Abstract
The stability constants of the 1 :1 complexes of trimethylenediamine‐N, N, N′ N′ tetraacetate ‘TMTA’ with rare earth trivalent cations have been checked. The heats involved in complex formation have been determined.The thermodynamic data show a similar trend as for those of the EDTA complexes, but the discontinuities in the plots ΔH vs. 1/r and ΔS vs. 1/r are displaced towards lower values of 1/r.A change of the coordination number and of the number of coordinated water molecules for the complexes along the series La3+ – Lu3+ is suggested. The simultaneous presence in solution of species differing only in the number of bonded H2O for some members of the series has been confirmed by spectrophotometric measurements in the temperature range 2 to 70°.All facts support the conclusion that in dilute solution all SEIII aquo ions exhibit the same coordination number.

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