The synthesis of a new trisbenzylsilanephosphine P{(o-CHCH)SiMeH} (1) is shown to proceed with high yields from P(o-tolyl). Compound 1 coordinates to the Rh and Ir dimers [MCl(COD)] (M = Rh, Ir) in a tetradentate or tridentate fashion, depending on the strict exclusion of water. The dimeric compounds [ClM(SiMeCH-o-CH)P(o-CH-CHSiMeH)], 2Rh and 2Ir, feature a tetradentate coordination of the starting ligand with P and two Si atoms as well as a non-classical agostic Si-H group. The presence of adventitious water in the solvents leads to the formation of two new complexes [(μ-Cl)M(SiMeCH-o-CH)P(o-CH-CHSiMeOSiMeCH-o-CH-)P(SiMeCH-o-CH)], 3Rh and 3Ir, which feature a siloxane bridge through Si-H bond breaking in 2. Reaction of [RhCl(COD)] with the bisbenzylsilanephosphine PhP{(o-CHCH)SiMeH} leads to the formation of compound 4Rh which features also a dimeric structure with the SiPSi ligand coordinated through the two silicon atoms, one of which occupies the apical position of a square-pyramidal geometry in the solid state, while the second is disposed equatorially trans to π-donor Cl. Finally, bidentate coordination of a PSi ligand is achieved by reaction of [RhCl(COD)] with PhP{(o-CHCH)SiMeH} which leads to the monometallic species [RhCl(SiMeCH-o-CH-PPh)], 5Rh, incorporating two chelating PSi ligands and maintaining a Cl ligand.

Incorporating pendant silicon functionalities into phosphine ligands enables to profit from the strong σ‐electron donor and trans influence properties of Si while enhancing the coordination ability of the ligand. Herein, we show that the introduction of bulky sigma donor substituents on the Si atoms and modulation of the number of Si–H functional groups in a series of phosphinobenzylsilanes allow either the stabilization of rare highly unsaturated 14‐electron rhodium(III) and iridium(III) species devoid of agostic interactions or the access to mixed‐valence MI–MIII complexes. Our findings using isopropyl‐substituted silicon are markedly different from those obtained when employing the methyl‐substituted Si series.