Structure and optical spectroscopy of holmium(III) triethylenetetraaminehexaacetate in single crystal and in solution

Abstract

The crystal structure of [C(NH₂)₃]₃[Ho(ttha)]·3H₂O (H₆ttha=triethylenetetraaminehexaacetic acid) has been determined. The coordination environment of the Ho^III ion is composed of five carboxylate oxygen atoms and four nitrogen atoms. Electronic absorption spectra of a [C(NH₂)₃]₃[Ho(ttha)]·3H₂O single crystal were measured at room and liquid helium temperatures and compared with those of the Ho^III–ttha (1:1) complex in solution. The results suggest that a species of coordination geometry similar to that of the [Ho(N₄O₅)]³⁻ type found in the crystal exists predominantly in solution at pH>5. At lower pH values, as for other Ln^III–ttha complexes, coordination equilibria are observed.