The cycloreversion process forming the propene adduct at 2.9 kcal/mol displays a barrier of ∆G ‡ = 15.5 kcal/mol. Note, that educts and products are not identical and no symmetry relations are present anymore. The most important observation is the inversion of the spin state ordering (∆G ST > 0) and the preference for the singlet state of the metallacycle compared to 7-Mn. This can be primarily attributed to the presence of the strong π-acceptor ligand CO as an additional ligand, but also to the changes in electronic structure induced by altering the ligand sphere. Consequently, the effect of the CO ligands is two-fold: It stabilizes the system in the singlet state and the octahedral coordination prevents structural relaxation that is observed as a deactivation channel in 5-fold coordinated species such as 1-Mn and 2-Mn. To achieve the desired stabilization of the singlet state, an intricate balance of ligands is required. The incorporation of carbon monoxide into the chemical structure has fundamental advantages but also potential downsides. Regarding the geometry of the olefin adduct 8-Mn in the triplet state, we observed that the obtained minimum is 22.1 kcal/mol higher in energy. However, small displacements led to significant structural rearrangements and CO migration to the carbene (see Figure ). A similar optimization behaviour was also found using other exchange-correlation functionals. Hence, it is not an artefact of the approximated DFT functional. This very stable species exhibits a Mn-C(H)(CH 3 )(C=O) motif and is located 2.0 kcal/mol above the singlet olefin adduct.