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.