σ-Aromaticity in polyhydride complexes of Ru, Ir, Os, and Pt

Abstract

Transition-metal hydrides represent a unique class of compounds, which are essential for catalysis, organic synthesis, and hydrogen storage. In this work we study IrH₅(PPh₃)₂, (RuH₅(PⁱPr₃)₂)⁻, (OsH₅(PⁱPr₃)₂)⁻, and OsH₄(PPhMe₂)₃ polyhydride complexes, inspired by the recent discovery of the σ-aromatic PtZnH₅⁻ cluster anion. The distinctive feature of these molecules is that, like in the PtZnH₅⁻ cluster, the metal is five-fold coordinated in-plane, and holds additional ligands at the axial positions. This work shows that the unusual coordination in these compounds indeed can be explained by σ-aromaticity in the pentagonal arrangement, stabilized by the atomic orbitals on the metal. Based on this newly elucidated bonding principle, we additionally propose a new family of polyhydrides that display a uniquely high coordination. We also report the first indications of how aromaticity may impact the reactivity of these molecules.