Topological analysis of the electron density in the bismuth–chalcogen di- and tri-iron carbonyl complexes: [EBiFe2(CO)6]− and [EBiFe3(CO)9]− (E = Se, Te)

DFT calculations were conducted to investigate bismuth–chalcogen di- and tri-iron carbonyl complexes: [E Bi Fen(CO)₆]⁻ (E = Se, Te, and n = 2, 3). The study employed the electron localization function and quantum theory of atoms in molecules to analyze the Fe–Fe, Fe–Se, Fe–Te, Fe–Bi, and Fe–CO bonding interactions. Additionally, a number of integral and local topological characteristics of the electron density related to these interactions were analyzed, along with the source function (SF). The topological properties and delocalization indices related to the Bi–Se and Bi–Te interactions, denoted as δ(Bi–E), suggest substantial direct Bi–E bonding in complexes 1 and 2, but only a minimal Bi–E interaction in clusters 3 and 4. The computed topological characteristics correspond well with the transition metal complexes documented in the existing literature. The topological parameters of the Fe–Fe bonds in complexes 1–4, where a localized bond has been identified, differ significantly from the Fe1–Fe2 interactions in clusters 3 and 4, where neither the bond critical point nor the bond path between the metal atoms could be identified. The SF contributions to the Fe–Fe bond critical points primarily arise from Bi atoms, which account for over 66.7%. Additionally, carbonyl O atoms contribute more than 15.7%, while E ligands contribute more than 7.6%. The topological properties and the delocalization indices associated with the Bi–Se and Bi–Te interactions, δ(Bi–E), imply significant direct Bi–E bonding in complexes 1 and 2, but only a very minor Bi–E interaction in clusters 3 and 4. The study also revealed notable π-back-donation from CO to Fe, as indicated by the Fe…OCO delocalization indices and SF calculations.