In the reaction of octacarbonyl dicobalt with 3-methyl-1,2-butadiene at room temperature dinuclear η3-allyl-type complexes are formed which contain 2 + n (n = 0, 1, 2, 3, ···) five-carbon units depending on the applied molar ratio. These complexes are the individual compounds in the octacarbonyl dicobalt-initiated living polymerization of 3-methyl-1,2-butadiene. The first three members in this series of complexes containing two (1), three (2), and four (3) five-carbon units have been isolated and characterized by IR, Raman, 1H, and 13C NMR spectroscopies, molecular weight, and for 1 and 2 also by single-crystal X-ray diffraction. The first two molecules of 3-methyl-1,2-butadiene establish the formation of two (η3-3,3-dimethylallyl)cobalt tricarbonyl parts which are linked together at the central allylic carbon by a carbonyl group. By the addition of 3-methyl-1,2-butadiene in excess, the monomer inserts into the unsubstituted allylic carbon−cobalt bond in a 1,2-fashion, pushing the cobalt atoms further apart. The X-ray structures of 1 and 2 disclose that the 1,2-polymer chain grows in a helical manner. The rate of the insertion of the monomer is first order with respect of both the 3-methyl-1,2-butadiene and the dicobalt complex and is negative first order with respect of carbon monoxide. The observed rate constants of the formation of 1−3 at 30 °C are 14.3 × 10-4, 1.75 × 10-4, and 0.79 × 10-4 s-1, reproducible to within 5, 10, and 7%, respectively.