Metal-thiolate bonds

Bond enthalpies reported by Dias and Martinho Simoes for a series of MCp2(XR)j complexes (where X = O and S) show that the M-S bonds are 24r-29 kcal/mol weaker than the corresponding M-0 bonds. For instance, the M-X bond enthalpy in Ti(Cp)2(SCgHj)2 is 86 kcal/mol, whereas the M-X bond enthalpy in the related alkoxo complex, Ti(Cp)2(OCgH5)2, is 110 kcal/mol. Thus, the early-metal-thiolate bond is weaker (or the early-metal-alkox-ide bond is stronger) than would be expected based on the H-X bond strengths. 

This greater strength of the late-metal-thiolate bond compared to the early-inetal-thi-olate bond or the late-metal-alkoxide bond has been rationalized in two ways. First, the late-metal-thiolate bond involves the favorable match of a soft ligand with a soft metal. Second, this bond involves a larger covalent component than the bond of a late metal to an alkoxide or an early metal to a thiolate. ... 

Phytochelatin a plant peptide produced in response to lieavy metals, e.g. cadmium, copper, mercury, lead and zinc. The structure is (Y-Glu-Cys) -Gly (n = 3-7). Like Metallothionein (see), P. form metal-thiolate bonds and thus sequester toxic metal ions. They are probably derived from glutathione rather than RNA-directed protein synthesis. 

Class I and/or II MTs have been described in all animals examined. Mammalian MTs have been some of the most extensively studied of the 61 or 62 amino acids, 20 are cysteine residues. Metal ions are bound to the MT exclusively through thiolate bonds involving all 20 cysteines (see Hamer, 1986). They associate with a wide range of metals in vitro, 18 different metals in the case of rat liver MT (Nielson etal., 1985). Divalent and trivalent metals exhibit saturation binding at 7 mole equivalents forming M7-MT, whereas copper (Cu(I)) and silver (Ag(I)) bind as monovalent ions forming M12-MT. The structure of these molecules is such that two metal-binding domains are formed an a-cluster from the carboxy-terminal portion of the protein, contains 11 cysteines which bind either 4 divalent or 6 monovalent ions the (3-cluster, the amino-terminal... 

Ligands generally form cr-donor bonds with metal ion sites. Several common ligands in bioinorganic chemistry also have strong re-donor interactions with the metal (thiolates and phenolates, in particular). 

Photoelectron spectroscopy (PES) has been shown to provide a convenient probe of metal ion effective nuclear charge and the nature of the metal-ligand bond via the energy of valence-electron photoionizations (16, 20, 22, 284, 285, 312, 332-334). Recently, PES spectroscopy has been employed in the study of oxo-molybdenum compounds of the type (L-A5)MoE(X,Y) [E = O, S, NO X, Y = halide, alkoxide, or thiolate] in order to evaluate the synergy between the axial (E) and equatorial (X,Y) donors in affecting the ionization energy of the HOMO localized on the Mo center (16, 284, 334). These studies have conclusively shown that equatorial dithiolene coordination electronically bulfers the Mo center in (L-A pMoEttdt) (Fig. 13) from the severe electronic perturbations associated with the enormous variation in the Ji-donor/acceptor properties... 

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