The precise nature of the excited states of Mn2(CO)lo leading to the well-known photochemistry-both Mn-Mn and Mn-CO bond breaking upon low-energy excitation-is still unclear. In order to identify possibly dissociative excited states (either Mn-Mn, Mn-CO,, or Mn-CO,,), the nature of the highest occupied Mn-3d orbitals is analyzed as well as the composition of the virtual orbitals. The following features are noted. (a) The low-energy excitations at 337-355 nm arise from B - CJ* and dn - u* excitations, while d - d excitations occur at much higher energy. (b) The Mn-Mn CJ bonding HOMO as well as the u* LUMO cannot simply be classified as arising from the 3d,2 components of eg parentage in the local octahedrons around Mn, they have little 3d,2 - 3d,2 (anti)bonding character but significant contributions come from M n - 4 ~ a~n d CO-2ne,, orbitals. Mn-Mn B antibonding is only strong in the B* orbital due to these contributions. (c) Due to the strong involvement of Mn-4p2, th 3d,2 orbital not only occurs in the B and u* orbitals but also in a higher set of virtuals, denoted d,d*, -1.5 eV above the u* orbital. Antibonding with axial CO’s is strong in these higher virtuals but absent or weak in the B and CJ* orbitals. CJ antibonding with equatorial CO’s is strong in the 3d2+ orbital of eg parentage, that is located very high in the virtual spectrum, -2 eV above the u* orbital. Mn-Mn dissociation will occur only from the B - B* excitation; CO loss will probably occur from the high-lying d - d excited states (excitations into d,d* and 3d2-,2). The observed photochemistry at low energy will have to be explained from curve crossings between the low-energy excited states and the photoactive states.
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