Structure of FeI₂·[16]aneS₄: Square-Planar or Octahedral Iron Coordination?

The adducts FeI₂·[16]aneS₄ (1) and FeBr₂·[16]aneS₄ (2) are high-spin iron(II) compounds that may be described as either octahedral or square-planar coordinated complexes. Mössbauer measurements on 1 at various temperatures and applied fields analyzed with the spin-Hamiltonian formalism together with spin-polarized molecular orbital calculations in local density approximation clearly favor the octahedral description. Calculations with different iron-iodine bond lengths elucidate the structural dependence of Mössbauer parameters and orbital energies during the gradual transition from octahedral to square-planar like coordination. A suggestion for the geometry of 2 (d_Fe,Br = 2.47 Å) is obtained by minimizing the total binding energy. MO calculations provide evidence that unusually large iron-sulfur bond distances in 1 and 2 are forced by the size of the macrocycle [16]aneS₄ and are connected with unusually large axial bond distances.