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7.2.3: The Kinetic Chelate Effect

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    207004

  • We discussed the increased thermodynamic affinity of chelating ligands in a previous section on the chelate effect. This thermodynamic benefit is related to a change in the rate of binding and dissociation events. 

    Recall that there is a relationship between the equilibrium constant and rate constants of any chemical reaction. The relationship is demonstrated below for a general reaction.

    \[aA+bB \rightleftharpoons cC+dD \]

    \[\text{forward rate} = k_f[A]^a [B]^b \]

    \[\text{reverse rate} = k_r[C]^c [D]^d \]

    At equilibrium, the rate of the forward reactions is equal to the rate of the reverse reaction. Therefore:

    \[ \text{forward rate}  = \text{reverse rate} \]

    \[k_f[A]^a [B]^b = k_r[C]^c [D]^d \]

    Rearrangement of the equation above gives:

    \[\dfrac{k_f}{k_r}=\dfrac{[C]^c [D]^d }{[A]^a [B]^b } = K_{eq} \]

    Because chelating ligands have larger values of \(K_{eq}\) than analogous monodentate ligands, there must also be a change in the relative rates of the forward (binding) and reverse (dissociation) reactions. In fact, there is a kinetic benefit that makes a metal bound to a polydentate ligand more inert than the analogous complex with monodentate ligands: this is called the kinetic chelate effect.

    The kinetic chelate effect is a result of a slower first dissociation step and faster re-association step relative to that of a monodentate ligand. The complete dissociation of a bidentate ligand, for example, would require two dissociation steps (see first reaction in figure below). It is the first dissociation step that is slower in a bidentate ligand (\(k_1 < k'_1\) in the figure below). This first dissociation step is slower in part because the chelate would have to rotate to move the free ligand away from the open coordinate site on the metal ion. For the same reason, the reverse of this step (association) is faster than the association of a monodentate ligand (\(k_{-1} > k'_{-1}\)).

    Image of step-wise dissociation of a bidentate ligand and analogous monodentate ligands. Top reaction shows the metal bound to a bidentate ligand through two nitrogen atoms in the presence of two ligands. The reaction rate is k-one. The first step requires replacement of one NH2 group with the ligand. A second step replaces the second NH2 group with the second ligand. The lower reaction shows a metal bound to two monodentate ligands in the presence of two ligands. The rate of the forward reaction k-one-prime is for replacement of one monodentate ligand. Compared to k-one, the k-one-prime is smaller such that dissociation of a monodentate ligand is faster than for a bidentate ligand. The k-one-prime-reverse rate constant is greater than the k-one-reverse such that association for a bidentate ligand is faster than the association of a monodentate ligand A second reaction replaces the second monodentate ligand.

    Figure \(\PageIndex{1}\): Step-wise dissociation of a bidentate ligand and analogous monodentate ligands. (CC-BY-NC-SA; Kathryn Haas)

     

    Sources:

    (1) Carter, M. J.; Beattie, J. K. Kinetic Chelate Effect. Chelation of Ethylenediamine on Platinum(II). Inorg. Chem.1970, 9 (5), 1233–1238. https://doi.org/10.1021/ic50087a044.

    Attribution:

    Modified or created by Kathryn Haas (khaaslab.com)

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