We just learned about the 18 electron rule and how it determines the stability of
transition metal complexes, in particular those that are octahedral or tetrahedral.
If you have not watched that tutorial yet, please do so now, as in this tutorial an understanding of
the general algorithm for applying this rule will be assumed. If you’re ready for more practice,
let’s dive right into some additional examples. First let’s try this ruthenium complex. We
have four carbonyls and a hydride, with a 1- charge on the complex.
As always, let’s start by separating the ligands. The carbonyls are neutral and the hydride is 1-,
so the 1- charge on the complex must be from this hydride, while the ruthenium atom is neutral.
Ruthenium is 5s14d7, which means it has 8 valence electrons.
Then we add up the electrons donated by the ligands, which will be two electrons for each
of them, times five ligands, equals ten electrons. Eight from the metal
plus ten from the ligands makes 18, so this is likely to be a stable complex.
Now let’s try this molybdenum complex. Here we see how important it is to have the common
ligands memorized, as for some students it may not be entirely clear what these are,
what their charges are, or how many electrons they donate. RO refers to an alkoxide ligand,
and there are three of these, and CMe refers to an alkylidyne ligand, so this is a 3- charge. That’s
a total of 6- from the ligands, and since this is a neutral complex, the molybdenum atom must be 6+.
Neutral molybdenum has 6 valence electrons, given that its electron configuration ends in 5s14d5,
so molybdenum 6+ has lost all of its valence electrons. So the only electrons to count
are from the ligands, that’s two per alkoxide times three, and then six for the alkylidyne,
since it makes a triple bond with the metal, and that gives us a total of 12. That is dramatically
less than the ideal count of 18, so this is likely to be a very unstable complex.
Steric crowding leads to coordinatively unsaturated complexes like this one,
which is important in catalysis, as we will see later in the series.
How about this other molybdenum complex, with seven methyl isocyanide ligands and an overall
2+ charge. That’s right, some complexes can even be heptavalent! All of these ligands are neutral,
so molybdenum must have an oxidation state of +2 to get to the formal charge on the complex.
We just mentioned that molybdenum has 6 valence electrons, so with an oxidation state of +2 it
must have 4 electrons. Each of these ligands contributes 2, and there are 7 of them,
so that’s 14, plus the 4 from the metal is 18 electrons total, so this complex should be stable.
Here’s just one more molybdenum complex. This one has four carbonyls, one bromide, and one
phenyl alkylidyne. Once again, the fact that this carbon has only one substituent is what reminds us
that it must make three bonds to the metal, so when in doubt, check how many of carbon’s bonds
are accounted for within the ligand, and the rest must go to the metal. The carbonyls are neutral,
the bromide is 1- and the alkylidyne is 3-, so that’s a total of 4-, and the complex is neutral,
so molybdenum must be 4+. Again, molybdenum has 6 valence electrons, so in the +4 oxidation state it
must have only two. So let’s take those two from the metal, plus a total of eight from the four
carbonyls, plus two from the bromide, plus six from the alkylidyne, and that gets us up to 18,
so this is likely to be a stable complex. Let’s try one more, this tungsten complex.
We have a Cp-, a carbonyl, a nitrosyl, and an eta-3 allylic ligand, with an overall
1+ charge on the complex. Here we really have to have our common charges memorized.
Cp- has a 1- charge, carbonyl is neutral, nitrosyl is 1+, and the allylic ligand is 1-.
That’s a total of 1- from the ligands, so to get to 1+ overall, the tungsten atom must be 2+.
Tungsten is 6s25d4, so losing two electrons leaves us with just the 4 d electrons. Now we add
everything up, 4 electrons from the metal, 6 from the Cp-, 2 from the carbonyl, 2 from the nitrosyl,
and 4 from the allylic ligand. That’s a total of 18, so this is a stable complex.
That’s enough practice with the 18 electron rule for now.
Things will certainly get more complicated, but a large portion of the complexes that
will be important in this series will look more or less like these, so we are
certainly well on our way to understanding the structure of transition metal complexes.