Investigation of iron(III) phosphites with regard to their hydrogen bonding

Abstract

Iron(III) phosphites, vic. Fe2(HPO3)3·9 H2O, FeH3P2O6·3 H2O, FeH6P3O9·H2O and Fe4H33P15O45·6 H2O were studied by means of powder X-ray, thermographic, IR and UV spectroscopy methods and by measurement of magnetic susceptibility. From the results obtained, and from analogy with phosphites studied earlier, the following structural model can be proposed: in the compounds studied, every iron atom is surrounded by six oxygen atoms of the water molecules and phosphite or, polyorthophosphite anions which form a weak ligand field of approximately octahedral symmetry. In Fe2(HPO3)3·9 H2O, symmetry of the anion is decreased from the point group C3v to the Cs group. This anion is characterised by two bonding distances between phosphorus and oxygen atoms,r PO=1,46 A andr PO 2=1,50 A, the respective force constants beingK PO=8.7 mdyn/A andK PO2=7.1 mdyn/A. Three types of hydrogen bonds occur in the crystal lattices of the compounds studied. The weakest bond (bond lengthr=2.86–2.88 A, bond energyE=4.6–5.0 kcal/bond) is formed between molecules of hydrate water, its energy approaching that of the hydrogen bond in liquid water. The stronger hydrogen bond (r=2.67–2.70 A,E=5.7 to 8.0 kcal/bond) is found between water molecules and phosphite or polyorthophosphite anions. The strongest hydrogen bond (r=2.55–2.64 A) is formed by polyorthophosphite anions, linking hydroxyl groups to oxygen atoms bound to different phosphorus atoms.