Ad imethylxanthene-based phosphine/borane frustrated Lewis pair (FLP) is shown to effect reversible CÀH activation, cleaving phenylacetylene, PhCCH, to give an equilibrium mixture of the free FLP and phosphoniuma cetylide in CD 2 Cl 2 solution at room temperature. This systema lso reacts with BÀHb onds although in ad ifferentf ashion:r eactions with HBpin and HBcatp roceed via CÀB/BÀHm etathe-sis, leading to replacement of the -B(C 6 F 5 ) 2 Lewis acid component by -Bpin/-Bcat, and transfer of HB(C 6 F 5 ) 2 to the phosphine Lewis base. Thist ransformationu nderpins the ability of the FLP to catalyze the hydroboration of alkynes by HBpin: the active species is derived from the HB(C 6 F 5 ) 2 fragment generated in this exchangep rocess.Scheme1.Stoichiometric reactions( a) between 1a and PhCCH;a nd (b) between 1b and PhCCH.