Metal-hydroxo bond

The kinetic results, which gave an indication of the relative reactivities of the aqua and hydroxo complexes, are thus in agreement with the metal-aqua and metal-hydroxo bond distances in these complexes (see Section IIIC) and illustrate the value of accurate structural parameters in predicting relative reactivities of similar complexes (208). 

In the case of copper complexes, the chemistry is less well understood. High valent copper-oxo species are not likely to be formed, consequently copper-hydroperoxo or copper-hydroxo species are usually proposed as active species in DNA oxidation. These species are thus more susceptible to homo-lytic cleavage of the peroxide or the metal-hydroxo bond and consequently, to 1-electron oxidation mechanism. However, the labeling of the product of deoxyribose oxidation at Cl by Cu(l,10-phenanthroline)2 clearly demonstrated that these complexes can mediate a 2-electron oxidation mechanism of DNA damage since the oxygen atom incorporated in DNA originates from H2O. 

Analogous reaction of Pt(Me)(OMe)(dppe) with CF2=CF2 in THF to give 6-methoxytetrafluoroethyl complex was proposed to involve migratory insertion step, without dissociation of OMe ion, though no direct stereochemical evidence has been provided [51], Apparent migratory insertion of alkenes to metal-hydroxo bond has also been observed in other metal complexes such as those of Ir [52]. 

More recently, the iridium complexes [Ir(PMe3)4]Cl, as well as the more electrophilic peroxo derivative [Ir(02)(PMe3)4]Cl, were catalyst precursors for a variety of nitriles RCN (R = Me, /7-NH2C6H4, p-OHCeHj) giving up to 800 turnovers in the hydration reaction realized at 140°C. Various general mechanisms for the hydration of nitriles are possible, two of which involve insertion into a metal-hydroxo bond (Scheme 26a) or nucleophilic attack of water upon coordination of... 

Methyl-Cobalamine and Related Compounds. There are three major types of cobalamine in animals and man, the methyl and hydroxo derivatives and the deoxyadenosyl derivative (coenzyme B12). The unique metal-carbon bond in the methyl derivative and in B12 is extremely photolabile, and their photolyses have been widely studied. However, radiolytic processes have not been widely studied. 

The centered position of the proton in the H3O2 bridges of compounds 15 to 20 is confirmed by the length of the two metal-oxygen bonds A bond M—O(OH) between a metal atom and a hydroxo-ligand is considerably shorter than the M—O(H2O) bond between the same metal and an aqua-ligand. In chromium(ni), for example. 

The 1,2-addition of C—H bonds across metal-heteroatom bonds has been reported for two different classes of complexes early transition metal d° complexes with imido ligands and late transition metal complexes with amido, hydroxo, and aryloxo ligands (Scheme 11.36). These transformations are potentially related to o-bond metathesis reactions discussed above however, the presence of a lone pair on the heteroatom that receives the activated hydrogen may impart important differences. 

Eyring and Wadsworth (50) and Little (51) present spectroscopic evidence that thiols bond to ZnO surfaces and xanthates bond to PbS surfaces by way of hydroxyl-exchange reactions in which metal-sulfide bonds are formed, and metal-hydroxyl bonds are broken during adsorption. We expect that trends exhibited in the relative stabilities of amino acid and hydroxo complexes of various dissolved cations should parallel trends in the relative reactivities and adsorption free energies, should this type of bonding control adsorption. 

The insertion of olefins into the metal-oxygen bonds of isolated alkoxo, phenoxo, or hydroxo compounds has been observed directly in a few cases. As will be noted in Chapter 11, a hydroxy or alkoxyalkyl group can be formed by insertion of an olefin into a metal-oxygen bond or by attack of hydroxide or an alkoxide on a coordinated olefin. Many studies described in Chapter 11 imply that this type of compound is formed by nucleophilic attack on a coordinatively saturated olefin complex, and this reaction has been proposed as the C-0 bond-forming step during oxidations of olefins catalyzed by palladium complexes. However, Henry provided some of the first evidence that the C-0 bond forms by insertion of olefins into metal-hydroxo and -alkoxo complexes under certain reaction conditions. ... 

The mechanism of the Wacker oxidation has been the subject of many mechanistic studies and much discussion for nearly 50 years. At this point, the identity of the elementary steps of this process appears to depend on the reaction conditions. The majority of the mechanistic discussion has focused on whether the C-O bond is formed by nucleophilic attack of water on a coordinated olefin or by insertion of an olefin into a metal-hydroxo complex. These elementary reactions were discussed in Qiapters 11 and 9, respectively. It appears that the mechanism involving nucleophilic attack occurs under conditions of high chloride concentration, and the mechanism involving olefin insertion occurs imder conditions of low chloride concentration. ... 

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