Structural characterization of the head-to-head isomers of the [Pd2(Ph2Ppy)2Cl2] and [PtPd(Ph2Ppy)2I2] complexes (Ph2Ppy = 2-(diphenylphosphino)pyridine)

The molecular structures of the thermodynamically unstable head-to-head isomers, HH-[Pd2(Ph2Ppy)2Cl2] and HH-[PtPd(Ph2Ppy)2I2], have been determined by single crystal X-ray diffraction. The two complexes have proved to be isostructural. The severe distortions of the bond angles from the ideal square planar geometry around the metal centers ligating the trans phosphorus donor atoms are indicative of a more pronounced internal strain in the HH isomers as compared to the HT counterparts. The enhanced internal strain is thought to be the major driving force responsible for the spontaneous conversion of the head-to-head isomers to their head-to-tail congeners. 13C NMR spectra in solution phase as well as solid-state 31P MAS NMR spectra have proved to be informative regarding the orientation of the asymmetric Ph2Ppy ligands.