Metal–ligand bonding in tricarbonyliron(0) complexes bearing thiochalcone ligands

Abstract

This quantum chemical study aims to address metal–ligand bonding interactions between iron and thiochalcones in a series of recently synthesized tricarbonyliron(0) complexes with hetaryl- and/or ferrocenyl-functionalized thiochalcone ligands. A wide variety of theoretical methods, including both topological and orbital approaches, were used to shed light on the bonding situation of the thiochalcones η⁴-coordinated through their 1-thia-1,3-diene fragment to the Fe(CO)₃ moiety. In general, the interaction of Fe(CO)₃ with thiochalcones is considerably weaker than in comparison to the interaction with butadiene as well as with 1-thia-1,3-diene (substituted with two methyl groups). The decomposition of the Fe(CO)₃–thiochalcone binding energy reveals the dominant covalent nature of the interaction between the Fe(CO)₃ and thiochalcone fragments. This is confirmed by a more detailed examination of diatomic interactions between the Fe center and the 1-thia-1,3-diene fragment of thiochalcones. Further analysis of the Fe(CO)₃–thiochalcone bonding indicates extensive π-back-donation from the occupied d-orbitals of Fe to the LUMO of thiochalcone (π*). This explains (i) changes in the bond lengths of the η⁴-coordinated fragment of thiochalcones and (ii) charge distribution among the Fe center and the ligands in the complexes. Despite its formal zero oxidation state, the Fe center bears a positive atomic charge, while the thiochalcone and carbonyl ligands acquire ancillary electron charge. π-Delocalization is observed within the 1-thia-1,3-diene fragment of the η⁴-coordinated thiochalcones, yet the central C–C bond of the fragment exhibits somewhat stronger π-character.