The Conjugate Base Mechanism

Other cases in which second-order kinetics seemed to require an associative mechanism have subsequently been found to have a conjugate base mechanism (called S ICB, for substitution, nucleophilic, unimolecular, conjugate base in Ingold s notation ). These reactions depend on amine, ammine, or aqua ligands that can lose protons to form amido or hydroxo species that are then more likely to lose one of the other ligands. If the structure allows it, the ligand Irons to the amido or hydroxo group is frequently the one lost. 

In the third step, addition of a ligand other than water is also possible in basic solution, the rate constant is Ihe equilibrium constant for the overall reaction is foH  

Additional evidence for the conjugate base mechanism has been provided by several related studies  

Base-catalyzed exchange of hydrogen from the amine groups takes place under the same conditions as these reactions. 

The isotope ratio ( 0/ 0) in the product in 0-enriched water is the same as that in the water regardless of the leaving group (X = CF, Br , N03 ). If an incoming water molecule had a large influence (an associative mechanism), a higher concentration of should be in the product, because the equilibrium constant K = 1.040 for the reaction 

Data fromT Matsubara, C. Creutz, Inorg. Chem., 1979,18,1956. 

Other evidence for the conjugate base mechanism includes  

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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. ... 

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 third test of the conjugate base mechanism that we put forward was based on the idea that the first step should be written as an equilibrium, and the reaction rate should show specific hydroxide ion catalysis. If this is indeed in equilibrium, and deuterium exchange studies say that it must be, then the rate of the reaction must depend on the hydroxide ion concentration, and on nothing else. 

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 conjugate-base mechanism, (55)-(57), originally proposed in 1937 (108), is now widely accepted, although there is still some fine structure to elucidate. 

The systems where fc2 fc i offer a direct demonstration of the conjugate base mechanism. In... 

Deprotonation trans to the leaving group is especially effective at promoting the dissociation step. The conjugate base mechanism cannot operate if a tertiary amine with no ionisable proton is placed trans to the leaving group as expected the rate of substitution is then slower and does not depend on [OH-]. 

The mechanism becomes more dissociative for 3d ions later in the series. Substitution rates may be increased by the conjugate base mechanism. 

That [OH] appears in the rate equation shows it has a rate-determining role. However, this is not because [OH] attacks the metal centre but rather because it deprotonates a coordinated NH3 ligand. Steps 25.39-25.41 show the conjugate-base mechanism Deb or SnIc mechanism). A pre-equilibrium is first established, followed by loss of X to give the reactive amido species 25.1, and, finally, formation of the product in a fast step. 

Two observations that are consistent with (but cannot rigidly establish) the conjugate-base mechanism are 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. 

Octahedral substitution is also affected by base catalysis according to the conjugate base mechanism (SjglCB). The rate constant for substitution of Cr in [Co(NH3)5CI] is over a million times faster for OH than it is for H2O. In fact, the rate law for the base catalysis reaction is complex second order first order in [Co(NH3)5CI] and first order in OH. In reality, however, the reaction takes place by proton abstraction, as shown by Equations (I7.32)-(I7.34) ... 

A more detailed understanding of the conjugate-base mechanism for the hydrolysis of kinetically inert amine complexes of cobalt(III) and rhodium(III) is possible if liquid ammonia is used as a solvent. Such techniques allow the separation of parameters for the pre-equilibrium step ( Tcb) and the rate-determining step. A polyamine complex generally contains more than one potentially acidic proton and each proton is characterized by its own acidity constant (Kcb)- exchange rates for... 

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