The (1-alkylcyclopropyl)carbynes (eta(5)-C5H5)(Co){P(OMe)(3)}Mo=CC(R)CH2CH2[R = CH2CH=CH2 (3a), CH3 (3b), SCH3 (3c), CH2C6H5 (3d)] have been prepared and characterized. Reaction of 3a-d with HCl.Et(2)O yields mixtures of the eta(4)-2,4-pentadien-1-al complexes (eta(5)-C5H5)Cl{P(OMe)(3)}Mo{CH2=CHC(R)=CH(CHO)} (4a-d) and the eta(2)-acyl complexes (eta(5)-C5H5)Cl-2{P(OMe)(3)}Mo{eta(2)-C(O)CH2C(R)CH2CH2} [R = CH3(5a), SCH3 (5b)]. Only one isomer of 4a-d forms initially, but over a period of days, there is interconversion of a series of isomers. Formation of dienal complexes 4a-d when R is an electron-donating group contrasts with the previously observed formation of cyclopentenones from derivatives of 3 where R = H, COR’, and CO(2)R’. These results are consistent with an electronic effect an the partitioning between reductive elimination and beta-hydrogen elimination in a metallacyclohexenone intermediate. Complexes 3b and 3c oxidize slowly in CHCl3 to give the dichloro derivatives (eta(5)-C5H5)Cl-2{P(OMe)(3)}-Mo=CC(R)CH2CH2 [R = CH3 (6a), SCH3 (6b)]. The structures of 4b and 6a have been determined by X-ray diffraction : (4b) monoclinic, P2(1)/n, a = 10.188(1) Angstrom, b = 11.666(1) Angstrom, c = 14.128(2) Angstrom, beta = 90.73(1)degrees, V = 1679.0(8) Angstrom(3), Z = 4, R = 4.33, R(w) = 5.09% for 2826 reflections I > 3 sigma(I); (6a) orthorhombic, P2(1)2(1)2(1), a = 9.480(1) Angstrom, b = 13.383(1) Angstrom, c = 13.585(1) Angstrom, V = 1723.5(3) Angstrom(3), Z = 4, R = 4.41, R(w) = 4.96%, for 2024 reflections I > 2 sigma(I).