Convenient syntheses are described for the five-, six-, and seven-membered phosphacycles PhP(CH2)x-1, where x = 5 (La5), 6 (La6), 7 (La7), and ButP(CH2)x-1 where x = 5 (Lb5), 6 (Lb6), 7 (Lb7). Treatment of [PtCl2(cod)] with La5-7 gives cis-[PtCl2(La5-7)2] (1a5-7), whereas with Lb5-7 a mixture of cis-[PtCl2(Lb5-7)2] (1b5-7) and trans-[PtCl2(Lb5-7)2] (2b5-7) is obtained. Metathesis of 1a7 with NaI gives a mixture of cis-[PtI2(La7)2] (3a7) and trans-[PtI2(La7)2] (4a7). The crystal structures of 1a5, 1a6, 1a7, and 4a7 have been determined. Comparison of the structures of 1a7 and 4a7 reveals that La7 has variable steric bulk, with the crystallographically determined cone angle ranging from 137° (smaller than La5) to 172° (larger than La6), depending on the particular twist-chair seven-membered-ring conformations adopted. The complex cis-[PtCl2(Lb6)] (1b6) is fluxional on the NMR time scale at ambient temperatures, as a result of restricted PtP rotation. Treatment of [Rh2Cl2(CO)4] with La5-7 or Lb5-7 gives the ...