Probing the donor strength of yldiide ligands: synthesis, structure and reactivity of rhodium complexes with a PC ylide N pincer ligand.

Control of the metal ligand interaction by changes in the ligand protonation state is vital to many catalytic transformations based on metal-ligand cooperativity. Herein, we report on the coordination chemistry of a new PC y (H)N pincer ligand with a central ylide as donor site, which through deprotonation to the corresponding yldiide changes from a neutral L 3 -type ligand to an anionic L 2 X-type PC Y N ligand. The isolation of a series of rhodium complexes showed that the strong donor ability of the neutral ylide PC Y (H)N is further increased upon deprotonation, as evidenced by one of the lowest reported CO stretching frequencies in complex [(PC Y N)Rh(CO)] (2) compared to other known rhodium carbonyl complexes. DFT calculations revealed that the high donor ability mostly results from the antibonding interaction of the p π orbital at the ylide with the d xz orbital at rhodium, which enhances the backdonation into the π* orbital of the CO ligand. This unique interaction results in a rather long metal-carbon bond, but still a strong activation of the CO ligand in order to minimize repulsion between the filled orbitals at the rhodium and the ylide ligand. Accordingly, CO by phosphine replacement leads to a strong deviation from the square-planar geometry in the analogous phosphine complexes [(PC Y N)Rh(PR 3 )] and an unusual reactivity with small alkyl halides, which upon oxidative addition add to the CO ligand, before inserting into the P-C bond in the pincer ligand. These results demonstrate the unique donor strength of yldiide ligands and their potential in the activation of strong bonds.