Density functional calculations have been performed on the ground and excited states of MnCl(CO)(5) in order to explain the photochemistry of MX(CO)(5) complexes (M = Mn, Re; X = Cl, Br, I). As found earlier for Mn-2(CO)(10) (Inorg. Chem. 1996, 35, 2886), the e(g)-type unoccupied 3d orbitals in the pseudooctahedral environment are located rather high in the virtual orbital spectrum, and the corresponding ligand-field (LF) excitations are more than 1 eV above the lowest excitations. Potential energy curves (PECs) nevertheless show that the lowest excited states, which involve transitions to the Mn-Cl sigma* orbital at equilibrium geometry, are dissociative for axial and equatorial CO loss. The mechanism is again, as in Mn-2(CO)(10), a strongly avoided crossing of the lowest excited state (a(1,3)E) with the higher dissociative LF states (different ones for COax and COeq dissociation) which rapidly descend upon Mn-CO bond lengthening. In spite of the lowest excitation being to the Mn-CI sigma*-orbital, Mn-Cl homolysis cannot occur out of the lowest excited state. The photochemical behavior of Mn-2(CO)(10), MnH(CO)(5), and MnCl(CO)(5) is compared. The mechanisms of CO loss are found to be very similar, but there is a large difference with respect to the breaking of the a bond (Mn-Mn, Mn-H, or Mn-CI). Only in the case of Mn-2(CO)(10), the lowest broad absorption band contains the sigma --> sigma* excitation and leads to a bond breaking.