In this video, we're going to take a look at cross-coupling reaction mechanisms. Now, the detailed mechanisms for many of these reactions are still debated, but it is accepted that all of them follow 3 stages. These stages include oxidative addition, transmetalation, and reductive elimination. Now all the coupling reactions will have some form of these three stages. Oxidative addition isn't always going to be the first step but it will be found somewhere in the beginning of these reaction mechanisms. Transmetalation will be found somewhere in the middle of all these coupling reactions and then reductive elimination will be near the end of these coupling reactions. Now starting out with oxidative addition, we're going to say a transition metal complex where 𝐱 represents a transition metal and ℓ is just a set number of ligands attached to it right from the beginning, usually 2 or 4, reacts with a carbon halide by inserting itself into the 𝐼₁𝑱 bond. So this represents our carbon halide.

Now, this step can happen by a variety of mechanisms. A concerted effort or concerted process means it happens all at once, so it's a one-step process. So here, if we're taking a look at a generic oxidative addition step, we have our carbon halide and we have our transition metal complex. Now, the transition metal has a lone pair from its 𝑨 orbitals, and it uses them to connect to the halogen or our 𝑱 group. While it's connecting to that 𝑱 group, at the same time, the bond between 𝐼₁ and 𝑱 will break and it will connect to the transition metal. So what winds up happening is we're going to have our transition metal connected to a set number of ligands and then we're going to have 𝐼₁ attached to the transition metal and 𝑱 as well. So this represents the oxidative addition step.

Now, in this process, we're going to say both of the new bonds formed, so the bond here and this bond here, they behave like 𝑱 type ligands. And as a result of behaving like 𝑱 type ligands, they cause the electron count to increase by 2. So as we do examples of oxidative addition, we'll see that the electron count of my transition metal complex is going to increase by 2. We're going to say here also recall this part of the cycle is driven by the 18 or 16 electron rule. Remember, transition metals want to try to reach 18 electrons ideally and 16 in other cases. So they'll welcome the addition of your 𝐼₁ and 𝑱 group to the transition metal complex because it gets them closer to these ideal values of 18 and 16 for the transition metal. Now that we've seen this generic layout of oxidative addition, click on to the next video and see how we approach the example where we're asked to create the new complex for palladium after oxidative addition.