For the past few tutorials we have been looking at transition metal complexes,
and the different kinds of ligands that can interact with metal centers. But until this point
we have only considered complexes with a single metal center. We must also be aware of polynuclear
transition metal complexes. These are complexes involving more than one central metal atom,
which we will frequently encounter. Each metal will be coordinated to some number of ligands,
and then the metals will also usually have one or more bonds between them.
As we mentioned in an earlier tutorial, this can be complicated by the presence of bridging
ligands. These are ligands that are coordinated to more than one metal center at the same time,
a feature that is expressed by the symbol mu preceding the ligand in the chemical formula.
As we may recall, one counterintuitive aspect of bridging ligands is that these
ligands will usually contribute the same number of electrons to the complex
as they would if coordinated to a single metal atom, even though we will depict two bonds
instead of one. So a bridging hydrogen will still contribute only two electrons, as we would expect
from hydride. While it may seem absurd to see two bonds coming from a hydrogen atom, those two bonds
actually contribute a total of only two electrons, which are distributed among two metal centers.
The exception is ligands with two or more lone pairs, like chlorine, which can actually donate
one lone pair to each metal, thereby contributing four electrons instead of the typical two.
It is often the case that each metal center in such a complex is bound to the same set
of ligands. Therefore, it can be easy to check whether the 18-electron rule is being satisfied
for each metal center. A common situation is that each metal will have an electron count of 17,
and then to get to 18 they will each contribute one electron to a covalent bond between them,
and this bond may be strong or weak, depending on a variety of factors. Here is one such situation.
We have two molybdenum centers, each of which is coordinated to two carbonyls,
one bidentate diphosphine ligand, and two bridging chlorines. When determining the charge
on the metal centers we must be careful. Apart from the neutral ligands, the bridging chlorines
have a negative charge, just as we would expect when coordinated to only one metal. But the two
negative charges are distributed amongst two metal centers. This means that each metal must have a +1
charge to balance things out and get to neutral charge overall. Molybdenum is 5s14d5,
so the +1 cation will be 4d5, thereby contributing 5 electrons to the count.
The bidentate phosphine contributes 4 electrons, two per phosphorus, the carbonyls contribute 2
electrons each, as do the bridging chlorines, as they each contribute two electrons to each metal.
That adds up to 17 electrons for each molybdenum center,
so we simply add one covalent bond between the two metals to bump the total up to 18.
Now that we understand how this works, let’s look at a few chemical formulae for polynuclear
complexes and see if we can predict the structure, presuming that the metals are coordinatively
saturated. This first one has a bridging alkylidene ligand, and then we see an iridium
center bound to a Cp ligand and a carbonyl, and because of the brackets here we see that
there are two of these. With this information we can start to put together the structure,
since we have two iridium atoms, each with the two ligands, and we can also put the bridging ligand.
We simply have to figure out how many bonds are between the metals. Now iridium is 6s25d7,
so it has 9 valence electrons. The carbonyl is neutral, the Cp ligand has a minus one charge,
and the alkylidene ligand has a minus two charge, which is distributed amongst the two metals,
because it is a bridging ligand, so we can think of it as minus one going to each metal.
That’s a total of minus two to each metal, so they must each be in the plus two oxidation state,
since the complex is neutral overall. 9 valence electrons minus two is 7. The 7 from the metal,
plus 6 from the Cp ligand, plus two from the carbonyl, plus two from alkylidene, since its
four electrons are split up between two metals, gives us a total of 17 electrons surrounding
each iridium center. Therefore we simply add one bond between the metals, and that will bring us to
18 for both of them, and we end up with this structure resembling a three-membered ring.
Let’s try one more. We have a binuclear rhenium complex, where each metal is bound to a Cp ligand
and a carbonyl, and then there are two bridging chlorines. Rhenium is 6s25d5, so the metals have 7
valence electrons. The Cp ligand is minus one, and the chlorines are each minus one, but coordinated
to two metals, so that’s minus one to each metal. With a total of minus two, each rhenium center
must be in the plus two oxidation state, so we drop the s electrons to give 5 valence electrons.
5 plus the 6 from the Cp, plus 2 from the carbonyl, plus 2 from each bridging chlorine,
or 4 total, gives us a total of 17 electrons. To get to 18 we will simply draw one bond
between the metals, and that’s our complex. Other complexes may have two or even three bonds
between the metal atoms if necessary to reach the appropriate electron count, which could be only
16 for certain complexes, and of course there are complexes with more than two metal atoms.
Just to see one example, this complex has three ruthenium atoms and 12 carbonyl ligands.
How can we make this work? Presuming symmetry, each ruthenium should have four carbonyls.
None of the ligands have any charge so the metals are neutral, which means they have all 8 valence
electrons. Plus 8 from the four carbonyls gets us to 16, so each metal needs two more electrons.
This can happen if each metal is bound to the other two, so we can set up a three-membered
ring like this, and the ligands occupy the other octahedral sites. It is also possible
to arrive at a viable structure without any metal-metal bonds and using bridging ligands,
however the structure shown here can be confirmed by spectroscopy. We may see interesting substances
like this or even stranger later in the series, but for now we will stop here with this brief
introduction so that we can move on to some other topics in organometallic chemistry.