plays a barrier of 12.7 kcal/mol for cycloaddition of ethylene and a barrier of 37.7 kcal/mol for decomposition via cyclopropanation, hence, a larger separation of the two pathways. Unlike, Ru complexes, the singlet-triplet energy splitting for the metallacyclobutane intermediate is one of the most crucial parameters in first-row TM metathesis and for 1-Mn was calculated to be ∆G ST = -5.7 kcal/mol with TPSSh and ∆G ST = -5.4 kcal/mol employing DLPNO-CCSD(T) in favor of the triplet state. A survey on the performance of various exchange-correlation functionals on the singlet-triplet splitting for metallacycle 1-Mn can be found in the electronic Supporting Information (see Table ). Essentially all tested functionals yielded similar trends. Coming back to 1-Mn, the quintet state of the metallacyclobutane is located just 0.4 kcal/mol above the triplet state and could also play a role in surface crossing events and deactivation processes. Similar to the reported iron system (1-Fe), the transition state for the [2+2]-cycloaddition in the singlet state also exhibits aromatic character with a NICS(1) value (NICS = Nucleus Independent Chem- Next, we investigated the reactivity and reaction energetics of 2-Mn, where Mn is coordinated by an anionic PCP pincer-type ligand. Similar to the iron CCC, PCN and CCN pincer complexes investigated by Solans-Monfort, the M=CH 2 unit in the initial carbene complex 2-Mn (as depicted in Figure ) is directed away from the PCP plane at an obtuse angle. In the triplet state, 2-Fe and 2-Mn both have an almost ideal square planar geometry. In sharp contrast to the reported Fe systems, the initial carbene complex 2-Mn in the triplet state is 20.5 kcal/mol more stable than in the envisioned singlet state. This finding is also supported by OPBE calculations with a ∆G ST value of -10.3 kcal/mol.