Phosphonium ylides and iminophosphorane were discovered more than 100 years ago by the same authors,1 and exploited since then in organic chemistry via Wittig type reactions for the formation respectively of C=C and C=N bonds.2 The strong electron density at the ylidic atom as pointed out by the zwitterionic canonical forms (Figure 1) makes them electron donating ligands in coordination chemistry. In this study we aimed at comparing for the first time the electronic properties of iminophosphorane and phosphonium ylides.3 For that purpose, we synthesized dicarbonyl rhodium(I) complexes featuring a rigid phosphine-ylide ligand formed on the ortho-phenylene skeleton In this communication we will compare the donating ability of both ylides based on experimental data as well as Density Functional Theory (DFT) calculations. In particular the influence of the substituent on the ylidic atom will be pointed out. [1] Staudinger, H.; Meyer, J. Über Neue Organische Phosphorverbindungen III. Phosphinmethylenderivate Und Phosphinimine. Helv. Chim. Acta 1919, 2 (1), 635–646. [2] Komen, C. M. D. Ylides and Imines of Phosphorus A.W. Johnson John Wiley & Sons Ltd, Chichester, Recl. Trav. Chim. Pays-Bas 1994, 113 (7–8), 382–382. [3] Popovici, I.; Barthes, C.; Tannoux, T.; Duhayon, C.; Casaretto, N.; Monari, A.; Auffrant, A.; Canac, Y. Inorg. Chem. accepté