Treatment of cobalt dichloride suspended in a mixture of cyclo-octa-1,5-diene, pyridine, and tetrahydrofuran with metal sodium affords a brown crystalline complex, Co(C8H13)(C8H12). Structural proof has been obtained which shows conclusively that the complex is π-cyclo-octenyl-π-cyclo-octa-1,5-dienecobalt. Thermally it is stable, and shows no tendency for valence tautomerism. Remarkable mobility of the hydrogen atoms of the ligand has been observed in some chemical reactions, and this is interpreted in terms of the molecular structure.