Half-sandwich type compounds C 5H 5RhLL′ (L= PMe 3, L′= PR 3, P(OR′) 3; L = PR 3, L′ = P(OMe) 3; L = C 2H 4, L′ = P(OMe) 3, SMe 2; L = L′ = CNR) are prepared starting from [(C 8H 14) 2RhCl] 2 and [(C 2H 4) 2RhCl] 2 by stepwise displacement of the olefinic ligands and final cleavage of the chloride bridges with NaC 5H 5. The compounds C 5H 5RhL 2 and C 5H 5RhLL′ react with CH 2I 2 form the carbenoid-rhodium(III) complexes C 5H 5RhCH 2I(L)I and [C 5H 5RhCH 2I(L)L′] +, the latter being isolated as the BF 4 or PF 6 salts. For L′ = P(OR) 3 (R = Me, Et), successive reaction with NaI gives the dialkylphosphonate compounds C 5H 5RhCH 2I(L)P(O)(OR) 2. The carbenoid complexes [C 5H 3RhCH 2I(PMe 3) 2]X and [C 5H 5RhCH 2I(PR 3)L]PF 6 isomerize thermally or on addition of NEt 3 to produce the yliderhodium(III) compounds [C 5H 5Rh(CH 2PMe 3)(PMe 3)I]PF 6 and [C 5H 5Rh(CH 2PR 3)(L)I]PF 6, respectively. In all cases, the ligand with the strongest donor properties migrates from the metal to the carbon. Kinetic studies of the reaction of [C 5H 5RhCH 2I(PMe 3)P(OMe) 3]PF 6 to give [C 5H 5Rh(CH 2PMe 3)(P(OMe) 3)I]PF 6 show a distinct dependence of the isomerization rate on the NEt 3 concentration and suggest an intramolecular exchange mechanism similar to that of dyotropic rearrangements. Substitution of the carbon-bonded iodide in [C 5H 5RhCH 2I(PMe 3) 2]I and C 5H 5RhCH 2I(PMe 3)P(O)(OMe) 2 leads to the formation of the ylide complexes [C 5H 5Rh(CH 2PMe 3)(PMe 3) 2]I 2 and [C 5H 5Rh(CH 2L)(PMe 3)P(O)(OMe) 2]I. The syntheses of the compounds [C 5H 5RhCH 2X(PMe 3) 2]PF 6 (X = Cl, CN), [C 5H 5RhCH 2X(PMe 3)P(OMe) 3]PF 6 (X = Cl, Br), [C 5H 5Rh(CH 2PMe 3)(PMe 3)Br]PF 6, C 5H 5RhCH 3(PMe 3)P(O)(OMe) 2 and C 5H 5RhCH 3(PMe 3)P(O)(OMe) 2 · NaI are also described.