We just learned about bonding in transition metal complexes, as well as a variety of ligands that
are commonly involved in them. Now it’s time to look at some interesting situations where ligands
bind to the metal atom in more complex ways. First let’s examine bidentate ligands.
Rather than the monodentate ligands we examined in the previous tutorial,
bidentate ligands are ligands which bind to the metal from two different parts of the ligand.
Take for example ethylenediamine. Here, each amino group behaves precisely as an amine would,
donating a pair of electrons to the metal, but since they are part of the same molecule,
it results in this five-membered ring type of structure. So this is a singular ligand
donating two pairs of electrons, after which the metal could bind with other different ligands.
Another bidentate ligand is the oxalate ion. This has two carboxylate moieties that can each
interact with the metal atom. It doesn’t stop at bidentate, we can also have polydentate ligands,
which make more than two interactions with the metal atom. Take this one for example,
ethylenediaminetetraacetic acid, or EDTA. This is technically a hexadentate ligand, as it interacts
with the metal atom from six different positions, making this octahedral complex.
When ligands bind from multiple positions we can also call this chelation, and the ligands which
can do this are chelating ligands. There are many such ligands, and we will become familiar
with some of them as we progress through the series. The denticity of a ligand is often
represented by the symbol kappa, and this will describe how many atoms are bound to the metal,
so a monodentate ligand has a denticity of one, a bidentate ligand has a denticity of
two, and so forth, where ligands may even exhibit differing denticity depending on the situation.
Next we are going to introduce a very interesting concept, that of the polyhapto ligand, where
hapticity is a word that describes the bonding mode of a ligand to a metal center. With these
polyhapto ligands, we will see a delocalization of the bond between the ligand and the metal.
Take for example this generalized alkene. This can behave as a ligand, donating p pi electron
density from its HOMO to the metal atom. However, the overlap that occurs is not very intuitive.
The way we will depict this is with the p orbitals in the alkene sitting parallel to the dz2 orbital
on the metal, and overlap occurring in this fashion, where neither of these individual
lobes are specifically overlapping, but rather the center of the pi bond is what is aligned.
And because of the specific orientation, we will depict this in line notation in this fashion,
with the arrow going from the center of the pi bond to the metal atom.
Then if possible, the LUMO, or p pi star orbital, will accept electron density from the metal
as a backbonding interaction, and both of these interactions are happening simultaneously.
Because electron density is being put into the antibonding orbital of the alkene, the double bond
lengthens and weakens. If this backbonding occurs, we will no longer depict this in linear fashion,
instead we may choose to represent it as a three-membered ring, like this. An alkyne
will behave quite similarly. The only difference is that an alkyne has two pi bonds, so it has two
sets of overlapping p orbitals. Here, if each pi bond interacts with the dz2 orbital of the metal,
but to only half the degree as in the previous example, this will still total to one sigma bond
for the interaction, as only two electrons are being donated. Additionally, the alkyne
can donate four electrons, which would be the case if one pi bond interacted with one orbital
and the other pi bond, being in a different plane, interacted with a different orbital.
Now things will get even a bit trickier as we investigate
ligands that have delocalized pi systems. Take for example this eta-3 allylic system,
C3H5-. This means that there is a formal negative charge which is completely delocalized.
Now what the eta-3 means is that all three carbon atoms are interacting with the metal. So this is
not just an interaction with a terminal carbon, as would be the case with an eta-1 allylic ligand.
This ligand will be depicted as such, with the bond connecting the metal atom with the middle
of the delocalized pi system on the ligand. To fully understand this, we must look at the
molecular orbitals from this side view. First we have a sigma bond resulting from an interaction
between these pi orbitals and the dz2 orbital on the metal. This is the lowest energy orbital for
the allylic system so there are no nodes, and all of these lobes overlap in-phase.
Then for the next orbital there will be one node, so these p orbitals will be of opposite phase,
and they can interact with this other d orbital in this way, generating a pi bond.
Then going one orbital further to the LUMO of the allylic system, here there will be two nodes,
and this time there will be backbonding from this other d orbital to further strengthen the bond. So
overall, this allylic anion donates two pairs of electrons to the metal, or four electrons total.
Then we have the cyclopentadienyl anion, or C5H5-. This is actually an exceptionally common ligand,
and it is an eta-5 ligand, so all five carbons are interacting with the metal, or essentially
the delocalized pi system. This ligand will donate three pairs of electrons to the metal,
and we depict it like this, with the five-carbon ring and a circle in the middle, representing the
pi system, and a line from the center of the pi system to the metal. Again we need to look at the
molecular orbitals to make sense of this. First we see the orbital with no nodes. This will allow
for easy overlap with the dz2 orbital of the metal to generate a sigma bond. Then there will be two
degenerate pi molecular orbitals, each of which have one node. These will look like this, and will
allow for overlap with these two perpendicular d orbitals on the metal to generate two pi bonds.
Going to the next set of empty molecular orbitals for the ligand, which each have two nodes,
we can see how backbonding from the metal is also possible, strengthening the interaction.
We should point out that we can have the same ligand bonding to the metal
in eta-3 fashion, meaning that only three carbons are interacting with the metal,
which would be depicted in this manner, and it will donate only two pairs of electrons.
The cyclopentadienyl anion, when acting as a ligand, will be abbreviated as Cp-,
and there are some slight variations we may see. Cp′ means there is a methyl group present,
Cp′′ means two methyls in any positions, and Cp* means methyls at all five positions.
Finally, benzene is also a very common ligand, which will donate up to three pairs of electrons,
or six electrons total, just like the cyclopentadienyl anion, though benzene
will be neutral. We can also see this written as eta-6 C6H6. In general, when the eta value is odd,
it usually indicates an anionic ligand, and when it is even, it usually indicates a neutral ligand.
Before moving on we should also mention another symbol, mu, and what it means when associated
with a ligand. This symbol means the ligand is coordinated to more than one metal center, and
is thus referred to as a bridging ligand. We will commonly see these with polynuclear complexes,
which have more than one metal atom interacting with one another in addition to all the ligands.
In such situations, the number next to the mu tells us how many metal atoms the
ligand is interacting with. The thing that will be the most counterintuitive at first
is that the number of electrons the ligand contributes will actually stay the same as what
we would expect under normal circumstances, it is simply that those electrons are being distributed
amongst more than one metal center, like the way that one hydrogen atom was able to
interact with two boron atoms even with just one pair of electrons in diborane,
as we saw earlier in the series. We will get a closer look at these trickier complexes later.
So with that we should have a better understanding of the ways in which ligands interact with metal
atoms in transition metal complexes. But how many ligands can a metal atom accommodate? What can we
say about the stability of a particular complex? Let’s move forward and get some more details.