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All I can find in the literature is that the cistrans interconversion proceeds via dissociation of one of the PPh3 ligands resulting in a square-pyramidal Cs intermediate, with the remaining PPh3 in an axial position.

I assume it proceeds via either dissociation of the CO trans to the PPh3, followed by PPh3 and CO binding again to form the trans isomer; or via a Berry pseudorotation-like conversion of the square pyramidal intermediate, however considering that the former seems unlikely due to the CO being a more strongly binding ligand than PPh3 and the latter would require quite a bit of energy, neither of these make sense to me.

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    $\begingroup$ More specifics are needed in the question, but it's possible that the square pyramid is in equilibrium with a trigonal pyramid (simple isomerization). That allows blurring of what it means to be cis and trans. $\endgroup$ – Zhe Mar 5 '19 at 22:22
  • $\begingroup$ I'm not sure what how to be more specific, any suggestions welcomed. Also I assume you meant trigonal bipyramid? $\endgroup$ – Funk Mar 5 '19 at 23:45
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    $\begingroup$ Oh, the complex is given in the title... Yes, I meant bipyramid. There aren't too many viable options here. I don't have a reference right now, but I remember seeing something similar before. $\endgroup$ – Zhe Mar 6 '19 at 0:45

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