Untersuchung von Eisen(III)-phosphiten im Hinblick auf die Wasserstoffbindungen

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 C_3v to the C_s group. This anion is characterised by two bonding distances between phosphorus and oxygen atoms,r _PO=1,46 Å andr _{PO 2}=1,50 Å, the respective force constants beingK _PO=8.7 mdyn/Å andK _PO2=7.1 mdyn/Å. Three types of hydrogen bonds occur in the crystal lattices of the compounds studied. The weakest bond (bond lengthr=2.86–2.88 Å, 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 Å,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 Å) is formed by polyorthophosphite anions, linking hydroxyl groups to oxygen atoms bound to different phosphorus atoms.