By using (h5-1-tert-butyl-2-methyl-1,2-azaborolyl)dicarbonylcobalt [h5-AbCo(CO)2] (1) as starting material, various phosphine and olefin derivs. of the type AbCo(CO)L are synthesized. The phosphines L = PMe3, PEt3, Pr3P, Bu3P, and P(OPh)3 give, due to the chirality of 1, the enantiomeric compds. 2-6, whereas the phosphines Me3CPMePh, P(OBu-sec)Ph2, and PPh3[OCHMeCO2Me] form the diastereoisomers 7-9. The orange-red, oily and volatile compds. are isolated with yields between 30 and 60%. The mechanism of formation of [h5-AbCo(CO)PMe3] (2) has been studied by means of intermediates at low temps. To avoid a 20e configuration when PMe3 is added to 1, the allyl complex [h3-AbCo(CO)2PMe3] (2a) is formed. With an excess of PMe3 it is converted to the monohapto complex [h1-Ab-Co(CO)2(PMe3)2] (2b) with a Co-C(3) s bond. 2A and 2b change to [h5-AbCo(CO)PMe3] (2) when warmed to room temp. In contrast to substitution reactions of [CpCo(CO)2] where intermediates have never been obsd., here an addn.-elimination process is proved. With di-Me maleate, maleic anhydride, and methylmaleic anhydride, 3 olefin complexes are obtained. However, the maleate complex contains the fumaric ester, due to a cis-trans isomerization during the complexation. X-ray structure analyses prove the mol. structures of the other 2 complexes. The original goal of this project, i.e., to prep. pure enantiomeric [h5-AbCo(CO)olefin] half-sandwich complexes to study the stereoselective influence of the Ab ring on addn. reactions to the olefin ligands could not yet be reached.