A new family of rhodium and iridium compounds with the bulky tris(ortho-methoxyphenyl) phosphine (PAn(3)) was synthesized and characterized by NMR methods. The X-ray crystal structures of RhCl(PAn(3))(COD)(1) and Ir[(PBz(3))(PAn(3))COD)]PF6 (4) have been determined. A stabilizing agostic interaction has been crystallographically observed in both compounds, due to the steric hindrance of the ortho-substituted phosphine ligand and its presence has been associated to the fluxional behavior shown by the complexes on the NMR timescale in solution. Iridium complexes containing PBz(3) and/or PAn(3) have been evaluated as catalyst precursors for the hydrogenation of trans-cinnamaldehyde (CNA), and their activities have also been compared to those of other iridium complexes containing bulky phosphine ligands, such as PTol(3) (tris-ortho-tolyl-phosphine). The catalytic experiments show that irrespectively of the phosphine combination, all of the evaluated catalysts prevalently hydrogenate the C=C moiety. However, the product selectivity can be tuned by changing either the substrate/catalyst ratio or the phosphine ligand at the metal center. The catalyst with PAn(3) proved to be more efficient and also gave higher yields of the enol product, indicating that stereoelectronic effects are responsible for the changes in selectivity. (C) 2008 Elsevier B.V. All rights reserved.