Synthetically more feasible model systems While it is well-known that present OM catalysts in their active form are usually 4-fold coordinate species with a low d-electron count, even simple base-metal model systems such as 1-Mn or 2-Mn are very challenging to realize experimentally. The preparation of low-spin Mn(I) complexes is commonly based on the reaction of a suitable carbonyl precursor such as Mn(CO) 5 Br or Mn 2 (CO) 10 with bidentate or tridentate ligands and subsequent chemical modifications. Moreover, chemical experience dictates that the presence of carbon monoxide is an integral component of most low-spin manganese complexes and that a coordination number of five or six is usually encountered in these molecules. Even though CO is usually not found as a co-ligand in active OM catalysts, it is worth to explore these possibilities computationally since CO ligands can be helpful in maintaining a singlet ground state throughout a catalytic cycle. Figure depicts a conceptual extension of the presented complexes 1-Mn and 2-Mn. A possible starting point for a synthetically feasible compound such as 6-Mn could be a Mn(I)