Correlated ab initio study of the excited state of the iron-coordinated-mode noninnocent glyoxalbis(mercaptoanil) ligand

The intriguing and theoretically unresolved magnetic coupling in the Fe(gma)CN (1) compound [gma = glyoxalbis(mercaptoanil)] has been investigated by means of first-principle correlated ab initio calculations. The low-energy spectrum of the complex has been studied using the difference dedicated configuration interaction method, which is a dynamically correlated multiconfigurational method. In agreement with available spectroscopic information, we found that the ground-state doublet is dominated by the coupling between an iron-centered quartet and the first excited triplet on the gma ligand. The open-shell character of the electronic structure of the ligand clarifies its noninnocent nature. The low-energy spectrum reveals the presence of a first excited quartet of different symmetry lying 200 cm(-1) above. The lowest excitation energy in the ground-state symmetry is found at 4790 cm(-1), thus ruling out the simple description of the system based on a Heisenberg Hamiltonian.