Eigen-Wilkins mechanism

In coordination chemistry, the Eigen-Wilkins mechanism describes the process of ligand exchange in metal complexes in aqueous solution from a mechanistic point of view. It was proposed by Manfred Eigen and Ralph G. Wilkins and named after them.

example

A chromium ³⁺ ion surrounded by six ammonia ligands

A Chrome ³⁺ - ion binds example, six water molecules to a Hexaaquakomplex . If ammonia (NH ₃ ) is added to an aqueous solution of this hexaqua complex , some or all of the water molecules in the complex are replaced by ammonia molecules in an equilibrium reaction, whereby the total number of ligands, i.e. 6, remains unchanged. If this exchange takes place quickly, the complex is also known as labile; if it takes place slowly, as inert .

The ligand exchange can be understood mechanistically according to the following scheme:

1. The ligand penetrates the outer solvate sheath (engl.  Outer sphere ) and is there by Coulomb interaction stabilizes (notation: M ^{n +}  ... L). None of the six ligands in the hexaqua complex are displaced. This process represents a "rapid pre-equilibrium". The equilibrium constant can be calculated according to the law of mass action : ${\displaystyle K_{\text{os}}}$

   ${\displaystyle K_{\text{os}}={\frac {[{\text{M}}^{n+}\cdots {\text{L}}]}{[{\text{M}}^{n+}]\cdot [{\text{L}}]}}}$
   
   If the mechanism tends to be associative ( i.e. if a negative activation volume is measured during exchange ), the transition state has a higher density than the starting materials and therefore requires less volume. According to this, associative mechanisms can be favored by increasing the pressure (see principle of le Châtelier ).

2. In the second, rate-determining step , the ligand displaces a solvent molecule from the outer solvation shell and a ligand pentaaqua complex is formed, in the example a chromium ³⁺ complex with one ammonia and five water ligands. The reaction rate  v _c is directly proportional to the metal ion and ligand concentration:

   ${\displaystyle {\begin{aligned}v_{c}={\frac {\mathrm {d} [{\text{Ligandpentaaquakomplex}}]}{\mathrm {d} t}}&=k\cdot [{\text{M}}^{n+}\cdots {\text{L}}]\\&=k\cdot K_{\text{os}}\cdot [{\text{M}}^{n+}]\cdot [{\text{L}}]\\&=k_{\text{eff}}\cdot [{\text{M}}^{n+}]\cdot [{\text{L}}]\end{aligned}}}$
   with the rate constant ${\displaystyle k.}$

literature

• M. Eigen, RG Wilkins: Mechanisms of Inorganic Reactions . In: Advances in Chemistry Series. No. 49, 1965, p. 55. American Chemical Society, Washington D. C.