Abstract
The solid‐state structures of two new homodinuclear chelate complexes with the late lanthanide(III) ions Yb and Lu, [Na~2~(Yb~2~OHEC)]^.^14.5H~2~O (1), and [Na~2~(Lu~2~OHEC)]^.^14.5H~2~O (2) (H~8~OHEC = 1,4,7,10,14,17,20,23‐octaazacyclohexacosane‐ 1,4,7,10,14,17,20,23‐octaacetic acid), have been determined by X‐ray crystal structure analysis. Each lanthanide(III) ion is coordinated by eight donor atoms of the ligand and the geometry of the coordination polyhedron approaches a bicapped trigonal prism. These structures are compared with those of the homodinuclear chelate complexes with the same ligand and the mid to early lanthanide(III) ions Gd, Eu, La and also Y. A distinctive structural change occurs across the lanthanide series. The centrosymmetric mid to early lanthanide(III) complexes are all ninefold‐coordinated in a capped square antiprismatic arrangement with a water molecule coordinated in a prismatic position. This structure is maintained in aqueous solution, together with an asymmetric minor isomer. The late lanthanide(III) OHEC complexes not only lack the inner‐sphere water, but the change of coordination sphere also results in a loss of symmetry of the whole complex molecule. The observed change of coordination mode and number of the lanthanide ion may offer a geometric model for the isomerization process in eight‐ and ninefold‐coordinated complex species that are isomers in a possible coordination equilibrium observed by NMR in aqueous solution. This model may also explain the intramolecular rearrangements necessary during water exchange in the inner coordination sphere of the complex [(Gd~2~OHEC)(H~2~O)~2~]^2−^ through a slow dissociative mechanism. Protonation constants of the H~8~OHEC ligand and complex formation constants of this ligand with Gd^III^, Ca^II^, Cu^II^ and Zn^II^ have been determined by solution thermodynamic studies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)