Synthesis and Crystal Structure of Ni(VO3)2·4H2O and -Ni(VO3)2·2H2O

Two new nickel metavanadate hydrates, Ni(VO3)2·4H2O and -Ni(VO3)2·2H2O, have been synthesized using a "chimie douce" process; their crystal structures have been solved from single-crystal X-ray diffraction data. The tetrahydrate Ni(VO3)2·4H2O crystallizes in the monoclinic space group C2/c with a unit cell of dimensions a=12.969(5) ?, b=9.817(3) ?, c=6.880(2) ?, = 112.14(3)°; Z=4; the structure was determined from 1055 independent reflections and refined until R=0.024 and Rw=0.034. An orthorhombic cell has been found for the dihydrate -Ni(VO3)2·2H2O: a=5.530(1) ?, b=10.621(10) ?, c=11.756(5) ?, Z=4, space group Pnma; the structure was determined from 1201 independent reflections until R=0.017 and Rw=0.018. The structures of the two materials mainly differ in the vanadium environment in the (VO3)n-n metavanadate chains. They are constituted of edge-linked VO5 square pyramids in the tetrahydrate and from vertex-linked VO4 tetrahedra in the dihydrate. As the hydrogen atoms were localized in these two compounds, all the hydrogen bonds existing in these structures were clearly evidenced.