The carbenerhodium(I) complexes trans-[RhCl(CPh₂)(L)₂] (L = PPrⁱ₃, 1a; SbPrⁱ₃, 1b) react with PF₃ by cleavage of the rhodium–carbene bond to give the corresponding PF₃ derivatives trans-[RhCl(PF₃)(L)₂] 5a,b, in good yield. In contrast, treatment of the half-sandwich-type compound [(η⁵-C₅H₅)Rh(CPh₂)(PPrⁱ₃)], 2a, with both PF₃ and P(OPh)₃ leads to the migratory insertion of the CPh₂ unit into one of the cyclopentadienyl C–H bonds to form the ring-substituted products [{η⁵-C₅H₄(CHPh₂)}Rh(PX₃)(PPrⁱ₃)] (X = F, 6a; OPh, 6b). The molecular structures of 6a and 6b have been determined by X-ray crystallography. The reaction of the stibine complex [(η⁵-C₅H₅)Rh(CPh₂)(SbPrⁱ₃)], 2b, with PF₃ proceeds by ligand displacement to afford the new carbenerhodium(I) compound [(η⁵-C₅H₅)Rh(CPh₂)(PF₃)], 7. The mechanism of the migratory insertion reaction is discussed.