On the structure and volumetric properties of solvated lanthanoid(III) ions in amide solutions

The coordination chemistry and the volumetric properties of three representative lanthanoid(III) ions--lanthanum(III), gadolinium(III), and lutetium(III)--have been studied in three amide solvents with gradually increasing spatial demand upon coordination: N,N-dimethylformamide (dmf) < N,N-dimethylacetamide (dma) < N,N-dimethylpropionamide (dmp). Large angle X-ray scattering (LAXS) and EXAFS have been used to determine the structure of the solvated lanthanoid(III) ions in solution, further supplemented with a crystallographic study on octakis(N,N-dimethylacetamide)lanthanum(III) triflate, [La(dma)8](CF3SO3)3. The selection of ions and solvents allows an estimate of the steric congestion effects on the resulting coordination number, CN, ranging from nine for lanthanum(III) ions in dmf to seven for the smaller lutetium(III) ion in space-demanding dma. The standard partial molar volumes of the solvated lanthanoid(III) ions in water and dmf are reflected in the CNs, as these solvent molecules are small enough to not interfere with each other upon coordination. However, the larger and more space-demanding dma displays a different pattern with an almost constant standard partial molar volume and a decreasing CN, counterbalancing the difference in ionic radius of the lanthanoid(III) ion.