dicarbonyl complex [Mn(PCP)(CO) 2 ], similar to as described by Tonzetich and coworkers. Depending on the stability of the depicted system under experimental conditions, either a direct reaction and formation of an olefin adduct or metallacycle (7-Mn) or alternatively loss of remaining CO to eventually form a 4-fold coordinate complex such as 2-Mn (vide supra) seem possible. The electronic effect of including a CO ligand in the carbene complex 6-Mn is clearly expressed in the stabilization of the singlet state and a ∆G ST value of 8.1 kcal/mol. Note that the corresponding complex without CO is favored in the triplet state. However, an adverse effect is observed for the singlet-triplet splitting in the metallacycle 7-Mn: A 6-fold octahedral coordination imposes strong geometrical constraints on the metallacycle and forces the metallacycle to adopt a distorted square pyramidal geometry rather than a trigonal bipyramidal coordination as observed naturally for 1-Mn and 2-Mn in the singlet state. As a consequence, the singlet state is destabilized in comparison to the triplet state. Accordingly, a singlet-triplet splitting of -6.3