A very common reaction for metal complexes is ligand substitution in which one ligand is exchanged with another. There are two common mechanisms for ligand substitution reactions the first of which is called the associative mechanism. In this mechanism the incoming ligand approaches the complex before departure of the leaving ligand. How can we spot an associative mechanism in experimental data, and what are some of the consequences of this mechanism? The prototypical mechanism of associative ligand substitution. The first step is rate-determining. A typical mechanism for associative ligand substitution is shown above. Let’s begin by examining the kinetics of the reaction. Reaction kinetics are commonly used to elucidate reaction mechanisms, and ligand substitution is no exception. Different mechanisms of substitution may follow different rate laws, so plotting the dependence of reaction rate on concentration of the species involved in the reaction often allows us to distinguish mechanisms. Associative substitution’s rate law depends on the concentrations of both starting materials. \[ L_nM–L^d + L_i → L_nM–L_i + L^d \] \[ \dfrac{d[L_nM–L^i]}{dt} = rate = k_1[L_nM–L^d][L^i] \] The easiest way to determine this rate law is to use pseudo-first-order conditions . Although the rate law is second order overall, if we could somehow render the concentration of the incoming ligand unchanging, the reaction would appear first order. The observed rate constant under these conditions reflects the consistency of the incoming ligand’s concentration (\(k_{obs} = k_1[L^i]\), where both \(k_1\) and \([Li]\) are constants). How can we make the concentration of the incoming ligand invariant, you ask? We can drown the reaction in ligand to achieve this. The small amount of the incoming ligand actually used up in the reaction has a negligible effect on the concentration of the “sea” of starting ligand we began with. The observed rate is equal to \(k_{obs}[L_nM–L^d]\), as shown by the purple trace below. By determining \(k_{obs}\) at a variety of \([L^i]\) values, we can isolate \(k_1\), the rate constant for the slow step. The red trace below at right shows the idea. Associative substitution under pseudo-first-order conditions. The reaction is “swamped out” with incoming ligand. In many cases, the red trace ends up with a non-zero y-intercept…curious, if we limit ourselves to the simple mechanism shown in the first figure of this post. A non-zero intercept suggests a more complex mechanism. We need to add a new term (called \(k_s\) for reasons to become clear shortly) to our first set of equations: \[rate = (k_1[L_i] + k_s)[L_nM–L^d]\] \[k_{obs} = k_1[L_i] + k_s\] The full rate law suggests that some other step (with rate ks[LnM–Ld]) independent of incoming ligand is involved in the mechanism. To explain this observation, we can invoke the solvent as a reactant. Solvent can associate with the complex first in a slow step, then incoming ligand can displace the solvent in a fast step. Solvent concentration doesn’t enter the rate law because, well, it’s drowning the reactants and its concentration undergoes negligible change! An example of this mechanism iis shown below. Associative substitution with solvent participation—a head-scratching mechanism for many an organometallic grad student! As an aside, it’s worth mentioning that the entropy of activation of associative substitution is typically negative. Entropy decreases as the incoming ligand and complex come together in the rate-determining step. Dr. Michael Evans ( Georgia Tech )