DENSITY-FUNCTIONAL STUDY OF GROUND AND EXCITED-STATES OF MN-2(CO)(10)

The precise nature of the excited states of Mn-2(CO)(10) leading to th e well-known photochemistry-both Mn-Mn and Mn-CO bond breaking upon lo w-energy excitation-is still unclear. In order to identify possibly di ssociative excited states (either Mn-Mn, Mn-COax or Mn-COeq), the natu re of the highest occupied Mn-Sd orbitals is analyzed as well as the c omposition of the virtual orbitals. The following features are noted. (a) The low-energy excitations at 337-355 nm arise from sigma --> sigm a and d pi --> sigma* excitations, while d --> d excitations occur at much higher energy. (b) The Mn-Mn sigma bonding HOMO as well as the s igma LUMO cannot simply be classified as arising from the 3d(z2) comp onents of e(g) parentage in the local octahedrons around Mn, they have little 3d(z2) - 3d(z2) (anti)bonding character but significant contri butions come from Mn-4p(z) and CO-2 pi(eq) orbitals. Mn-Mn sigma antib onding is only strong in the sigma orbital due to these contributions . (c) Due to the strong involvement of Mn-4p(z), th 3d(z2) orbital not only occurs in the sigma and sigma orbitals but also in a higher set of virtuals, denoted sigma,'sigma', similar to 1.5 eV above the sigm a orbital. Antibonding with axial CO's is strong in these higher virt uals but absent or weak in the sigma and sigma orbitals. sigma antibo nding with equatorial CO's is strong in the 3d(x2)-y(2) orbital of e(g ) parentage, that is located very high in the virtual spectrum, simila r to 2 eV above the sigma orbital. Mn-Mn dissociation will occur only from the sigma --> sigma excitation; CO loss will probably occur fro m the high-lying d --> d excited states (excitations into sigma',sigma and the 3d(x2-y2)). The observed photochemistry at low energy Will h ave to be explained from curve crossings between the low-energy excite d states and the photoactive states.