It should not go unmentioned that migratory insertion of highly nucleophilic ligands such as alkyl groups is also a well-known concept in manganese carbonyl chemistry in the ground state. The quintet states for the adduct and the metallacycle showed dissociative character with respect to ethylene or CO, and thus could not be fully converged. We assume that both observed behaviors, the migratory insertion-like reactivity of the nucleophilic alkylidene in the triplet state and the dissociation of CO or olefin in higher spin states could lead to early deactivation of the catalyst or pre-catalyst. Another potential side-reaction could be insertion of CO into the metallacycle forming a metallacyclopentanone species. However, we have not observed such a behaviour in the singlet or triplet states of our investigated Mn complexes.