Electronic Structure of Open-Shell Tetrahedral {Fe(NO)2}9 Dinitrosyliron Complexes

High-level ab initio excited-state theory is employed to investigate the electronic structure of doublet {Fe(NO)2}9 species in the ground state and compared with the results obtained by density functional theory. Both of the approaches consistently suggest that the linear NO ligands in dinitrosyliron complexes (DNICs) feature a radical character. Theoretical calculations also predict that the cyanide-supported DNIC anion of [(NC)2Fe(NO)2]− features C2v symmetry with a Fe–C–N bonding motif, and multireference theories suggest a minimal active space of CAS(9,9) to describe these {Fe(NO)2}9 compounds, while larger CAS(13,13) calculations do not tend to significantly improve the geometries. Experimental vibration modes of NO ligands are also accurately assigned due to second-order n-electron valence state perturbation theory.