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 o f MX(CO)(5) complexes (M = Mn, Re; X = Cl, Br, I). As found earlier fo r Mn-2(CO)(10) (Inorg. Chem. 1996, 35, 2886), the e(g)-type unoccupied 3d orbitals in the pseudooctahedral environment are located rather hi gh in the virtual orbital spectrum, and the corresponding ligand-field (LF) excitations are more than 1 eV above the lowest excitations. Pot ential energy curves (PECs) nevertheless show that the lowest excited states, which involve transitions to the Mn-Cl sigma orbital at equil ibrium geometry, are dissociative for axial and equatorial CO loss. Th e 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 d escend 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 ar e found to be very similar, but there is a large difference with respe ct 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 si gma --> sigma excitation and leads to a bond breaking.