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From here

I can see why hapticity changes here, but not why it is necessary for the benzene ring to bend. The resonance energy of benzene is pretty large, and there's no balancing increase in stability elsewhere--unless the decrease of hapticity increases stability, which I doubt (more delocalization=more stability, not vice versa). Also, the concept of a "bent benzene" is a bit strange to me.

I'd like to know what exactly is going on in this reaction and why it happens.

@F'x mentioned that the bent benzene may just be a way of drawing the molecule to clearly show the change of overlap. If this is the case, I'd still like to know how the hapticity changed (because my initial logic behind change of hapticity is invalid if the structure doesn't actually bend).

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    $\begingroup$ Do you have any indication that the benzene ring actually distorts that way, or is it merely a convenient way of drawing it because it clearly shows η⁴ hapticity? $\endgroup$
    – F'x
    May 8 '12 at 13:29
  • $\begingroup$ @F'x: Is it? Hmmm. If you explain why the hapticity changes in that case (I suspect I know why), then I think it qualifies as an answer(I can edit the question to accommodate that possibility). $\endgroup$
    – ManishEarth
    May 8 '12 at 13:32
  • $\begingroup$ I don't know for sure, but I suspect it might be the case… $\endgroup$
    – F'x
    May 8 '12 at 13:41
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That diagram is misleading because it’s technically in reference to bis(hexamethylbenzene)ruthenium(0). This is a famous example by the late and great E.O. Fischer. This Ru(0) complex is 18e, three methyl resonances were observed for the bent ring at −10 °C. Crystallography later confirmed this. This structure is surprising to the new student, perhaps, because the classic 18e bis(benzene)chromium is sandwiched between two planar arenes.

This is not an organic chemistry question. It is deep in the realm of organometallic chemistry, with a proper explanation requiring a molecular orbital energy diagram and then one has to invoke a Jahn Teller instability argument that requires lowering symmetry from $D_{6d}$ or $D_{6h}$. Suffice it to say, stability is increased by changing the hapticity.

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