The catalytic C(sp3)-H deuteration of tricyclohexylphosphine (PCy3) with excess benzene-d6at 60 °C was achieved using the iridium catalyst precursor Ir(κ2Si,Si-xantsil)(H)2(PCy3)Cl ( 1 ), in the presence of (Me3Al)2·DABCO (xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl), DABCO = 1,4-diazabicyclo[2.2.2]octane). In this reaction, the 3,5-equatorial and 4-axial C-Hs of PCy3were selectively deuterated, chiefly affording P(Cy-d3)3. To gain insight into the reaction mechanism, iridium complexes bearing an asymmetric and bulkier bis(silyl)-xanthene chelate ligand with SiMe2and Si(p-Tol)2coordinating moieties, namely, Ir(κ2Si,Si-xantSiMeSip-Tol)(H)2(L)Cl (L = pyridine ( 2-py ) or PCy3( 2-PCy )), were synthesized, and similar reactions using them were examined under stoichiometric and catalytic conditions. The stoichiometric reaction of 2-py with (Me3Al)2·DABCO at 60 °C resulted in dehydrochlorination to give the dinuclear complex [Ir(μ-κ1Si:η6,κ1Si-xantSiMeSip-Tol)(H)]2( 3 ). On the other hand, the analogous reaction of 2-PCy in benzene at 60 °C afforded the thermally unstable Ir(xantSiMeSip-Tol)(H)(PCy3) ( 4 ). When this reaction was performed in benzene-d6, the partially deuterated analogue Ir(xantSiMeSip-Tol)(D)(PCy3-dn) ( 4-D ) was formed due to the deuteration of Ir-H and some C(sp3)-Hs of PCy3of 4 via H/D exchange with benzene-d6. Moreover, 2-PCy showed catalytic activity similar to that of 1 .