The structures of solvated complexes of Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and In(III) ions in 1,1,3,3-tetramethylurea (TMU) have been determined by means of EXAFS (extended X-ray absorption fine structure) and electronic spectroscopy. The solvation structures in TMU are square pyramidal for the Mn(II) and Ni(II) ions, distorted tetrahedral for the Co(II) and Cu(II) ions, tetrahedral for the Zn(II) ion, and octahedral for the Cd(II) and In(III) ions, while in water all these metal ions are six-coordinated octahedrons. The solvation structure of Fe(II) ion is square pyramidal or trigonal bipyramidal. In the bulky TMU solvent, the coordination number should be reduced for relaxation of the sterically repulsive interaction around the solvated metal ions. The metal-oxygen (M-O) bond lengths of solvated metal ions in TMU are

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The structures of copper(II) complexes with ethylenediamine (en) in aqueous and neat ethylenediamine solutions have been determined by the EXAFS (extended X-ray absorption fine structure) method. The copper(II) ion in en has an axially elongated octahedral structure with three en molecules. The Cu-N bond lengths in the four equatorial and two axial positions are 204(1) and 239(2) pm, respectively. The dynamic exchange reaction between en molecules in the bulk and en molecules bound to the copper(II) ion has been studied by the nitrogen-14 NMR line-broadening method. The rate constant and activation parameters for the ethylenediamine solvent exchange are as follows: kn = (1.4 ± 0.3) X 107 s"1 at 25 °C, AH* = 9.2 ± 2.5 kJ moH, and AS* = -77 ± 10 J moH K_1. The exchange reaction may be activated via the dissociative-interchange mechanism, followed by the formation of an intermediate with two monodentate en molecules and the occurrence of substantially fast axial-equatorial interconversion in the intermediate.

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The structure of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) and its complexes with alkali metal ions in aqueous solution has been investigated by X-ray diffraction and Raman spectroscopic methods at 25 °C. The X-ray scattering data and Raman spectrum for an aqueous 18-crown-6 solution show that free 18-crown-6 has a conformation of C1 or D3d symmetry. The molecule seems to be flexible and may be present as a mixture of the two conformations in aqueous solution. The structure of the lithium complex was not conclusive because of a weak scattering power of lithium atoms and weak complex formation between lithium ion and 18-crown-6. The sodium 18-crown-6 complex is estimated to have a structure similar to that found in crystal. The 18-crown-6 ring within the complex adopts the C1 conformation, where five oxygen atoms within 18-crown-6 coordinate to sodium ion at the equatorial position and an oxygen atom within 18-crown-6 and a water molecule at the axial one. The structure of the potassium complex is also similar to that in crystal in which the D3d conformation is taken. Potassium ion is located at the center of the mean plane of 18-crown-6 and one or two water molecules solvate potassium ion above and/or below the plane of 18-crown-6. It is suggested that the structure of the caesium complex is with either C1 or D3d symmetry, where caesium ion is apart from the mean plane of 18-crown-6. The rubidium complex was not examined because of a strong fluorescent X-ray emission from rubidium atoms when studied. The 18-crown-6 ring in the sodium and potiassium complexes is rather rigid probably because the cavity of the ring well fits to the metal ions, while the ring coordinating to large caesium ion becomes more flexible than that in the sodium and potassium complexes due to weaker interaction with caesium ion.