Ligands’ σ-donation and π-backdonation effects on metal-metal bonding in trinuclear [M3(Tr)2(L)3]2+ (M = Fe, Ni, Pd, Pt, Tr = tropylium and L = CO, HCN and C2H4) sandwich compounds: Theoretical investigation

Abstract

Abstract DFT calculations with full geometry optimization using GGA BP86 and meta-GGA M06L functionals have been performed on the [M3(Tr)2L3]2+ (M = Fe, Ni, Pd, Pt, Tr+ = C7H7+ and L = CO, HCN and η2-C2H4) sandwich complexes. The M3 triangular core is encapsulated between two tropylium cations which tends to establish M–L bonding with regards to the metal nature and the spin state. It turned out that each tropylium cation is connected to the M3 triangular core through an η2, η2, η2 coordination mode engaging six carbon atoms amongst seven. For the Fe singlet structures, the calculated HOMO–LUMO gaps by both BP86 and M06L are small, thus favouring the triplet one. The [Fe3(Tr)L3]2+ singlet structures are predicted to have two formal Fe Fe single bonds and one formal Fe Fe triple one within the Fe3 triangle based on the bond distances and the Wiberg bond indices (WBIs), while their analogues of triplet state exhibit two formal Fe Fe single bonds and one formal Fe Fe double bond. For the nickel, palladium and platinum d10 metal structures, they display large HOMO–LUMO gaps and adopt three formal metal–metal single bonds within the M3 core, in accordance with the small WBIs. The different binding capabilities of the isoelectronic CO, HCN and C2H4 auxiliary ligands are highlighted by the corresponding MOs’ diagrams and their donation and backdonation amounts.