Revisiting the polytopal rearrangements in penta-coordinate d⁷-metallocomplexes: modified Berry pseudorotation,...

This paper provides a first-principles theoretical investigation of the polytopal rearrangements and fluxional behavior of five-coordinate d⁷-transition metal complexes. Our work is primarily based on a potential energy surface analysis of the iron tetracarbonyl hydride radical HFe˙(CO)₄. We demonstrate the existence of distorted coordination geometries in this prototypical system and, for the first time, introduce three general rearrangement mechanisms, which account for the non-ideal coordination. The first of these mechanisms constitutes a modified version of the Berry pseudorotation via a square-based pyramidal C_4v transition state that connects two chemically identical edge-bridged tetrahedral stereoisomers of C_2v symmetry. It differs from the classical Berry mechanism, which involves two regular D_3h equilibrium structures and a C_4v transition state. The second mechanism is related to the famous "tetrahedral jump" hypothesis, postulated by Muetterties for a number of d⁶ HML₄ and H₂ML₄ complexes. Here, our study suggests two fluxional rearrangement pathways via distinct types of C_2v transition states. Both pathways of this mechanism can be described as a single-ligand migration to a vacant position of an "octahedron", thus interchanging (switching) the apical and basal ligands of the initial quasi-square pyramidal isomer, which is considered as an idealized octahedron with a vacancy. Accordingly, we call this mechanism "octahedral switch". The third mechanism follows a butterfly-type isomerization featuring a key-angle deformation, and we thus call it "butterfly isomerization". It connects the quasi-square pyramidal and edge-bridged tetrahedral isomers of HFe˙(CO)₄ through a distorted edge-bridged tetrahedral transition state of C_s symmetry. Our paper discusses the overall features of the isomers and rearrangement mechanisms as well as their implications. We rationalize the existence of each stationary point through an electronic structure analysis and argue their relevance for isolobal analogues of HFe˙(CO)₄.