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When a bond is broken into ionic fragments, and I'm specifically thinking about organometallic or coordination chemistry, the two products carry opposite charges and the potential energy curve should, in my humble opinion, be attractive everywhere (at every point where the distance is larger than its equilibrium value).

Does this mean that the dissociation of a compound like ferrocene into $\ce{FeCp+}$ and $\ce{Cp-}$ (in the gas phase) occurs without a barrier?

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    $\begingroup$ What is IMHO? If the potential energy is a curve, does it not already imply an energy barrier? $\endgroup$ – Satwik Pasani Jan 11 '16 at 15:14
  • $\begingroup$ Yeah, what you say implies exactly the opposite. $\endgroup$ – Mithoron Jan 11 '16 at 15:33
  • $\begingroup$ IMHO = "in my humble opinion". The fact that the potential energy curve is attractive everywhere just means that the energy rises the further you pull the two ions apart. A barrier implies that the energy rises up to the transition state and then falls again as you approach the product valley. You could re-phrase my question as: Is there a transition state? $\endgroup$ – Ben Winston Jan 11 '16 at 20:48
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In my theoretical research I have often seen that, indeed, there is no barrier for such reactions. In modeling the kinetics you take the bond dissociation energy as the forward barrier and assume a zero barrier going backward. This will also be true for radical recombination reactions.

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The discussion stands and falls with your definition of a barrier. Let me note that your’s is a slightly more obscure one in my opinion. I would have understood a barrier as any movement that requires an energy input to proceed.

In your example of a heterolytic dissociation, there is — as you noted — always an attractive force between the compounds you are separating, and thus there is always an incentive for the system to return to the undissociated state. With increased separation, the energy difference per separation length grows smaller, so at the beginning you need more force to separate a smaller distance. To me, that is a barrier.

If you really define a barrier as a local maximum on the potential energy surface, then please note that all dissociations would be barrierless. The typical homolytic dissociation curve does not have any local minimum; it only converges to a constant value at infinite separation.

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