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.