Negative Ion Photoelectron Spectroscopy of Exotic Gaseous Iron-Cyanide Dianions [Fe(CN)n]2- (n = 4-6): Experiment and Many-Body Perturbation Theory.

Iron-cyanide complexes with unsaturated ligand coordination and unusual metal oxidation states are transient and highly reactive in the condensed phase, posing significant experimental and theoretical challenges for investigating their intrinsic properties. In this work, a series of exotic gaseous iron-cyanide complex dianions, [Fe(CN)n]2- (n = 4-6), are generated by electrospraying ferricyanide solution and studied using photoelectron spectroscopy. The GW-based many-body perturbation theory for the first time is applied to interpret the electronic structure of isolated 3d transition-metal complex ions. The experimental adiabatic/vertical detachment energies (ADEs/VDEs) for [Fe(CN)4]2-, [Fe(CN)5]2-, and [Fe(CN)6]2- are 0.35/0.65 eV, 1.54/1.74 eV, and 2.61/2.73 eV (±50 meV), respectively. The evGW calculation, which outperformed a hierarchy of GW methods in predicting VDE and fitting spectral envelope, assigns predominant isomers as quintet tetrahedral [Fe(CN)4]2-, quartet trigonal bipyramidal [Fe(CN)5]2-, and triplet octahedral [Fe(CN)6]2-. The covalency of the ligand field and Fe-CN orbital interactions are further assessed by supplementary analysis.