Heteropolyanions [WM₃(H₂O)₂(XW₉O₃₄)₂]^12–(X = M = Zn or Co^II) form on ageing concentrated solutions of sodium salts of the well known polyanions [Zn(H₂O)(ZnW₁₁O₃₉)]^8– and [Co^II(H₂O)(Co^IIW₁₁O₃₉)]^8–. Their high-yield syntheses are reported. An X-ray crystallographic study of Na₁₂[WZn₃(H₂O)₂(ZnW₉O₃₄)₂]·46H₂O [Space group p2₁/c, a= 13.027(4), b= 17.788(5), c= 24.124(4)Å, β= 118.94(2)°, Z= 2, R′= 0.051 for 3783 reflections] and its high-resolution ¹⁸³W NMR spectrum (ten lines, one with half the intensity of the others) reveal that the polyanion is isotypical of the earlier described [M₄(H₂O)₂(PW₉O₃₄)₂]^10– anions (M = Zn or Co^II) and elucidate the puzzling analytical composition Zn₅W₁₉. However, the lower point-group symmetry C2 (instead of C2h) points to the existence of enantiomers. This anion and the homologous cobalt(II) anion are always obtained as racemic mixtures. Both parent anions transform into substitution derivatives by replacement of atoms in M positions (Two or three for Zn, two only for Co) by transition elements: derivatives of Mn^II or Mn^III, Fe^II or Fe^III, Co^II, Ni^II, Cu^II, Zn, Pd^II, Pt^II and V^IV are reported. The structures of D,L-Na₁₂[WCu₃(H₂O)₂(ZnW₉O₃₄)₂]·48H₂O and D,L-K₁₂[WZnV₂O₂(ZnW₉O₃₄)₂]·30H₂O are also described and compared. Chemical features, comparative electronic UV–VIS spectra, and ¹⁸³W NMR spectra of these series are discussed.