To further investigate the influence of the donor strength and design of the supporting pincer ligands, we considered three additional frameworks. As for OM, the metallacyclobutane structure is the key intermediate and should ideally be in the singlet state. Hence, we investigated the impact of these ligands on ∆G ST in the corresponding metallacyclobutane complexes (see Figure ). For 3-Mn a singlet-triplet splitting of -18.3 kcal/mol was found, for 4-Mn a splitting of -7.2 kcal/mol and for 5-Mn a value of -8.6 kcal/mol, respectively. In all these cases (including 2-Mn, vide supra) the triplet state remains the more favorable one, but a strong influence of the ligands can be observed. Similar to the computational studies of iron complexes, the effect of the central donor is pivotal, but the stabilization of the singlet state in Mn systems is much more difficult to achieve than in related Fe systems -at least based on the limited existing data. It is also particularly noteworthy at this point that the deviations from ideal <S 2 > in the calculated triplet states are significantly higher for the calculated Mn pincer species than for the corresponding Fe pincer species -indicating a more complex electronic structure of the Mn systems. Earlier investigations by Chirik and coworkers already revealed the complexity of the electronic structure of pincer coordinated Fe carbene complexes. It would certainly be of interest to examine the influence of various pincer ligands and substituent effects 53 in more detail, but this is beyond the scope of this current article. spectively. Applying this concept to the metallacyclobutane structure derived from 1-Mn, we computed δ(α) = 119 ppm and δ(β) = -11 ppm. A similar and even better result is obtained for 2-Mn with δ(α) = 115 ppm and δ(β) = 4 ppm. Even though these rules were primarily derived from Ru, Os, Mo, W and Ir complexes, the values obtained for both model complexes were found to match well and are in line with our assumption on an isodiagonal relationship.