LS2. Mechanistic Possibilities

There are two basic steps in ligand substitution: association and dissociation. Association, in this case, refers to the binding of a ligand to the metal. The ligand donates an electron pair to the metal and the two molecules come together to form a new bond. Dissociation, in this case, refers to the release of a ligand from a metal. The metal-ligand bond breaks and the ligand leaves with its electron pair.

Two mechanistic possibilities seem pretty obvious. Either the new ligand binds first and then the old one leaves, or the old ligand leaves first and then the new one binds.

In an associative mechanism, the new ligand starts the reaction off by associating with or binding to the transition metal. That's the bond-making step. The bond-breaking part, or dissociation, comes later.

Figure LS2.1. An associative substitution mechanism.

In a dissociative mechanism, the old ligand goes ahead and dissociates from the transition metal before the new ligand has arrived. That's the bond-breaking step. The bond-making part, or association, comes later.

Figure LS2.2. A dissociative substitution mechanism.

• Associative mechanism is association first. The new ligand binds and then the old one leaves.
• Dissociative mechanism is dissociation first. The old ligand leaves and then the new one binds.

Knowing the mechanism is important because the mechanism has an impact on what factors affect the reaction. For example, if the reaction is associative, adding lots more new ligand may speed up the reaction, because then it becomes more likely that the new ligand will find the metal complex and bind with it. However, if the old ligand is supposed to leave before the new ligand arrives, then it doesn�t matter how much new ligand is around. It has to wait for the old ligand to leave before it can bind, anyway, so adding a lot more new ligand won't speed things up.

a) Which kind of step costs more energy: bond-making or bond-breaking?

b)  What would be the rate-determining step in the associative mechanism?

c)  What would be the rate-determining step in the dissociative mechanism?

d)  What would be the rate law for the associative mechanism?

e)  What would be the rate law for the dissociative mechanism?

Problem LS2.2.

Draw a mechanism, with arrows, for the following substitution.  Assume an associative mechanism.

Problem LS2.3.

Draw a mechanism, with arrows, for the following substitution.  Assume a dissociative mechanism.

Problem LS2.4.