# Explain associative interchange reactions? What is the mechanism?

Aug 14, 2017

Associative interchange, or ${I}_{a}$ (as opposed to dissociative interchange, ${I}_{d}$), since:

• the departing ligand $X$ is assumed to have a fairly weak bond with the metal $M$ that the incoming $Y$ ligand can easily disrupt. That is, $Y$ is a good nucleophile and the $M - X$ bond is fairly weak.
• the departing ligand $X$ and incoming $Y$ basically swap by the time the reaction is over.

However, there are two accepted variations of this mechanism that I know of.

Consider a square planar complex $\text{MXL"_2"T}$, where $T$ is the ligand trans to the departing ligand $X$.

The regular process is described in $\left(a\right)$, and a solvent-assisted process is described in $\left(b\right)$, in the figure below.

Typically, a 5-coordinate transition state forms, but a 6-coordinate transition state is also known, with assistance from solvent (Inorganic Chemistry, Miessler et al., pg. 458).

A two-term general rate law is frequently used to describe a square-planar substitution process:

$r \left(t\right) = {\overbrace{{k}_{1} \left[\text{Cplx"])^((b)) + overbrace(k_2["Cplx}\right] \left[Y\right]}}^{\left(a\right)}$

where:

• the ${k}_{1}$ portion describes the pseudo-first-order solvent-assisted process.

• the ${k}_{2}$ portion describes the regular, second-order association process. The complex referred to pertains to $\text{MXL"_2"T}$.

Referring to the diagram above:

• In $\left(a\right)$, $Y$ comes in and forms a 5-coordinate transition state, and displaces $X$ with assistance from $T$ due to the trans effect. See here for a discussion on the trans effect. As both the complex and $Y$ participate in one step, this is second order overall.

• In $\left(b\right)$, the solvent $S$ comes in and displaces $X$ first. This step is presumed slow, while the second step, in which $Y$ displaces the solvent $S$, is presumed fast.

Since the solvent is presumed in large excess, the order of $S$ is approximately zero, and thus, the solvent-assisted mechanism is pseudo-first-order in the complex.