Hydroboration of allyl(dimesityl)phosphane with HB(C(6)F(5))(2) gives the intramolecular five-membered P-B Lewis pair 7, that was characterized by X-ray diffraction. Similarly, HB(C(6)F(5))(2) addition to the substrates (mesityl)(2)P-CR[double bond, length as m-dash]CH(2) (R = CH(3), Ph) yield the corresponding (mesityl)(2)P(micro-CHRCH(2))B(C(6)F(5))(2) products 9a (R = CH(3)) and 9b (R = Ph) that show a weak intramolecular P...B interaction. The activation energy of the (reversible) P...B cleavage of these substrates was determined by dynamic (19)F NMR spectroscopy (9a: Delta G(not equal)(inv) (280 K) = 11.7 +/- 0.4 kcal mol(-1)). Compounds 9b and 9c show similar values. Compound 9c was prepared by HB(C(6)F(5))(2) addition to (mesityl)(2)P-CH=CHSiMe(3). Compound 9a reacts rapidly with dihydrogen (2.5 bar) at room temperature in pentane to give the zwitterionic H(2)-activation product (mesityl)(2)PH(+)(micro-CHMeCH(2))BH(-)(C(6)F(5))(2) (11).