A novel class of aromatic compounds: Metal-centered planar cations [Fe(Sb₅)]⁺ and [Fe(Bi₅)]⁺

Iron enters the pentagon: Quantum chemical calculations using gradient-corrected DFT predict that the cations [Fe(Sb₅)]⁺ and [Fe(Bi₅)]⁺ in the electronic singlet state have planar (D_5h) equilibrium geometries (see scheme). Analysis of the electronic structure shows that the molecules are metal-centered six-π-electron aromatic species with strong iron-ligand π bonds which involve the d(π) atomic orbitals of the Fe center and the degenerate π orbital of the ring. The calculated ⁵⁷Fe NMR chemical shifts indicate extremely high deshielding of the metal nucleus.