A number of studies of the vibrational spectra of the [M(terpy)X2] species have been described, all of which support the formulation as isomorphous five-coordinate complexes (124, 171, 225, 371). The copper(II) complexes [Cu(terpy)Cl2] are isostructural, and a number of studies of the paramagnetic species doped into a host matrix of [Zn(terpy)Cl2] have been reported (23, 213). The zinc complex [Zn(terpy)Cl2] exhibits absorption maxima at 22,650 and 18,000 cm (23, 348). The mercury(II) halide adducts are not so well characterized, but may be prepared by the direct reaction or HgX2 with terpy (171) or by trans-metallation of [Ph2Sn(terpy)Cl2] with HgCl2 (471). They are thought to possess similar, five-coordinate structures. The structures of the 1 1 adducts of the nitrates M(N03)2 terpy are not known with any certainty (171,328). A Cd NMR study of Cd(N03)2 terpy has been reported (430). 

Some metals (cadmium, zinc, copper, and mercury) induce special protein complexes called metallothio-neines. Iron forms a number of other protein complexes (ferritin, hemosidern, and transferrin). 

The uncertainty for the cadmium complex has been increased considerably by the review due to the uncertainty in the value of the equilibrium constant. Aruga concurrently determined the corresponding quantities for the sulphate complexes and calculated theoretically the difference between the enthalpy change at / = 0 and 0.5 M to be 4.05 kJ-mol. The correction was estimated by the review to be (4.40 0.20) kJ-mol. A comparison of the sulphate data extrapolated to standard conditions with other calorimetric data in [73POW] indicates that the results reported by Aruga are low by about... 

A structural analog for the Cys4Zn motif of the ADA protein is constituted by the thiolate complex [ZnSPh(Tmph)].132 Related cadmium complexes have been also described.149 These studies are also pertinent to other enzymes that feature cysteine thiolate alkylation such as methionine synthase, methanohcoenzyme methyltransferase, farnesyl transferase. 

The extent of tht formation of a complex, denoted by the symbol n, is defined as being the average number of ligands bound to the metallic ion. It is equal to the ratio of the difference between the total ligand concentration and that remaining free, on the one hand, and the analytical concentration of the metallic ion on the other. In the case of the cadmium complexes, it is written as... 

Human exposure mainly arises from combustion of fuels, plants, and waste, and consumption of adventitious cadmium present in food and water [83]. Humans and animals breathe cadmium-containing particles (mainly the oxide) and ingest cadmium complexes with their food and drinks. Cigarette smoking is a major route of uptake, whereas skin contact is not widespread owing to the natural dilution of cadmium, except for occupational settings. Dietary cadmium is more concentrated in some food items such as shellfish, offal, grains, and seeds. Some crops, such as rice, soybeans, or wheat, are more likely to accumulate cadmium firom polluted soils than others. 

Other important complexones, such as polyaminopolyphosphonic ligands, cannot be modelled alone in the same way, since available literature data on their cadmium complexes are not sufficient to propose a reliable model. 

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