NMR Studies on Zerovalent Metal π-Complexes of Dibenzylideneacetone. I. Ligand Conformation and bonding in the Binuclear Palladium Complex

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Abstract
To elucidate the ligand conformation and the metal–olefin bonding nature in the binuclear palladium complex of dibenzylideneacetone, Pd2[(C6H5CH=CH)2CO]3, 1H NMR studies have been performed on the deuteriated complexes Pd2[(C6D5CH=CH)2CO]3, Pd2[(C6D5CD=CH)2CO]3, and Pd2[(C6D5CH=CD)2CO]3. Examination of the spectra has revealed that the three dibenzylideneacetone ligands which triply bridge the two Pd atoms are all in the s-cis,trans conformation. The most probable bonding scheme of the three ligands to the two Pd atoms is as follows: one Pd atom is ligated by one s-trans and two s-cis olefinic moieties and hence the other by one s-cis and two s-trans to yield a metal–olefin bonding stronger in the former combination than in the latter. A linear correlation between the couplings and the mean chemical shifts of the trans olefinic protons suggests a dominant role of metal between the couplings and the mean chemical shifts of the trans olefinic protons suggests a dominant role of metal to olefin π-back donation in differentiating the olefinic moieties in their bonding strength to the metal atoms and in retaining the characteristic rigid structure of the complex.