Source of materialCrystals suitable for X-ray diffraction analysis were obtained by the slow evaporation at 278 Κ of an aqueous mixture of ammonium heptamolybdate, (ΝΗ4)6Μογθ24 · 4H20, tartaric acid, C4H4O6, guanidinium carbonate, (CNìHóhCOa and an excess of a 35 wt.% hydrogen peroxide solution. Experimental detailsTo keep a reasonable data/parameters ratio, only the Mo atoms were refined with anisotropic temperature factors. The light atoms were refined isotropically. All the Η atoms were calculated with AFIX and included in the refinement with a common isotropic temperature factor (Ui S0 = 0.08 Â 2 ). The Η atoms of the four water molecules were not localized. DiscussionIn the frame of research aimed at developing new synthetic procedures of oxide materials, the so-called chelate method, which consists in the thermal decomposition of solid metal precursors obtained from a solution of the complexed metals, was shown to be * Correspondence author (e-mail: tinant@chim.ucl.ac.be) an appropriate way to get multimetallic oxides [1,2]. The title compound was isolated during attempts to synthesize new alkali metal-free precursors for Mo-based oxides, in order to avoid any contamination of the final material after removal of the surrounding ligands during the calcination step leading to the oxide. Guanidinium derivatives are therefore particularly welcome. The structure of the bridged dinuclear [Mo202 (02)