Theoretical Investigation on the Electronic Structure of Pentacyano(L)ferrate(II) Complexes with NO+, NO, and NO- Ligands. Redox Interconversion, Protonation, and Cyanide-Releasing Reactions

Reaction pathways for the one- and two-electron reductions of [Fe(CN)5NO]2- have been investigated by means of a density functional theory (DFT) approach combined with the polarized continuum model (PCM) of solvation. In addition, UV−vis spectroscopic data were obtained using ZINDO/S calculations including a point-charge model simulation of solvent effects. DFT methodologies have been used to assess the thermodynamical feasibility of protonation and cyanide-release processes for the reduced species. We conclude that [Fe(CN)5NO]3- is a stable species in aqueous solution but may release cyanide yielding [Fe(CN)4NO]2-, consistent with experimental results. On the other hand, the [Fe(CN)5NO]4- complex turns out to be unstable in solution, yielding the product of cyanide release, [Fe(CN)4NO]3-, and/or the protonated HNO complex. All the structural and spectroscopic (IR, UV−vis) predictions for the [Fe(CN)5HNO]3- ion are consistent with the scarce but significant experimental evidence of its presence as an intermediate in nitrogen redox interconversion chemistry. Our computed data support an FeII(LS) + NO+ assignment for [Fe(CN)5NO]2-, an FeII(LS) + NO assignment for the one-electron reduction product, but an FeI(LS) + NO+ for the one-electron product after dissociation of an axial cianide, and an FeII + singlet NO- for the two-electron reduction species.