Condensation of manganese atoms with cyclopentadiene, cyclohexa-1,3-diene, cycloheptatriene, and cyclo-octa-1,3-diene at –196 °C, followed by warming to 20 °C in the presence of CO, has given organomanganese carbonyl complexes in yields of 5–10% based on Mn. The new compounds [Mn(?4-C6H8)2(CO)], [Mn(?4-C7H8)2(CO)], and [Mn(?3-C8H13)(CO)4] have been isolated. A single-crystal X-ray diffraction study of [Mn(?4-C6H8)2(CO)] shows that the manganese atom adopts a square-pyramidal configuration with the carbonyl ligand in the axial position. The two hexadiene rings are each folded equatorially away from the Mn atom and the molecule has C2v symmetry. As in other diene complexes, the two inner C atoms are slightly closer to the metal [Mn–C (mean) 2.078(8)Å] than are the outer C atoms [Mn–C (mean) 2.159(7)Å]. The crystals are orthorhombic, space group P212121(no. 19) with Z= 4 in a unit cell with a= 8.570(1), b= 17.220(1), and c= 7.562(1)Å. The structure has been refined to R 0.045.