Why is iron pentacarbonyl known, but iron(III) hexacarbonyl is not?

Why is the neutral complex $$\ce{[Fe(CO)5]}$$ known, but the cationic carbonyl complex $$\ce{[Fe(CO)6]^3+}$$ is not known?

• It might be a short question and look simple, but there is more to it than many people might see at first. We have the 18 valence electron rule we also have the problem of when should we use the 18 VE rule and when should we use ligand field theory. – Nuclear Chemist Jan 6 '20 at 9:02
• @andselisk why 3+ instead of +3? – Haha Hahaha Jan 6 '20 at 10:25
• @HahaHahaha because it's 'number before sign' – Zenix Jan 6 '20 at 11:24
• @HahaHahaha +3 implies oxidation number (and it's placed above the symbol of a chemical element). 3+ is used for a charge number. Which one do you think is applicable here? – andselisk Jan 6 '20 at 11:49
• Ohh, i did not know that nomenclature, i they were interchangeable, so if in an ionic reaction i were to write say $\ce{Fe(III)}$ is what i intend to write, then is 3+ better or +3, because is the os and charge – Haha Hahaha Jan 6 '20 at 14:40

There are several things which are needed to be known to be able to deal with this question.

Firstly there is the 18 Valence Electron (18VE) rule (description at libretexts.org). The iron pentacarbonyl is an 18VE complex. It is coordinatively saturated. A low valent transition metal complex with strong ligand field ligands is considered to be coordinatively saturated if the metal has 18 valence electrons (noble gas electron count).

An iron atom has eight valence electrons, each carbon monoxide (carbonyl) ligand adds two electrons to the valence count. Thus we have a total of 18 in total.

Each positive charge which the complex has causes it to have one less valence electron, so the cationic complex will have an iron which will have 17 VE, this alone would make it less stable.

But there is another issue which is more important, the bonding of carbon monoxide to transition metals is dominated by the withdrawal of electron density from the metal atom to the carbon monoxide. So the less electron-rich the metal the less able the carbon monoxides are to bind.

The fact that the iron in the +3 oxidation state will make it less able to bind to $$\ce{CO}$$ molecules. This is the main reason why it will not be possible to make the complex.

If you consider the series of complexes $$\ce{[V(CO)6]-}$$, $$\ce{[Cr(CO)6]}$$, and $$\ce{[Mn(CO)6]+}$$, the strength of the bonding between the $$\ce{CO}$$ and the metal decreases along the series $$\ce{V, Cr}$$ and $$\ce{Mn}$$ ($$\ce{Mn}$$ has the weakest bonds).

Secondly, we should only use the 18VE rule when the oxidation state of the metal is low and the ligands are strong ligand field ligands. I do not think that the iron(III) is sufficiently low in oxidation state for the chemistry to be dictated by the 18 VE rule. I would predict that if I was to bubble $$\ce{CO}$$ through a solution of iron(III) in dilute acid that nothing would happen.

However, if I was to make a solution of an iron(II) complex of a dialkyl glyoxime then it would be able to react with $$\ce{CO}$$. Here the iron is in a lower oxidation state and has more electron density on it.

• In place of 18VE rule, more fancy term is Sidgwick's effective atomic number rule. – Zenix Jan 6 '20 at 11:27
• I have never heard of Sidgwick's rule, the problem with named reactions is that they give no indication in the name to what they are. I know that every chemist wants a reaction named after them. But things like the aldol reaction and the 18 VE rule are better names as they give an indication in the name of what the rule relates to. – Nuclear Chemist Jan 6 '20 at 12:23
• @nuclearchemist it's even worse in astronomy. – Oscar Lanzi May 2 '20 at 14:21