Chiral (cyclopentadienone)cobalt complex I (R, R1 = H, Me), prepd. from sodium carbomethoxy(methyl)cyclopentadienide and ICo(CO)4, followed by photocycloaddn. of 2 molar equiv Me3SiC.tplbond.CH, was methylated with di-Me sulfate to give a cobaltocenium salt (not isolated) which underwent, diastereoselectively, nucleophilic addn. of R2Li (R2 = Me3SiC.tplbond.C, 1-octynyl, EtOC.tplbond.C) giving cyclopentadienecobalt complex II. The crystal structure of I was detd.