We just learned about ligand exchange, so let’s check out another basic organometallic reaction
called oxidative addition. This is a way that new ligands can attach to a metal. When this occurs,
it will change the oxidation state of the metal, so it will be the most common with metals
which have stable oxidation states that are two apart, like neutral palladium and palladium +2,
rhenium +1 and +3, or iridium +1 and +3. The greater oxidation state is usually more stable. In
addition, there must be coordination sites open, unless ligand dissociation is to occur first..
Speaking generally, this reaction involves some metal in the n
oxidation state, reacting with some substance A-B, where the bond between A and B will break,
as each fragment becomes a separate ligand on the metal. This results in an oxidation state of
n+2 for the metal, and the ligands A and B could be either cis or trans to one another depending
on the situation. The most common situation would be cis-concerted oxidative addition. This is when
the two groups end up cis to one another, and the addition occurs all at once, in concerted fashion.
This is very common for molecular hydrogen as well as carbon-halogen bonds.
How does this work mechanistically? Well let’s take a look at this square planar iridium complex,
with a chloride, carbonyl, and bidentate phosphine ligand. The dz2 orbital on the metal projects its
lobes along the axis that has open coordination sites. Let’s say molecular hydrogen approaches
this orbital in perpendicular fashion. Hydrogen can donate electron density from this sigma bond
into this d orbital, which weakens the sigma bond. It can also accept
backbonding into its sigma star antibonding orbital, which weakens the bond further.
Because of this, the molecule will split, and we will get two hydrogens on the iridium center,
cis to one another. This also means that one ligand must get pushed to the other axial
position, and in this case that will be the carbonyl. We now have an octahedral complex,
and the iridium center has a oxidation state that is 2 greater than it was before.
There can also be stepwise oxidative additions. Whereas the concerted mechanism is more common for
nonpolar bonds like C-H or H-H bonds, the stepwise mechanism will be more common for polar bonds.
This is a two-step process, and for the first step the metal will interact with the less
electronegative end of this polar molecule, kicking off the rest. The A fragment will now
be a ligand on the metal, which will now have an oxidation state that is two greater than before,
and this intermediate will have a formal charge that is one greater than before as well.
Then the nucleophilic portion will attack and coordinate, thus restoring the original charge.
An example of this is as follows, with this platinum(II) complex, which reacts with methyl
iodide. The platinum atom attacks the methyl group yielding an SN2-like transition state,
and kicks off the iodide, so now there is a methyl ligand, and the complex has a 1+ charge.
Then the iodide will coordinate, returning the complex to the original charge. This could also
operate via a radical intermediate instead of a cation. We must however be aware of electron
count, because the metal may or may not be able to accommodate the additional ligand.
With the basic concept understood, let’s take a look at some examples. Here is a square planar
platinum complex with a bidentate diphosphine and two chloride ligands. If this reacts with
molecular hydrogen, it is quite easy to draw the product. Since the two hydrogens must end up cis
to one another, one of these chloride ligands gets pushed to one side, and this is what we get. There
are no other possible products. Now let’s look at a similar iridium complex with the same bidentate
diphosphine, a carbonyl, and a bromide. Let’s say it is to react with this triethylsilane. Now it
is certainly the weak silicon-hydrogen bond that will undergo oxidative addition, so let’s pull the
molecule apart there and put both groups on the metal. Remember that they must end up cis to one
another on the product, so either the carbonyl or bromide must get pushed to another location,
since the bidentate ligand is unlikely to do so. Also, the two groups being added can do so
in either orientation. So that actually leaves us with a mixture of four possible products.
We can have the carbonyl pushed to the apical position with the two groups added in either
arrangement, and then we can have the bromide pushed down with the two groups added in either
arrangement. So we must always be aware of the potential for a mixture of stereoisomers.
We should be aware that in addition to the H-H and Si-H bonds we just examined,
this mechanism also commonly involves B-B, B-H, Sn-H, Sn-Sn, N-H, O-H,
S-H, and C-X bonds, so we should be prepared to see this type of chemistry when these bonds are
present. C-H activation is more difficult, but it is synthetically very important.
C-C bond activation is rare but has been shown in some cases. We should also note that d0
metals can’t participate in oxidative addition because they can’t be oxidized in the same way
as metals with many d electrons, as they have no d electrons to lose. And that covers an introduction
to oxidative addition. Let’s move forward and examine some other important reactions.