Polyoxoanions assembled by the condensation of vanadate, tungstate and selenite: solution studies and crystal structures of the mixed metal derivatives (NMe 4_{4}) 2_{2} Na 2_{2} [WVI^{VI} 4_{4} V V^{V} 2_{2} O 19_{19} ]·8H2_{2}O and (NMe4_{4} )4.83_{4.83} [(SeIV^{IV}WVI^{VI}4.57_{4.57} VV^{V}4.43_{4.43}O33_{33})2_{2} (WVI^{VI}(O)(H2_{2}O))(V V^{V}O) 2.6_{2.6} ]·10.57H2_{2}O

The reaction of acidified aqueous solutions of Na2WO4, NaVO3 and SeO2 in a 16 : 4 : 3 molar ratio gives a complex mixture of polyoxometalates (POMs) with both WVI and VV addenda, formulated as [(SeIVWVIxVV9−xO33)2(WVIO(H2O))(VO)m]n− with x = 4, 5, 6; m = 2 or 3. In solution, the sandwich-type complexes [(SeIVWVIxVV9−xO33)2(WVIO(H2O))(VO)m]n− slowly lose Se and convert into [WVI4VV2O19]4− and further to [WVI5VVO19]3− with a Lindqvist-type structure. These transformations were monitored with 51V and 77Se NMR, electrospray mass-spectrometry and capillary electrophoresis. Crystals of (NMe4)2Na2[(W4V2)O19]·8H2O (1) and (NMe4)4.83[(SeW4.57V4.43O33)2(W(O)(H2O))(VO)2.6]·10.57H2O (2) were isolated and structurally characterized