I guess you are all familiar with the oxidative addition or reductive elimination reaction in organometallic chemistry. If you read further into the topic you may find different pathways like via radicals, nucleophilic substitution, ionic paths or even the famous concerted mechanism.

In general we get a σ-type donation from the, let's take dihydrogen as the approaching ligand, hydrogen's occupied bonding orbital into an empty metal-centered orbital. Some sources say it's a p-orbital but I think the energy gap would be a bit too large so the $\ce{d_{z²}}$-orbital seems a bit more likely to me. On the other hand we get a back-donation from the occupied metal d-orbitals into the σ* of the H-H molecule.

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Now while this may be likely for something as symmetric as hydrogen I still have my doubts about this. Firstly the question remains for all reactions if something is really concerted or if two electrons are ever donated at once or if there is a very fast cascade of two single-electron transfers. And related to this, can this actually happen the way it is graphically shown in literature or is the orbital overlap simply too poor for a good transition? You may also ask this for the Dewar-Chatt-Duncanson model or any other insertion reaction. As the metal d-orbitals approach from an angle the overall overlap should be lower than in a head-on σ-type bond. And I think there are already papers, where for the alkene case a pre-coordination via a hydrogen is proposed:

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I remember that my professor used to mention a so called 'slip-mechanism' but I cannot find anything related to that. If I remember correctly it was just about this topic. As the overlap is quite poor it seems more likely that the addition process is carried out in two fast single-electron transfer steps where you have a 'slipping' towards one side followed by a slipping towards the other side.

I tried to draw it in chemdraw. This is of course too much but it shows what I mean by 'slipping'. Imagine there is still a weak connection between the orbitals even though it slips to the other side much like in π-bonds (pink dashed lines) but at the moment where the electron is actually transfered it slightly slips and twists to increase the orbital overlap

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Has anyone heared about something like that before? That the mechanism may not be a 100% concerted but actually two very fast single electron transfers?

  • $\begingroup$ A mechanistic question like this immediately begs two questions: (1) what different outcomes would you expect and (2) can you design an experiment to check for the difference? $\endgroup$
    – Zhe
    Jan 14 '19 at 14:13
  • $\begingroup$ (1) it would not change the outcome probably, (2) for ultra-fast reactions like these you'd have to trap the intermediate species (if you don't trust in computational calculations). Doesn't seem very likely to me that you could do that. The question is more related to the fact that we take these simple views of having everything aligned in a square or hexagon (Grignard) and accept that it will happen in a single step so easily. The problem with catalysis is that you need to know about every single step and it's perhaps even microscopic reversibility. $\endgroup$ Jan 14 '19 at 17:36
  • $\begingroup$ That is at least how I was taught the organic (not the inorganic 'black box' view) of organometallics in catalysis. And the question is if that mattered like for alkynes or alkenes where the angle of the antibonding orbitals is much different it could perhaps have an effect on the actual choices for the reaction conditions or the catalyst itself. To be honest I don't know if there would be any consequence to it but I just imagine that it doesn't remain stiff like it's often depicted but may also be quite dynamic. $\endgroup$ Jan 14 '19 at 17:39
  • $\begingroup$ So it sounds like you're asking about more than whether this reaction is concerted; you're asking if this addition is symmetric. For example, in inverse-electron-demand Diels-Alder reactions, the reactions are typically concerted but highly asymmetrical in terms of the progression of bond formation. That may be the case here depending on the alkene or alkyne of interest. Again, whether or not that has a discernible impact on the outcome determines how much we should care. Interesting question nevertheless. $\endgroup$
    – Zhe
    Jan 14 '19 at 18:03
  • $\begingroup$ That would probably summarize it quite well. $\endgroup$ Jan 14 '19 at 19:07

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