Thiols, coordinated

Nitrosothiols are also readily decomposed by mercuric ion to the corresponding coordinated thiols and nitrous acid (in acid solution) as in equation 16. This reaction has been used as the basis of an analytical procedure for thiol determination30. Mechanistic... 

Some of the reactions of coordinated donor atoms are quite unexpected. Thus, coordinated thiol will react with alkyl bromides to form sulfonium salts. Here, there are lone pairs on the donor atom in addition to the one involved in forming the coordinate bond. Their qualitative reactivity is hardly affected by coordination. Ammonia which has been coordinated to platinum (IV) will form iV-chloro derivatives when treated with hypochlorite. This reaction involves an N-H bond rather than the lone pair. Coordinated PCI3 will undergo hydrolysis to give coordinated P(0H)3, a reaction which involves the P-Cl bonds rather than the lone pair. 

The reactions of coordinated thiols have been examined in more detail. In one of the first reports on this subject (17), the alkylation of (2-aminoethylthio)diethylgold to give a complex sulfonium ion was found to occur quite readily ... 

A kinetic study of the alkylation of coordinated thiol in NH2CH2-CH2SH and related ligands revealed that the reaction proceeded without the breaking of the metal-sulfur bond (11). The nucleophilicity of the coordinated sulfur was found to depend on the central metal involved but was greater than that of the sulfur in a thiol. It was found that sulfur in such ligands did not react with alkyl halides when it occupied a bridging position between two metal atoms in a polynuclear complex. 

Coordination of a ligand to a metal changes its properties by polarization of electron density by the positive charge on the metal ion but also can affect properties by more subtle effects of particular bonding type, such as ir back-bonding. Coordinated metals often stabilize molecules, as strikingly illustrated in the oxidation of a coordinated thiol to the corresponding coordinated sulfenic acid ... 

FIGURE 10.5 Potential energy diagram (enthalpy of reaction in kcal/mol) for oxidative addition of thiols. The first step involves the formation of a reversible 19 e coordinated thiol adduct that has an estimated enthalpy of binding of —3 to —6 kcal/mol. The transition state occurs when this is attacked by a second mole of chromium radical. The measured enthalpy of activation is near 0 kcal/mol since the exothermic enthalpy of binding in the first step is canceled by the activation enthalpy of the second step. Adapted with permission from reference 79. Copyright 1996, American Chemical Society. 

One mechanism consistent with many of the experimental observations [3] is shown in Figure 4. The initial activation of the binuclear site by reduction, protonation and consequent dissociation of water primes the site for binding dihydrogen. The activation of dihydrogen involves the formation of a hydride (proposed to bridge between the iron and nickel) and a coordinated thiol. In this manner dihydrogen is formally cleaved heterolytically into H- and H+. 

Soft metals such as Ag+, Cu+ and MeHg+ coordinate to Co SR" complexes and uncoordinated thioethers with about equal ease. Thus it seems that Co" and R" " have similar effects on the binding power of RS , reducing it by about 10 (Table 90 equation 167). This property has implications for electron transfer since ligation of coordinated thiol to a second metal centre is probably the first step in inner-sphere electron transfer. Data given in Table 90 demonstrate that Co" —SR" retains some Lewis basicity towards soft metals. These studies have utilized [Co(R-—S)(en)2] systems (R—S = cyst, tga) but the metals Ru"" and Co" also form very stable... 

The significance of this discovery is that control of the degree of polymerization and of coordination modes of the thiolate precursors is hkely to affect the shape of the final nanoproduct in a solventless method. Since traditional thiol-metal coordination chemistry is well established, this affords a great opportunity to join the traditional coordination chemistry with an increased variety of nano-... 

PBE dendrons coordinate to the surface of II-VI semiconductor nanocrystals (e.g., CdSe [33] and CdSe/ZnS core/shell structure [34, 35]) to modulate the photoluminescence of the nanocrystals [32]. Trioctylphosphine oxide (TOPO)-capped II-VI semiconductor nanocrystals of several-nanometers diameter have been synthesized by a pyrolysis reaction of organometallics in TOPO [33-35]. The capping ligand (TOPO) can be replaced by stronger ligands such as thiol compounds [36], suggesting that dendrons bearing sulfur atom(s) at the focal point replace TOPO as well. 

Thiols are known to be excellent hgands for the stabilization of gold and platinum nanoparticles. In this respect, we did not observe any Iluxional behavior [31,52] in solution NMR experiments for thiols coordinated to the surface of noble metal particles (Fig. 8). However, in the case of rutheniiun, we foimd the slow catalytic formation of alkyl disulfides [31]. After exclud-... 

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