Crystal structure and Raman spectra of [Fe(H2O)6]3+(ClO4-)3•3H2O

Single crystal X-ray diffraction is used to study the structure of colorless crystals isolated from the saturated aqueous solution of trivalent iron perchlorate (TIP) in 67.5% perchloric acid. It is found that the compound crystallizes in the trigonal crystal symmetry; parameters of the hexagonal unit cell: a = b = 16.079(2) , c = 11.369(2) , alpha = beta = 90A degrees, gamma = 120A degrees, space group R{ie907-1}(S (6)), Z = 6, rho(calc) = 2.021 g/cm(3). The structural form of the crystal hydrate is [Fe(H2O)(6)](3+)(ClO (4) (-) )(3)center dot 3H(2)O. The structure contains two independent complex iron cations. Each of them is in the special position {ie907-2}, but retains the regular octahedral structure: average bond lengths are r(Fe-O) = 1.997(1) , {ie907-3}O-Fe-O bond angles differ from 90A degrees by only 0.93A degrees. Independent [Fe(H2O)(6)](3+) cations form short H-bonds (OaEuro broken vertical bar O 2.64 ) with three crystallization water molecules and somewhat longer H-bonds (OaEuro broken vertical bar O 2.73 ) with three ClO (4) (-) anions. The ClO (4) (-) anion is disordered over two positions with occupancies of 0.62(2) and 0.37(2). Both positions correspond to the general position. The outer-sphere crystallization water molecule is characterized by the tetrahedral direction of H-bonds, which it forms with two anions and two independent [Fe(H2O)(6)](3+) cations. All water molecules are in the general position. The Raman spectroscopic study of polycrystalline samples reveals weak bands belonging to the internal vibrations of two types of water molecules. The least broad bands are assigned to the transitions of crystallization water molecules whose symmetry is insignificantly lowered by two anion-molecular Hbonds. Anomalously broad bands are assigned to the transitions of a coordinated water molecule whose symmetry is more lowered by intermolecular and anion-molecular H-bonds.