Structure and energetics of Fe-2(CO)(8) singlet and triplet electronic states

The singlet and triplet state potential energy surfaces (PES) of Fe-2(CO)(8) are explored by means of density functional theory (DFT) techniques. The two PES have different global mimima: the dibriged C-2v isomer for the singlet and the unbridged D-2d isomer for the triplet. The sign of the energy gap between singlet and triplet global minima depends on the type of adopted DFT functional: hybrid functionals predict the triplet is more stable than the singlet, but the opposite applies to generalized gradient approximated (GGA) functionals. The analysis of the computed CO stretching frequencies demonstrates that the experimental data for the unbridged form is compatible also with the unbridged triplet D-2d isomer. Starting from these two facts, the electronic structure of unbridged D-2d Fe-2(CO)(8) is discussed herein. Single-point energy computations at the coupled-cluster single and double (CCSD) level favor the D-2d triplet state.