Potential energy curves (PECs) have been calculated for a number of excited states of Mn-2(CO)lo, along the Mn-Mn bond dissociation coordinate and along Mn-COax and Mn-COeq coordinates, in order to understand why irradiation into the sigma --> sigma* band does not only lead to Mn-Mn bond breaking but also to Mn-CO dissociation. Mn-Mn bond homolysis can straightforwardly occur along the dissociative sigma --> sigma* B-3(2) PEC. The sigma --> sigma* excited state is not itself Mn-CO dissociative. CO dissociation occurs since PECs that correspond at equilibrium geometry to d(pi)* --> sigma* (1,3)E(1) excited states (nearly degenerate with the sigma --> sigma* excited state) are Mn-COax dissociative (both (1)E(1) and (3)E(1), (1,3)E in C-4 upsilon) or Mn-COeq dissociative (only just, and only the b(3)A’ component (in C-s) of (3)E(1)). The Mn-CO dissociative character has been traced to the precipitous lowering of the initially high-lying Mn-CO sigma-antibonding (3d(e(g))-like) orbitals upon Mn-CO bond lengthening, making them considerably lower than sigma* in Mn-2(CO)(9). Excitations to these orbitals (the ligand-field (LF) excitations) are at high energy in Mn-2(CO)(10), much higher than the sigma --> sigma* and d(pi)* --> sigma* excitations. However, the energy of these LF excited states very rapidly goes down upon Mn-CO bond lengthening, they cross the alpha --> alpha* and d(pi)* --> sigma* excited states, and the energy lowering of the LF excitation energy in Mn-2(CO)(9) with respect to the lowest excitation energies in Mn-2(CO)(10), to B-1,3(2) sigma --> sigma* and (1,3)E(1) d(pi*) --> sigma*, provides the energy for the Mn-CO bond breaking.