Now that we have a general understanding of the structure of transition metal complexes
and some other background information, we are ready to start learning about the types
of chemical reactions these complexes will undergo.
The first type is fairly straightforward conceptually.
It is called ligand substitution, and it involves one or more ligands being exchanged for that
same number of some other ligand, as is abbreviated here.
We have two ligands, L and L prime, and the number of L ligands, represented by x, is
reduced by n, while n L prime ligands join the complex.
To keep things simple, let’s look primarily at situations in which only one ligand is
exchanged.
This will most commonly be a two-electron process, and this can occur in one of three
ways.
It can be an associative process, which is sort of like SN-Ac.
The new ligand joins the complex, forms a more ligated intermediate, and then some other
ligand leaves.
It can also be a dissociative process, which is sort of SN1-like.
A ligand leaves, and then some other ligand joins the complex.
Or it can be like an SN2 reaction, with a new ligand coordinating simultaneously to
another ligand leaving, which we can call an interchange reaction.
The associative process is preferred by 16-electron complexes, as association of another ligand
forms an 18-electron complex, which is stable.
This is also encouraged by open coordination sites that may be on the metal, such as with
square planar complexes that can be attacked from the top or bottom.
The dissociative and interchange processes are preferred by 18-electron complexes because
the metal is already stable and usually crowded in terms of coordinating ligands, so one ligand
must leave to make room for another.
For example, here is an octahedral molybdenum complex with six carbonyl ligands.
If one of these is to be substituted with trimethylphosphine, a carbonyl will first
dissociate to generate a 16 electron complex, then the new ligand will associate to restore
the 18 electron count.
In addition, there can be one-electron processes.
An activation can occur by way of oxidation or reduction, where the complex can either
gain or lose one electron, thus priming it for ligand exchange.
If the LUMO is nonbonding or modestly antibonding, reduction is likely, while if the HOMO is
nonbonding, oxidation is likely.
The gain or loss of an electron can also occur simultaneously to ligand exchange, whether
associative, if the coordination environment is uncrowded, or dissociative, if the coordination
environment is crowded.
When we combine these concepts, we can identify four common pathways.
First, oxidative activation results in MLx+, followed by association of a nucleophilic
ligand, and then subsequent loss of another ligand.
This is called SON2, S for substitution, O for oxidative activation, N for nucleophilic
ligand, and 2 for bimolecular intermediate, like the bimolecular transition state in SN2.
SOE1 maintains oxidative activation, but is followed by dissociation of a ligand to form
an electron-rich intermediate, and then subsequent coordination of an electrophilic ligand, so
the 1 is for unimolecular intermediate, like in SN1.
SRN1 utilizes reductive activation to form MLx-, followed by dissociation of a ligand
to form an electron-poor intermediate, and subsequent coordination of a nucleophilic
ligand.
So again the 1 refers to dissociation of a ligand first, then another ligand binding.
And lastly SRE2 includes reductive activation, association of an electrophilic ligand, and
subsequent loss of another ligand.
Now there is a bit to talk about in terms of metal-ligand bond strength.
First of all, these bonds are much weaker than those between carbon and these same ligands.
They have much lower bond dissociation energies.
That’s what makes the ligands fairly easy to replace, and the more easily they are displaced,
the more labile we can say they are.
But generally speaking, these bonds get stronger going down a group, most likely because as
the valence shells get larger, the d orbitals on the metal extend farther into space, allowing
for more overlap.
So 5d metals make stronger bonds than 4d, which are stronger than 3d.
In addition, polyhapto ligands like benzene or the cyclopentadienyl anion make stronger
interactions than single donor ligands like amines.
In addition, certain polyhapto ligands can shift from an eta-5 coordination to eta-3
coordination, which leaves two fewer electrons coordinated to the metal, and opens up a coordination
site for a new ligand.
So this slippage phenomenon is another interesting way a complex can be primed for ligand exchange.
Now we must address an important question.
If a transition metal complex has several different ligands, and it is to undergo ligand
exchange, will the exchange we want to carry out be thermodynamically favorable, and which
ligand will be replaced?
There is a lot to discuss here, so let’s look first at something called trans influence.
This applies especially to square planar complexes so let’s look at those now.
Ligands that are trans to one another in a transition metal complex, meaning those that
are directly opposite one another, along one line through the metal center, are interacting
with the same orbital on the metal, so they influence one another and compete for orbital
overlap.
The stronger one bond is, the weaker the bond in the trans position.
