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

The crystal structure of [C(NH2)(3)](3)[Ho(ttha)] . 3H(2)O (H(6)ttha = trie thylenetetraaminehexaacetic acid) has been determined. The coordination env ironment 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)] . 3H(2)O single crystal were measured at room and liquid helium temp eratures and compared with those of the Ho-III-ttha (1:1) complex in soluti on. The results suggest that a species of coordination geometry similar to that of the [Ho(N4O5)](3-) type found in the crystal exists predominantly i n solution at pH > 5. At lower pH values, as for other Ln(III)-ttha complex es, coordination equilibria are observed.