Raman study of ferric nitrate crystalline hydrate and variably hydrated liquids

A Raman investigation on crystalline ferric nitrate enneahydrate (FNEH) has shown that ${\mathrm{NO}}_3^{-}$ and ${\mathrm{Fe(H}}_2\mathrm{O}{)}_6^{\text{3+}}$ ions are present in two distinct environments in the monoclinic unit cell of the crystal. In FNEH melt, it has been found that ${\mathrm{NO}}_3^{-}$ and Fe³⁺ ions form a complex in which water is also coordinated to the Fe³⁺ ion. Raman evidence is presented which indicates that ${\mathrm{NO}}_3^{-}$ ion in the nitrate complex is coordinated to the Fe³⁺ ion in monodentate fashion, with an appreciable covalency in the ${\mathrm{Fe}}^{\text{3+}}{\mathrm{–ONO}}_2^{-}$ bond. The addition of water to FNEH melt causes dissociation of the nitrate complex. This complex almost ceases to exist when the water content of the melt is raised to 28 mole of water per mole of the salt. A weak polarized band at ∼407 cm⁻¹ in the Fe(NO₃)₃ · xH₂O system (x ≥ 28) has been attributed to the presence of the hydrolysis product of Fe³⁺ ions. The effect of dilution and of temperature on this band has also been investigated. It is suggested that the hydrolysis product of the salt exists in the form of a polymeric species.