STRUCTURE OF FEI2-CENTER-DOT[16]ANES(4) - SQUARE-PLANAR OR OCTAHEDRALIRON COORDINATION

The adducts FeI2 .[16]aneS(4) (1) and FeBr2 .[16]aneS(4) (2) are high- spin iron(II) compounds that may be described as either octahedral or square-planar coordinated complexes. Mossbauer measurements an 1 at va rious temperatures and applied fields analyzed with the spin-Hamiltoni an formalism together with spin-polarized molecular orbital calculatio ns in local density approximation clearly favor the octahedral descrip tion, Calculations with different iron-iodine bond lengths elucidate t he structural dependence of Mossbauer parameters and orbital energies during the gradual transition from octahedral to square-planar like co ordination. A suggestion for the geometry of 2 (d(Fe,Br) = 2.47 Angstr om) is obtained by minimizing the total binding energy. MO calculation s provide evidence that unusually large iron-surfur bond distances in 1 and 2 are forced by the size of the macrocycle [16]aneS(4) and are c onnected with unusually large axial bond distances.