Cyclic polyvanadates incorporating template transition metal cationic species: Synthesis and structures of hexavanadate [PdV6O18](super 4-), octavanadate [Cu2V8O24](super 4-), and decavanadate [Ni4V10O30(OH)(sub 2)(H2O6)](super 4-)

Three types of heteropolyvanadates, [(C2H5)(sub 4)N](sub 4)[PdV6O18], [(C2H5)(sub 4)[Cu2V8O24], and [(C6H5)(sub 4)P](sub 4)[Ni4V10O30(OH)(super 2)(H2O)(sub 6)] were synthesized through the reaction between the [VO3]- anion and metal template cations of Pd(II), Cu(II), and Ni(II). Structural studies of cyclic heteropolyoxovanadates have allowed gaining insight into the metal template effect on the formation of the cyclic polyoxovanadate species in acetonitrile.