In the reaction of octacarbonyl dicobalt with 3-methyl-1,2-butadiene at room temperature dinuclear eta(3)-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, H-1, and C-13 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 (eta(3)-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 degrees C are 14.3 x 10(-4), 1.75 x 10(-4), and 0.79 x 10(-4) s(-1), reproducible to within 5, 10, and 7%, respectively.