Here is a list depicting the relative strengths of the trans influence.
Hydrides are very strong, then phosphines, then thiocyanates, then relatively equal are
iodide, methanide, carbonyl, and cyanide, followed by bromide, chloride, amine, and
hydroxyl.
If two ligands are opposite one another, the one that is higher up on this chart will be
making the stronger bond to the metal.
Now in determining which ligand will act as a leaving group in ligand exchange, we have
to learn the trans effect.
This is similar to trans influence, except we are not specifically looking at thermodynamics
but rather kinetics.
The trans effect is the effect of other ligands in accelerating the rate of ligand replacement
of the group trans to themselves.
The main difference is that it takes into account strong pi-bonding and backbond accepting
ligands such as carbonyl and cyanide ligands.
This will be the more important trend to memorize, so take a look at this.
We have carbonyl, cyanide, and ethene at the top.
These will strongly promote the leaving of the ligand across from themselves.
This is followed by phosphines and hydride, then methanide, then the cyclopentadienyl
anion, nitrite, iodide, and thiocyanate, followed by bromide and chloride, then pyridine, ammonia,
hydroxide, and water.
Let’s apply this to an example to help cement the concept.
Here is a square planar platinum complex with four chloride ligands.
One of these is to be replaced by ammonia in associative ligand exchange.
Since they are all the same, it doesn’t matter which one leaves.
If a second exchange with ammonia is to then take place, now there is a difference.
We can either get the two ammonia ligands cis, meaning next to each other, or trans,
meaning across from each other.
Chloride is stronger than ammonia in terms of the trans effect, so the ligand across
from ammonia is not as likely to be replaced as one across from chloride.
For this reason, we will get the cis isomer, also known as cisplatin, which is a chemotherapeutic
agent.
To be thorough, let’s take another platinum complex, this time with four ammonia ligands.
If one of these is to be replaced by chloride, at first it doesn’t matter which.
But then if this is to happen a second time, we must look at the trans effect.
Again, chloride is higher up on the trend than ammonia, so the ligand opposite the chloride
is most likely to be replaced.
This results in transplatin, which is clinically ineffective.
We must also mention a related concept called the cis effect.
This is the tendency of a ligand to encourage the dissociation of a neighboring ligand,
one in a cis position.
This is the general trend for the cis effect.
The reason these ligands exhibit this behavior can be different from one to another.
For the acetate ion, for example, this occurs because the ligand can become bidentate and
stabilize the intermediate through chelation at the site where the cis ligand is lost.
For thiolate, chloride, and bromide, this is because they are able to form bridges where
they weakly interact with a second metal atom, and for two of these interactions to occur
with separate intermediate complexes, ligand dissociation must have occurred at the cis
position for each of them.
Still other ligands encourage dissociation by stabilizing square-pyramidal intermediates
through subtle electronic effects.
At any rate, this trend can be referred to in order to determine the ability of a ligand
to promote the cis effect, however it should be noted that the cis effect is much weaker
than the trans effect, and we will tend to ignore it unless it is the only phenomenon
that is relevant in a complex.
With all of this understood, let’s just look at a couple of examples and make sure
we can predict reaction products.
Here is a platinum complex with one carbonyl and three chloride ligands.
Which ligand will ammonia replace?
Well the carbonyl is stronger than chloride in terms of the trans effect, so the ammonia
will end up trans to the carbonyl.
Then let’s try this other platinum complex, with ammonia and three bromides as ligands.
Where will the next ammonia go?
Here, bromide beats ammonia, so although ammonia could replace itself, that would not yield
a new product, so instead let’s have one of these bromides that are cis to ammonia
do the directing, and the new ammonia ligand will go here.
We should be able to do this with a sequence as well.
Here is platinum with four chloride ligands.
If nitrite reacts, followed by ammonia, what will happen?
First the nitrite will displace any chloride.
Then, because nitrite beats chloride in terms of the trans effect, ammonia will end up trans
to the nitrite.
So that gives us a basic introduction to ligand exchange and some guidelines that we can follow
to have a good sense of what will happen in any given situation.
This is only the first of many types of organometallic reactions, but it is a very important one
in catalysis.
It is frequently the case that substrates become ligands on the metal in order to undergo
transformation, so reactions involving ligand association and dissociation are an integral
part of catalytic cycles.
However, before we dive into the specifics of these catalytic cycles, we must continue
to learn about some other kinds of organometallic reactions, so let’s move forward and do
just that.