Reaction of the eta(3)-allyl iron tricarbonyl anion, 1, with alkyl halides (RX,R=-CH3,-CH(2)Ph,-(CH2)(3)CH3, -CH(CH3)(2),-CH2CH=CH2) followed by treatment with PPh(3) gives eta(4-)(CH3CH=CHC(O)R)Fe(CO)(2)PPh(3) complexes, 2a-e, respectively, in good yields. P(OPh)(3) and P(OCH3)(3) also serves as effective trapping ligands. Low-temperature H-1 NMR studies show that CH3I and PhCH(2)Br react with 1 to give eta(3-)(CH2#CH#CH2)Fe(CO)(3)-R(7,R=-CH3; 8, R = -CH(2)Ph). These complexes react with PPh(3) in less than 5 min at -78 degrees C to give acyl complexes eta(3-) (CH2#CH#CH2)Fe(CO)(2)PPh,C-3(O)R (R = -CH3, -CH(2)Ph) which undergo acyl migration at 21 degrees C (Delta G(*) ca. 21 kcal/mol) to give the observed eta(4)-enone-Fe(CO)(2)PPh(3) products 2a and 2b, respectively. Free enones were obtained from 26 and 2c by reaction with CH3CN. Reaction of either syn- or anti-1-methallyl-Fe(CO)(3)- with CH3I followed by trapping with PPh(3) yields the same set of products in identical ratios : eta(4)-(E)-(CH3CH=(CH3)C(O)CH3)Fe(CO)(2)PPh(3), 13a, eta(4)-(Z)-(CH3CH=C(CH3)C(O)CH3) Fe(CO)(2)PPh(3), 13b, and eta(4)-(CH2=C(CH2CH3)C(O)CH3)Fe(CO)(2)PPh(3), 13c. Similiar results were obtained using PhCH(2)Br. Product structures indicate highly regioselective acyl migration to C-1 when PPh(3) is used as the trapping ligand. Moderate regioselective migration to C-3 is observed when CH3CN or CO is used as the trapping ligand in these systems. When CH3CN is used as the trapping ligand, the major product isolated are the free beta,gamma-enones formed from interception of the beta,gamma-enone complexes prior to 1,3-hydrogen shift and formation of the conjugated alpha,beta-enone iron complexes. A low-temperature in situ H-1 NMR study of this reaction using anti-methallyl-Fe(CO)(3)(-) allows the NMR observation and determination of the rates of conversion for the complete set of sequentially formed intermediates : eta 3-anti-(CH3CH#CH#CH2)Fe(CO)(3)-CH3, anti-17, --> eta(3)-anti-(CH3CH#CH#CH2)Fe(CO)(2)(PPh(3))(C(O)CH3), anti-18, --> eta(3)-syn-(CH3CH#CH#CH2)Fe(CO)(2)(PPh(3)) (C(O)CH3), syn-18, --> eta(4-)(CH2=CHCH(CH3)C(O)CH3)Fe(CO)(2)PPh(3), 19, --> 13a-c. : eta(4)-(E)-(CH3CH=(CH3)C(O)CH3)Fe(CO)(2)PPh(3), 13a, eta(4)-(Z)-(CH3CH=C(CH3)C(O)CH3) Fe(CO)(2)PPh(3), 13b, and eta(4)-(CH2=C(CH2CH3)C(O)CH3)Fe(CO)(2)PPh(3), 13c. Similiar results were obtained using PhCH(2)Br. Product structures indicate highly regioselective acyl migration to C-1 when PPh(3) is used as the trapping ligand. Moderate regioselective migration to C-3 is observed when CH3CN or CO is used as the trapping ligand in these systems. When CH3CN is used as the trapping ligand, the major product isolated are the free beta,gamma-enones formed from interception of the beta,gamma-enone complexes prior to 1,3-hydrogen shift and formation of the conjugated alpha,beta-enone iron complexes. A low-temperature in situ H-1 NMR study of this reaction using anti-methallyl-Fe(CO)(3)(-) allows the NMR observation and determination of the rates of conversion for the complete set of sequentially formed intermediates : eta 3-anti-(CH3CH#CH#CH2)Fe(CO)(3)-CH3, anti-17, --> eta(3)-anti-(CH3CH#CH#CH2)Fe(CO)(2)(PPh(3))(C(O)CH3), anti-18, --> eta(3)-syn-(CH3CH#CH#CH2)Fe(CO)(2)(PPh(3)) (C(O)CH3), syn-18, --> eta(4-)(CH2=CHCH(CH3)C(O)CH3)Fe(CO)(2)PPh(3), 19, --> 13a-c.