Thermochemistry of Lanthanide Complexes

Abstract

VALUES are now available for ΔH₁, the heat content change in the reaction where Ln represents a lanthanide element and Y a complexing agent such as the ethylenediaminetetraacetate (EDTA) or nitrilo-triacetate (NTA) ion. The measurements usually refer to dilute solutions of molar concentrations of ∼ 0.01 or less. The dependence of ΔH₁ on the atomic number of the lanthanide is often rather complicated, a plot of one quantity against the other showing both a maximum and a minimum. This is true, for example, of the EDTA¹ and 1:1 NTA complexes^2,3 and the 1:1 and 1:2 diglycollate and dipicolinate complexes⁴. There are, however, grounds for believing that a complicating factor may be a change somewhere in the middle of the series in the effective hydration number of the uncomplexed lanthanide ion, consequent on the lanthanide contraction. This factor can be eliminated by measuring the integral heats of solution of a series of isomorphous lanthanide salts to give suitably dilute solutions. Such series of salts covering all the lanthanides are not numerous, but two well known ones are the bromates and the ethylsulphates, both of which crystallize as enneahydrates. If, for example, ΔH₂ is measured for the process then ΔH₁ + ΔH₂ = ΔH₃, where ΔH₃, is the heat content increase for the process from which the simple hydrated lanthanide ion has been eliminated.