Density Functional Study of the Primary Photoprocesses of Manganese Pentacarbonyl Chloride (MnCl(CO) ₅ )

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 CO(ax) and CO(eq) dissociation) which rapidly descend upon Mn-CO bond lengthening. In spite of the lowest excitation being to the Mn-Cl 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 sigma bond (Mn-Mn, Mn-H, or Mn-Cl). Only in the case of Mn(2)(CO)(10), the lowest broad absorption band contains the sigma --> sigma excitation and leads to sigma bond breaking.