Applications of extended X-ray absorption fine structure spectroscopy to the study of polyoxometalates

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to characterise polyoxometalates of the type M(m)O(y)(n-) and X(x)M(m)O(y)(n-) in the solid state and in solution. The best fit to the low-temperature (80 K) niobium K-edge EXAFS of K-7[HNb6O19]. 13H(2)O was obtained with one terminal oxygen at 1.79 Angstrom (M-O-t), four doubly bridging oxygens at 1.97 Angstrom (M-O-b), four niobiums at 3.32 Angstrom and one niobium at 4.76 Angstrom. The Nb-O-b-Nb bond angle was determined as 113 degrees following a multiple-scattering calculation for this unit. Analysis of the niobium K- and tungsten L(III)-edge EXAFS of salts of NbxW6-xO19(x + 2-) (x = 2-4) yielded Nb-O bond lengths of 1.74-1.76 and 1.98-1.99 Angstrom, W-O bond, lengths of 1.72-1.74 and 1.89-1.91 Angstrom, and non-bonded M ... M distances of 3.31-3.35 and 4.74-4.78 Angstrom. The first co-ordination spheres also comprise one six-fold bridging oxygen at about 2.3 A but this shell is poorly defined due to static and thermal disorder in the bond. A combination of vanadium K- and tungsten L(III)-edge EXAFS of [V2W4O19](4-) yielded W ... W separations of 3.27 Angstrom but shorter V ... W separations of 3.20 Angstrom, showing a slight distortion in the hexametalate structure that is not observed for the niobotungstates. Molybdenum K-edge EXAFS analysis of [NBu(4)(n)](3)[PMo12O40] in the solid state at 80 K revealed systematic asymmetry in the Mo-O-b bonds with two oxygens at 1.81 and two at 1.97 Angstrom. The structure is not perturbed when the salt is dissolved in acetonitrile. Likewise-the molybdenum K-edge EXAFS of [TeMo6O24](6-) in water and [Mo6O19](2-) in acetonitrile analysed for the same co-ordination spheres as those of the parent salts Na-3[TeMo6O24]. nH(2)O and [NBu(4)(n)](2)[Mo6O19].