## Posts Tagged ‘**steady state approximation**’

## Dissociative Ligand Substitution

Associative substitution is unlikely for saturated, 18-electron complexes—coordination of another ligand would produce a 20-electron intermediate. For 18-electron complexes, dissociative substitution mechanisms involving 16-electron intermediates are more likely. In a slow step with positive entropy of activation, the departing ligand leaves, generating a coordinatively unsaturated intermediate. The incoming ligand then enters the coordination sphere of the metal to generate the product. For the remainder of this post, we’ll focus on the kinetics of the reaction and the nature of the unsaturated intermediate (which influences the stereochemistry of the reaction). The reverse of the first step, re-coordination of the departing ligand (rate constant *k*_{–1}), is often competitive with dissociation.

### Reaction Kinetics

Let’s begin with the general situation in which *k*_{1} and *k*_{–1} are similar in magnitude. Since *k*_{1} is rate limiting, *k*_{2} is assumed to be much larger than *k*_{1} and *k*_{–1}. Most importantly, we need to assume that variation in the concentration of the unsaturated intermediate is essentially zero. This is called the **steady state approximation**, and it allows us to set up an equation that relates reaction rate to observable concentrations Hold onto that for a second; first, we can use step 2 to establish a preliminary rate expression.

(1) rate = *k*_{2}[L_{n}M–◊][L^{i}]