Bonding and Electronic Structure Analyses of Homoleptic 18-Electron Sandwich Complexes of Fe and Cr with Peristylane-like Cages and Their Aza and Oxa Analogues

A comprehensive computational investigation of bonding, electronic structure, and stability of homoleptic 18-electron sandwich complexes of iron (Fe) and chromium (Cr) with [5]- and [6]-peristylanes, and their aza- and oxo-substituted derivatives, is carried out. Utilizing the density functional theory (DFT) methodology, optimization and vibrational frequency analyses at the PBE0-D4/def2-TZVPP and ωB97M-V/def2-QZVPP levels of theory, and analyses of the nature of the metal-ligand interactions with the natural adaptive orbital (NAdO), the extended transition state-natural orbitals for chemical valence (ETS-NOCV), and the quantum theory of atoms in molecules (QTAIM) analyses have been carried out. The study reveals these complexes to possess partial covalent character, and the eclipsed conformation is more stable than the staggered one. The work tries to deepen the theoretical understanding of transition metal sandwich complexes with strained organic molecules like the peristylanes.