Conjugate base mechanism hydrolysis

Fig. 2.12 Examples of non-linear Arrhenius (or Eyring) plots (a) 1u(A oh)7 " ) vs T for the base hydrolysis of trans-Co(en)2ClJ. Curvature may result when k, k2 and A// , not equalling A// in the conjugate-base mechanism (Sec. 4.3.4). Reprinted with permission from C. Blakeley and M. L. Tobe, J. Chem. Soc. Dalton Trans. 1775 (1987). (b) nk vs T for iron removal from C- and N-terminal monoferric transferrin (lower and upper scales respectively). Transferrin contains two iron binding sites = 35 A apart. Either of the two sites, designated C- and N-terminal, can be exclusively labelled by Fe(lll) ions and these may be removed by a strong ligand such as a catechol (see Sec. 4.11). Reprinted with permission from S. A. Kretschmar and K. N. Raymond, J. Amer. Chem. Soc. 108, 6212 (1986). (1986) American Chemical Society.

There is no reason to believe that the conjugate base mechanism does not apply with the other metal ions studied. Complexes of Cr(III) undergo base hydrolysis, but generally rate constants are lower, often 10 —10 less than for the Co(III) analog, Table 4.10. The lower reactivity appears due to both lower acidity (A"i) and lower lability of the amido species (kf) in (4.49) (provided k i can be assumed to be relatively constant). The very unreactive Rh(III) complexes are as a result of the very low reactivity of the amido species. The complexes of Ru(III) most resemble those of Co(III) but, as with Rh(III), base hydrolyses invariably takes place with complete retention of configuration. ... 

Examine the volume profile for the base hydrolysis on the basis of a conjugate base mechanism ... 

Acid hydrolysis of an octahedral metal ion complex is typically a dissociative or SNl-type reaction. In the case of base hydrolysis, reactions tend to display SN2-type reaction mechanisms, although others take place by what is termed an SnI-conjugate base mechanism. The latter involves attack by an electrophile to abstract a proton... 

Figure F shows the conjugate base mechanism for base hydrolysis. Dr. Tobe suggests essentially that in base hydrolysis, the hydroxide ion occupies a unique position for one of several reasons. Perhaps the hydroxide ion is hydrogen bonded...

The second critical test of this conjugate base mechanism is based on the fact that this five-coordinated intermediate, if indeed it exists, would not always have to react with the solvent, though the solvent would be what it would react with under most circumstances. We have run this type of base hydrolysis in the presence of many anions of high concentration, and the only thing that we can find is the hydroxo complex so at least in water solution, water seems to be what this five-coordinated intermediate picks up. But in dimethylsulfoxide it certainly is possible to throw in various anions, and since dimethylsulfoxide is not as good as water in coordination, other nucleophiles may react. We do find in dimethylsulfoxide that a base, such as hydroxide ion, speeds up the rate of base hydrolysis but the product, instead of being a hydroxo compound, is the complex corresponding to whatever anion we have added, such as nitrite ion, azide ion, and thiocyanate ion. 

Dr. Halpern This could be used in stabilizing, say an activated complex. The point about the hydrolysis observation is that this refers to the octahedral complex, whereas the explanations that have been offered for the effect of amide in the conjugate base mechanism are concerned, not with weakening of the binding, but with stabilizing a five-coordinated intermediate. I wondered if the role of the hydroxide in promoting water substitution might be of the same nature. 

The kinetics of substitution of bipy or phen into Co (467, 574), Ni (144), and Pt (697) complexes have been reported. Several studies of the hydrolysis of complexes of form [Co(III)(bipy)2XY] may be found. For both cis and trans isomers where X = Y = CY, hydrolysis is instantaneous (581), whereas for the cis isomer with X = acetate and Y = acetate or OH , reaction is very slow as a conjugate base mechanism cannot operate and the first-order reaction is therefore independent of [OH ] (124). One NO2 group in acid dependent and under acidic conditions is thought to proceed via protonation of one nitro group (289, 472). The interconversion of the cis and trans isomers, where X = N02 and Y = H2O, has an overall rate constant equal to + A [H+], implying reactions for both OH and... 

The Green-Taube experiment provides an elegant demonstration that a conjugate-base mechanism operates when base hydrolysis (with a fixed concentration of [OH]") of [Co(NH3)5X] + (X = Cl, Br, NO3) is carried out in a mixture of H2( 0) and H2( 0), it is found that the ratio of [Co(NH3)5( OH)]2+ to [Co(NH3)5( OH)] + is constant and independent of X . This provides strong evidence that... 

The systems where k2> k-i offer a direct demonstration of the conjugate base mechanism. In the reactions of the trans-RS and trans-RR(SSp isomers of the [Co(2,3,2-tet)Cl2] complex, examination of the reaction product showed that the act of base hydrolysis was accompanied by the exchange of one secondary amine proton. All other exchange was shown to take place after this product was formed. Comparison of the amount of proton exchange in recovered unreacted tra 5-[Co(en)2Cl2] with that in the recovered reaction product likewise indicated that an act of base hydrolysis required the removal of one amine proton. 

Dissociative mechanisms lead to products where the stereochemistry may be the same or different than the starting complex. Table 12.9 shows that cw-[Co(en)2L(H20)] is a hydrolysis product of both ct5 -[Co(en)2LX] and trfl 5-[Co(en)2LX] in acid solution. While these aquation reactions with pure ci5-[Co(en)2LX] lead exclusively to cis products, retention of the trans ligand orientation in fra 5-[Co(en)2LX] depends on both L andX. The conjugate base mechanism is unlikely in these reactions they are carried out in acidic solution. 

One other structural feature that has recently been explored in Cr(III) chemistry is the influence of the flat sec-NH proton on the rate of hydrolysis. The incorporation of this feature into the skeleton of a Co(III) chloro amine complex causes rate increases of up to 10" with respect to analogous systems where this feature is absent. However, mcr-[CrCl(en)(dpt)] has a base hydrolysis rate of only 5 times that of mer-[CrCl(en)(5Medpt)] (Table 6.7). On the basis of the activation volume data now becoming available for base hydrolysis it is possible that this process is more associative than previously thought and an interchange conjugate base mechanism for haloaminechromium(III) complexes has been proposed. 

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