| Sumario: | The coordinating properties of the diaminocarbene (A) and phosphonium ylide (B) ligand types have been investigated systematically through a test family of C,C-chelating ligands containing two moieties of either kind. The overall character of ο-CHAB ligands (a + b = 2) has been analyzed from the lR CO stretching frequencies of isostructural complexes [(η²CHAB)Rh(CO)][TfO]. The test moieties A = NCHN(Me)C and B = PhPCH were first considered. While the ligands bearing at least one diaminocarbene end (AA, a = 2 and AB, a = 1) could be generated (and trapped by complexation), the bis-ylide case BB (a = 0) proved to be awkward: treatment of the dication CH(PPhMe) with n-BuLi indeed lead to the Schmidbaur's carbodiphosphorane PhPCPPhMe, through an unprecendented ylidopentacoordinated phosphorane which could be fully characterized by NMR techniques. The bis-ylide ligand type CHB could however be generated by bridging the phosphonium methyl groups by a methylene link (B = (PPhCH)CH), preventing the formation of the analogous highly strained carbodiphosphorane. The three complexes [(η²-CHAB)Rh(CO)][TfO] were fully characterized, including by X-ray diffraction analysis and Rh NMR spectroscopy. Comparison of their lR spectra indicated that the A type bis-NHC ligand is less donating than the hybrid AB type, which is itself less donating than the B type bis-ylide ligand. The excellent linear variation of the VCO frequencies vs a (= 0, 1, 2) shows that the coordinating moieties act in a pseudoindependent way. This was confirmed by DFT calculations at the B3PW91/6-31G**/LANL2DZ*(Rh) level, It is therefore demonstrated that a phosphonium ylide ligand is a stronger donor than a diaminocarbene ligand.
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