Phosphonium Ylides vs Iminophosphoranes: The Role of the Coordinating Ylidic Atom in cis -[Phosphine-Ylide Rh(CO) 2 ] Complexes.

The coordinating properties of two families of ylides, namely, phosphonium ylides and iminophosphoranes, differently substituted at the ylidic center (CH 2 - vs N i Pr - ), have been investigated in structurally related cationic phosphine-ylide Rh(CO) 2 complexes obtained from readily available phosphine-phosphonium salt precursors derived from an ortho -phenylene bridge. However, while the Rh(CO) 2 complex bearing the P + -CH 2 - donor moiety proved to be stable, the P═N i Pr donor end appeared to induce lability to one of the CO groups. All of the Rh I carbonyl complexes in both ylide series were fully characterized, including through X-ray diffraction analysis. Based on the experimental and calculated infrared (IR) CO stretching frequencies in Rh(CO) 2 complexes, we evidenced that the phosphonium ylide ligand is a stronger donor than the iminophosphorane ligand. However, we also found that the difference in the intrinsic electronic properties can be largely compensated by the introduction of an i Pr substituent on the N atom of the iminophosphorane, hence pointing to the noninnocent role of the peripheral substituent and opening novel possibilities to tune the properties of metal complexes containing ylide ligands.