Crystal structure and spectroscopic properties of Na(2)K(6)(VO)(2)(SO(4))(7).

Red-brown crystals of a new mixed alkali oxo sulfato vanadium(V) compound Na(2)K(6)(VO)(2)(SO(4))(7), suitable for X-ray determination, have been obtained from the catalytically important binary molten salt system M(2)S(2)O(7)-V(2)O(5) (M = 80% K and 20% Na). By slow cooling of a mixture with the mole fraction X(V(2)O(5)) = 0.24 from 325 degrees C, i.e., just below the liquidus temperature, to the solidus temperature of around 300 degrees C, a dark reddish amorphous phase was obtained containing crystals of the earlier described V(V)-V(IV) mixed valence compound K(6)(VO)(4)(SO(4))(8) and Na(2)K(6)(VO)(2)(SO(4))(7) described here. This compound crystallizes in the tetragonal space group P4(3)2(1)2 (No. 96) with a = 9.540(3) A, c = 29.551(5) A at 20 degrees C and Z = 4. It contains a distorted VO(6) octahedron with a short V-O bond of 1.552(6) A, a long one of 2.276(5) A trans to this, and four equatorial V-O bonds in the range 1.881(6)-1.960(6) A. The deformation of the VO(6) octahedron is less pronounced compared to that of the known oxo sulfato V(V) compounds. Each VO(3+) group is coordinated to five sulfate groups of which two are unidentately coordinated and three are bidentate bridging to neighboring VO(3+) groups. The length of the S-O bonds in the S-O-V bridges of the two unidentately coordinated sulfato groups are 1.551(6) A and 1.568(6) A, respectively, which are unusually long compared to our earlier measurements of sulfate groups in other V(III), V(IV), and V(V) compounds.