AbstractThe reaction of the bis(triflates) [F3CSO3‐(CH)m‐O3SCF3] [m = 5–10, 12, 14, 16 (1a–i) with Na2[Os(CO)4] in dimethyl either affords the osmacycloalkanes 2a, 2b and 2f–i (m = 5, 6, 10, 12, 14, 16), diosmacycloalkanes 3a–i (m = 5‐10, 12, 14, 16), and triosmacycloalkanes 4a–i (m = 5–10, 12, 14, 16). The structure of 3f was investigated by an X‐ray structural analysis. This 22‐membered disomacycle crystallizes in the space group Pā with Z = 1. If the unsaturated cis‐4‐octen‐1,8‐diyl bis(trifluoromethanesulfonate) (5) is treated with Na2[Os(CO)4] the diosmacyclooctadeca‐5,14‐diene 6 is obtained. By treating the bis(triflates) 1a, b with Na2[Fe(CO)4] the corresponding ferracycloalkanes are formed only in situ. Insertion of carbon monoxide into the M–C σ bonds leads to the cyclic ketones 7a, d and to the diketone 8d.