[Bis(diisopropylamino)phosphino](trimethylsilyl)carbene (I) adds one equiv of dimesitylfluoroborane to give [(Me2CH)2N]2PR:C(SiMe3)BR12 (II; R = mesityl, R1 = F) (III) as an air-stable solid which was characterized by an x-ray crystal diffraction study. In III the B-C1 bond length is 152.5 pm, while the P-C1 distance is 169.6 pm. The bonding planes of the boron and the C1 atom are twisted by only .apprx.31.5 Deg. Further evidence for partial boron-carbon double bond character is provided by NMR spectroscopy. In soln., rotation about the B-C(1) bond is restricted, leading to chem. different environments for the two mesityl groups. The rotation barrier can be estd. to be DG.thermod. > 90 KJ mol-1, as predicted by theor. calcns. I also reacts with trimethoxyborane giving C-borylated phosphorus ylide II (R = R1 = OMe) (IV). However, according to NMR spectroscopy, B-C p-interactions with the ylidic carbon atom in IV are negligible.