1-Aryl-2-methylenecyclopropanes reacted with [RhCl(PPh3)3] at 50â °C to give [RhCl(η⁴-CH2.dbd6;CArCH.dbd6;CH2)(PPh3)2] (Arâ =â C6H5 1a, C6H4F-p 1b, C6H4Me-p 1c or C6H4OMe-p 1d) via ring opening isomerization of the substrate and its subsequent co-ordination to Rh. The diene-co-ordinated rhodium complexes have been characterized by X-ray crystallography and NMR spectroscopy. Similar reaction at 0â °C afforded the rhodium(I) complexes with Ï -co-ordinated 1-aryl-2-methylenecyclopropane, [RhCl(η²-CH2.dbd6;.ub1s;CCH2C.ub1e;HAr)(PPh3)2] (Arâ =â C6H5 2a, C6H4F-p 2b, C6H4Me-p 2c or C6H4OMe-p 2d). Exchange of the ligand of 2a with added 1-aryl-2-methylenecyclopropanes occurs reversibly at 30â 45â °C with the thermodynamic parameters of the reactions 2aâ +â CH2.dbd6;.ub1s;CCH2C.ub1e;HC6H4X-pâ â â 2b (or 2c)â +â CH2.dbd6;.ub1s;CCH2C.ub1e;HC6H5 being Î Hâ °â =â â 10.3 kJ mol^{â 1} and Î Sâ °â =â â 32 J K^{â 1} mol^{â 1} for Xâ =â F and Î Hâ °â =â 2.2 kJ mol^{â 1} and Î Sâ °â =â â 2.6 J K^{â 1} mol^{â 1} for Xâ =â Me, respectively, at 298 K. The structure of a PEt3 co-ordinated analog, [RhCl(η²-CH2.dbd6;.ub1s;CCH2C.ub1e;HC6H4Me-p)(PEt3)2] 3c, has been determined by X-ray crystallography. The reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh3)3] at 25â °C gave a mixture of 1a and 2a. Heating of a benzene solution of 2a at 50â °C turned it into 1a in low yield (<7%), while the reactions of 1-methylene-2-phenylcyclopropane with 2a at the same temperature gave 1a (10%) and 2-phenylbuta-1,3-diene (14%). The amounts of the products formed via ring opening isomerization in these reactions are much smaller than those in the reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh3)3] at 50â °C.