Tricarbonyl(η 5 -2,4-pentadienyl)manganese (1) and tricarbonyl(η 5 -2,4-dimethyl-2,4-pentadienyl)manganese (2) photochemically add 2,3-dimethyl-1,3-butadiene (3). Mediated by manganese, 6,7-dimethyl-2,6-cyclononadiene-1-yl and 1,3,6,7-tetramethyl-2,6-cyclononadiene-1-yl complexes are formed. Η-shifts from the methyl groups of the substituted CC double bonds convert the primary products into tricarbonyl-η 3:CH -l-methylene-2-methyl-6-cyclononen-2-ylmanganese (4), and tricarbonyl-η 3:CH -1-methylene-2,5,7-trimethyl-6-cyclononen-2-yl-manganese (6), respectively. In addition tetracarbonyl-η 3 -1-methylene-2-methyl-6-cyclononen-2-yl-manganese (5), and tetracarbonyl-η 3 -1-methylene-2,5,7-trimethyl-6-cyclononen-2-ylmanganese (7) are obtained. The latter are also formed from 4 and 6 by addition of CO under ambient conditions. In the complexes 4 and 6 the hydrocarbon ligands are coordinated by enyl units, encompassing C 1, C2, and the exocyclic methylene group, and by a C-H-Mn 2-electron-3-center bond of the 2-methyl group to the manganese. They exhibit temperature dependent 1 H NMR spectra due to a hindered rotation of the 2-methyl group with a barrier of activation of AG* = 40-43 kJ · mol -1 . There is evidence for an equilibrium of the complexes 4 and 6 with the isomers tricarbonyl-η 3:CH -1-methylene-2-methyl-5-cyclononen-2-yl-manganese (4′), and tricarbonyl-η 3:CH -1-methylene-2,5,7-trimethyl-5-cyclononen-2-yl-manganese (6′). At higher temperatures the isomers 4, 4′ and 6, 6′ are interconverted by a 1,4-H shift with a barrier of activation of AG* = 75 kJ · mol -1 . The crystal and molecular structure of 4 was determined by X-ray structure analysis. The constitutions of the complexes 4-7 were deduced from IR and 1 H NM R spectra.