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I have done some work with diiridium complexes (affectionately known as "scheisschemie" in our research group). The addition of the activated alkyne dimethylacetylenedicarboxylate (DMAD) to the iridium compound 1 yields compound 2, in which the alkyne acts as a parallel bridging, dianionic 4 electron donor.
This, in turn, rearranges to form compound 3, in which the phosphines have bent back to a cis- arrangement, and the alkyne acts as a perpendicular bridging, neutral 4 electron donor.
This is one of the first instances in which an alkyne goes from the parallel to the perpendicular bridging modes without the loss of one of the other ligands (i.e., going from Ir2(CO)3(DMAD)(dppm)2 with a parallel alkyne to Ir2(CO)2(DMAD)(dppm)2 with a perpendicular alkyne would have been the expected reaction). The differences between the two isomers are astounding.
Below are two three dimensional representations of isomer 3, as well as an ORTEP representation obtained from x-ray crystallographic data. (Images generously provided by Dr. Robert McDonald. Of course, he also first synthesized this compound, and grew the diffracting crystals, back in the days when he was still a grad student, so he actually deserves more credit than I'm giving him).
Please click on the thumbnail to see the full-size images.
A large difference can also be found in the reactions of the two isomers with tetraflouroboric acid. In the case of isomer 2, acidic protons will attack one of the alkyne carbons (forming a vinyl, which I wish I could have grown crystals out of, but alas, it kept oiling out of solution), while in isomer 3, the proton attacks one of the Ir metal centres, forming a hydride.
All the research described above has been published in the Canadian Journal of Chemistry (pp 2289-2303, Vol 74, #11, Nov 1996). Full details of the compounds and reactions are in the paper.