In my organometallic chemistry class at the university, the doctor told us that compound μ³-CO-[Fe₄(CO)₁₂]²⁻ had the structure that appears in the image. As you can see, there is a pentavalent carbon.

enter image description here

When we asked her, the doctor told us that it was because one of the iron atoms donated electrons to carbon through a coordination bond and that carbon was still tetravalent. I am not sure if this is the correct explanation, if I misunderstood the doctor or if my chemistry knowledge is worse than I thought and I haven't done enough research.

In any case, is it possible that this is the structure?

  • 1
    $\begingroup$ Useful links for text and formula formatting (not to be applied to titles): Notation basics , Formatting of math/chem expressions and upright vs italic $\endgroup$
    – Poutnik
    Commented Feb 18, 2022 at 6:32
  • 2
    $\begingroup$ $\ce{[Fe4(CO)13]^2-}$ is a canonical example among metal carbonyl clusters that has been synthesized by Hieber in 1930 and the structure and bonding have been determined and discussed in numerous publications later. doi.org/10.1021/ja00973a018 gives a good start. $\endgroup$
    – andselisk
    Commented Feb 18, 2022 at 7:34
  • $\begingroup$ Try to write chemical formulas ( or equations) like μ³-CO-[Fe₄(CO)₁₂]²⁻ rather by MathJax as $\ce{\mu^3-CO-[Fe4(CO)12]^2-}$ displayed inline as $\ce{\mu^3-CO-[Fe4(CO)12]^2-}$ or with double dollars to display it separately as $$\ce{\mu^3-CO-[Fe4(CO)12]^2-}$$ $\endgroup$
    – Poutnik
    Commented Feb 18, 2022 at 7:53
  • $\begingroup$ Heres a related discussion on hypervalent carbon: chemistry.stackexchange.com/questions/4027/carbon-with-5-bonds/… $\endgroup$
    – BrownBag
    Commented Feb 18, 2022 at 10:31
  • $\begingroup$ @Poutnik Ironically, μ³-CO-[Fe₄(CO)₁₂]²⁻ is better than $\ce{\mu^3-CO-[Fe4(CO)12]^2-}$ because the letter mu should be upright. Both formulas aren't IUPAC-complaint at least because the number of metal centers linked by the bridging atom should be denoted with right subscript (i.e. μ₃-, not μ³-). The superscript is used with eta notation for hapticity. And "CO-" part is equally misleading in both formulas. $\endgroup$
    – andselisk
    Commented Feb 24, 2022 at 8:47

1 Answer 1


If you have only ever heard of Lewis diagrams,* then you might misinterpret the shown representation.

Instead of bonding the focus of this diagram is more on structure, even though it compromises on that when showing a double bond in the carbonyl moiety, and to show that this particular carbon has four closest neighbours.
You can almost instantly spot this as lone pairs are omitted completely.

I do not want to dive deep into the bonding of this complex, it is far better to consult the literature for this, but one way of looking at is to look at the ligand first.
A Lewis diagram of carbon monoxide is $\ce{^\ominus|C#O|^\oplus}$, which emphasises on the triple bond nature of the compound. There are other resonance structures. This molecule is very versatile and has many bond modi. One important feature is that it is a σ-donor through carbon.
Carbon monoxide bond in μ3-fashion to the iron cluster. This is a highly delocalised bond; a multi-centre-two-electron bond in first order approximation. The important take-away is that carbon still only utilises eight electrons for bonding.

In a more bonding complete diagram, there should be a lone pair and a positive formal charge at oxygen, a triple bond between oxygen and carbon, and a single line pointing to the middle of the vertex established by the iron cluster. Obviously you'll sacrifice some structural information. But diagrams do that anyway.

One more important point is, that you should not confuse valence with coordination. It is better to avoid valence all together, as its use has been and will remain inconsistent at best.
According to the IUPAC definition, carbon is tetravalent. Always. It is a constant specific to this element.


* In a strict interpretation of a Lewis diagram, all electrons are shown and all lines represent two bonds. There is no difference whether these bonds covalent, ionic, or anything in between, there is no discrimination "from where the bond originates". There are no arrows in true Lewis diagrams.

† Valence in the Gold Book: https://doi.org/10.1351/goldbook.V06588


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