We’ve learned a few types of organometallic reactions, and all of these involved changes
occurring with metal-ligand bonds.
Now let’s take a look at reactions that can occur on the ligands themselves, as these
will also be quite common.
First, some ligands may be susceptible to nucleophilic attack.
Take for example this generalized complex with a carbonyl ligand.
Some external nucleophile may attack the carbonyl carbon, just as we are familiar with from
organic chemistry, which will leave us with this more familiar-looking version of the
carbonyl.
In doing so, electron density has been pushed onto the metal, so it will exhibit an oxidation
state that is one lower as a result of the reaction.
This nucleophile is likely to have been a hydride, carbanion, hydroxide, alkoxide, or
amine.
We can see the same thing occurring with other types of ligands, such as this isonitrile.
The nucleophile attacks just as it did the carbonyl, leaving us with this imine, and
the lower oxidation state for the metal.
From here, if there is a source of acidic protons, the metal can actually provide another
bond to the carbon, allowing this pi bond to pick up the proton, leaving us with this
amino group.
So as you can see, a ligand can change quite a bit as the result of nucleophilic attack.
We can see how this works on carbene and carbyne metal complexes as well.
This is actually a bit more straightforward, as the nucleophile will simply attack the
carbon, and a pi bond will be pushed onto the metal, resulting in a lower oxidation
state for the metal.
For this type of reaction common nucleophiles will include carbanions, alkoxides, thiolates,
amines, and phosphines.
Finally, metal-ligand sigma bonds can be attacked, such as here where a metal from some complex
attacks this carbon atom, thus kicking off this other complex entirely, like an organometallic
version of the SN2 reaction.
Let’s also take a look at electrophilic attack on ligands.
Take for example this metal complex with an alkyl ligand.
If introduced to this trityl cation, the metal atom can push electrons onto this carbon,
forming a pi bond, which will cause a hydride to attack the trityl cation to reduce it.
This will result in a positive charge on the metal complex.
We can also see how metal-ligand pi bonds can interact with electrophiles.
For example, here is a metal with a triple bond to this alkylidyne.
One of these pi bonds can interact with an electrophile, most commonly a proton, leaving
this alkylidene ligand, and a positive charge on the complex.
Now in both of these situations chemistry is occurring at the position alpha to the
metal, but chemistry can occur at the beta position as well.
Here we see an alkyl ligand and the trityl cation, but this time, the metal can attack
this beta carbon, kicking off a hydride from this position to protonate the electrophile.
This will result in an ethene ligand, with pi to metal dative bonding as shown here.
Additionally, the beta position can attack when there is an alpha-beta pi bond.
Here, the metal atom generates a pi bond with the adjacent carbon, thus pushing this other
pi bond to coordinate with the electrophile.
This will result in an increase in the oxidation state of the metal.
This works not just for carbon-carbon pi bonds, but for carbonyls as well, where the oxygen
atom will coordinate to the electrophile.
And finally, chemistry can even occur at the gamma position.
Say there is a beta-gamma pi bond, and the metal attacks the beta position, pushing the
pi bond out to attack the electrophile.
This will result in a substituted alkene ligand that is now bound via pi to metal dative bonding.
With that understood, let’s check out some examples.
Here is a manganese complex, reacting with methyllithium.
What might this do?
Well this methyl is a very strong nucleophile, and we have some carbonyls here, so it is
quite clear that the methyl will attack one of these carbonyls to produce an acylmetal
species.
Here’s another example, a rhenium complex reacting with lithium phenoxide.
Again, this reagent is nucleophilic, so where might it attack?
Well we have this alkylidene, so the most likely reaction is attack on this carbon,
which pushes the pi bond up onto the metal, to yield this alkylmetal species.
Now here’s a ruthenium complex reacting with the trityl cation.
As we might guess, this cation will act as an electrophile, so what is the nucleophile?
Well these phosphines and carbonyls don’t look promising, but we have this benzyl ligand,
which means there is a CH2 adjacent to the metal.
The metal can push a pi bond onto the carbon and send a hydride over to coordinate to the
trityl cation, leaving us with this, and the complex will neutralize in the process.
As with the other reactions we’ve been learning about, the synthetic utility of such transformations
will become apparent a bit later in the series.
For now let’s move forward and wrap up this section on organometallic reactions.