The synthesis and structural characterization of a mixed-sandwich (eta(5)-C5Me5)Rh(III) complex of the cyclometalated phosphine PMeXyl(2) (Xyl = 2,6-C6H3Me2) with unusual kappa(4)-P,C,C',C '' coordination (compound 1-BArf; BArf = B(3,5-C6H3(CF3)(2))(4)) are reported. A reversible kappa(4) to kappa(2) change in the binding of the chelating phosphine in cation 1(+) induced by dihydrogen and hydrosilanes triggers a highly efficient Si-H/Si-D (or Si-T) exchange applicable to a wide range of hydrosilanes. Catalysis can be carried out in an organic solvent solution or without solvent, with catalyst loadings as low as 0.001 mol %, and the catalyst may be recycled a number of times.