The paramagnetic 15-electron alkyls Tp(t-Bu-Me)Co(R) (R = Me, Et, n-Bu) have been prepared, and the closely related Tp(t-Bu)Co(Me) has been structurally characterized. Hydrogenation of Tp(t-Bu,Me)Co(N-2) produced the analogous hydride, Tp(t-Bu,Me)Co(H), the crystal structure of which has been determined. Reactions of Tp(t-Bu,Me)Co(N-2) with ethylene and propane yielded the corresponding olefin complexes Tp(t-Bu,Me)Co(C2H3R) (R = H, Me). Tp(t-Bu,Me)Co(C2H4) has been structurally characterized by X-ray diffraction as well. The alkyls are thermally stable and show no tendency to beta-eliminate; upon exposure to CO homolytic cleavage of the Co-C bond is observed rather than migratory insertion.