Explain associative interchange reactions? What is the mechanism?
1 Answer
Associative interchange, or
- the departing ligand
X is assumed to have a fairly weak bond with the metalM that the incomingY ligand can easily disrupt. That is,Y is a good nucleophile and theM-X bond is fairly weak. - the departing ligand
X and incomingY 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
The regular process is described in
Inorganic Chemistry, Miessler et al., pg. 458
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(t) = overbrace(k_1["Cplx"])^((b)) + overbrace(k_2["Cplx"][Y])^((a)) 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"MXL"_2"T" .
Referring to the diagram above:
-
In
(a) ,Y comes in and forms a 5-coordinate transition state, and displacesX with assistance fromT due to the trans effect. See here for a discussion on the trans effect. As both the complex andY participate in one step, this is second order overall. -
In
(b) , the solventS comes in and displacesX first. This step is presumed slow, while the second step, in whichY displaces the solventS , 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.