Density Functional Study of the Photodissociation of Mn2(CO)10

Potential energy curves (PECs) have been calculated for a number of excited states of Mn2(CO)10, along the Mn−Mn bond dissociation coordinate and along Mn−COax and Mn−COeq coordinates, in order to understand why irradiation into the σ → σ* 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 σ → σ* 3B2 PEC. The σ → σ* excited state is not itself Mn−CO dissociative. CO dissociation occurs since PECs that correspond at equilibrium geometry to dπ* → σ* 1,3E1 excited states (nearly degenerate with the σ → σ* excited state) are Mn−COax dissociative (both 1E1 and 3E1, 1,3E in C4v) or Mn−COeq dissociative (only just, and only the b3A‘ component (in Cs) of 3E1). The Mn−CO dissociative character has been traced to the precipitous lowering of the initially high-lying Mn−CO σ-antibonding (3d(eg)-like) orbitals upon Mn−CO bond lengthening, making them considerably lower than σ* in Mn2(CO)9. Excitations to these orbitals (the ligand-field (LF) excitations) are at high energy in Mn2(CO)10, much higher than the σ → σ* and dπ* → σ* excitations. However, the energy of these LF excited states very rapidly goes down upon Mn−CO bond lengthening, they cross the σ → σ* and dπ* → σ* excited states, and the energy lowering of the LF excitation energy in Mn2(CO)9 with respect to the lowest excitation energies in Mn2(CO)10, to 1,3B2 σ → σ* and 1,3E1 dπ* → σ*, provides the energy for the Mn−CO bond breaking.