MOLECULAR-STRUCTURE DETERMINATION BY EXAFS OF [Y(NCS)(6)](3-) UNITS IN SOLID-STATE AND IN SOLUTION - A COMPARISON WITH DENSITY-FUNCTIONAL THEORY CALCULATIONS

The structure of one of the rare octahedral Y3+ complexes, hexakis(thi ocyanato-N)yttrate(III), has been elucidated with extended X-ray absor ption fine structure (EXAFS) spectroscopy and confirmed by density fun ctional theory (DFT) calculations. The analysis of EXAFS spectra indic ates coordination through nitrogen atoms, already suggested by IR and NMR data, and provides information about the linear arrangement of the NCS- ligands with the yttrium atoms inside the complex. This arrangem ent emphasizes multiple scattering contributions to the EXAFS signal, due to the focusing scattering effect, and allows the accurate determi nation of the structure of the whole complex up to the third coordinat ion shell, which is distant by more than 5 Angstrom from the absorbing atom, Y, a resolution without precedent in the use of the technique. The best reproduction of the solid state and acetonitrile solution spe ctra was achieved with the same structure: a symmetric octahedron with coordination distances equal to 2.36(1), 3.5(1), and 5.1(2) Angstrom for Y-N, Y-C, and Y-S shells, respectively, the only difference betwee n both spectra being the higher dynamic disorder of the solution spect rum. DFT calculations predict this geometry as the most stable, discar ding other arrangements in which the coordinating atom is sulfur. The agreement between EXAFS data and DFT optimized structure is quite high , and differences' between predicted and experimental IR bands are bel ow 5%.