The first production reactions of a characterized metallacyclobutene complex with alkenes are reported. Thus, the metallcyclobutene complex (eta(5)-C5H5)(PPh3)Co[kappa(2)-(C,C)-C(SO2Ph)=C(Si(CH3)(3))CH(CO2CH2CH3) ] (2) undergoes reaction with alkenes to give 1,4-diene complexes with a high degree of regio- and stereoselectivity. A mechanism is proposed in which the metallacyclobutene generates a cyclic vinylcarbene intermediate that undergoes [4+2]-cycloaddition reactions with activity quantum mechanical methods.