Syntheses and magnetic properties of a heteropolyoxotungsten(VI)iron(III) cation and [LFeIII(.mu.-MO4)3FeIIIL] complexes (M = chromium(VI), molybdenum(VI)). Crystal structures of L2Fe2(CrO4)3.cntdot.H2O and [L3Fe3W4O14(OCH3)3](ClO4)2.cntdot.0.5H2O (L