Sodium tetraisopropylcyclopentadienide or lithium tri(tert-butyl)cyclopentadienide reacts with cobalt(II) chloride in THF to give the corresponding dimers [(C 5 HR 4 )Co(μ-Cl)] 2 (1a, R=CHMe 2 ) or [(C 5 H 2 R 3 )Co(μ-Cl)] 2 (1b, R=CMe 3 ) in good yield. The nitrosyl complexes [(C 5 HR 4 )Co(μ-NO)] 2 (2a), [(C 5 HR 4 )Co(μ-Cl)(μ-NO)Co(C 5 HR 4 )] (3a), and [(C 5 HR 4 )Co(NO)Cl] (4a) (R=CHMe 2 ) or the tri(tert-butyl)cyclopentadienyl derivatives 2b, 3b, and 4b have been obtained from CoCl 2 , the respective cyclopentadienide, and nitric oxide. From the disproportionation of 1a or 1b with carbon monoxide, the salt [(C 5 HR 4 )Co(μ-Cl) 3 Co(C 5 HR 4 )] + 2 [Cl 2 Co(μ-Cl)] 2 2 - (5a, R=CHMe 2 ) and the dicarbonyl complex [(C 5 HR 4 )Co(CO) 2 ] (6a, R=CHMe 2 ) or the tri(tert-butyl)cyclopentadienyl derivatives 5b and 6b have been isolated. Compounds 6a and 6b were converted to the carbonyl-bridged dimers [(C 5 HR 4 )Co(μ-CO)] 2 (7a, R=CHMe 2 ) or [(C 5 H 2 R 3 )Co(μ-CO)] 2 (7b, R=CMe 3 ) by UV irradiation. Compound 1a was cleaved with acetonitrile to yield the novel 17 valence electron cation [(C 5 HR 4 )Co(MeCN) 2 ] + (8, R=CHMe 2 ), which in acetonitrile solution could be further oxidized to [(C 5 HR 4 )Co(MeCN) 3 ] 2 + (9, R=CHMe 2 ) by electrochemical means or chemically with ferrocenium hexafluorophosphate. Compound 1a gave the neutral 17 VE complex [(C 5 HR 4 )Co(PMe 3 )Cl] (10, R=CHMe 2 ) with trimethylphosphane, which could be converted to the methyl-substituted odd-electron compound [(C 5 HR 4 )Co(PMe 3 )CH 3 ] (11, R=CHMe 2 ) by treatment with methyllithium. Hydrolysis of complex 11 proceeded with phosphane loss and resulted in formation of the hydroxo-bridged dimer [(C 5 HR 4 )Co(μ-OH)] 2 (12, R=CHMe 2 ). Crystal structure analyses have been carried out for 1a, 3b, 5a, 7a, 7b, 8, 10, and 12. For growth of single crystals tetraisopropylcyclopentadiene turned out to be a well-suited solvent.