2-Phenyl-1-methylenecyclopropane is isometized into 2-phenyl-1,3-butadiene and 1-phenyl-1,3-butadiene in the presence of catalytic amounts of RhH(CO)(PPh3)(3) and RhH(PPh3)(4). The Rh-containing product of the reactions has a 2-phenyl-1-methylallyl (or 1-phenyl-1-methylallyl) ligand, and is formed also from the reaction of 2-phenyl-1,3-butadiene with RhH(CO)(PPh3)(3). RhH(CO)(PPh3)(3) promotes ring-opening isomerization of 4-phenyl-1-methylenespiro[2,2]pentane to afford Rh[eta(3)-anti-CH2C{C(=CH2)Ph}CHCH3](PPh3)(2) (2) at 50 degrees C and Rh{kappa(1),eta(2)-CH2CH(Ph)C(=CH2)CH=CH2}(CO)(PPh3)(2) (3) at -35 degrees C. X-ray crystallography of 2 shows the pi-allylic ligand having a methyl group at anti position. The mechanism for formation of 2 via intermediate 3 is discussed. (c) 2005 Elsevier B.V All rights reserved.