Oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands: The role of counterions and water of crystallization

•Crystal structures of three oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands were revealed.•Dianionic malate ligands adopt a unique bidentate coordination mode via alkoxy and α-carboxylate groups in an oxo-bridged dinuclear anionic Mo(VI) complex.•Unprecedented icosameric water clusters trapped in the crystal lattice of a mononuclear W(VI) oxo complex were uncovered.•High affinity of oxo Mo(VI) and W(VI) complexes towards water makes the final outcome of their crystal structures unpredictable.

Crystal structures of three oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands were solved, namely [(CH3CH2)4N]2[Mo2O5(Hmal)2(H2O)2] (H3mal = malic acid) (1), Na6[Mo2O5(cit)2)]·10.5H2O (H4cit = citric acid) (2) and Na2[WO2(H2cit)2]·10H2O (3). In 1, dianionic malate ligands adopt a unique bidentate coordination mode via alkoxy and α-carboxylate groups in the oxo-bridged dinuclear anionic complex, in which two terminal oxo ligands and a water molecule complete the distorted octahedral geometry around the Mo(VI) centre. In compound 2, a similar oxo-bridged dinuclear core, [Mo2O5]2+, is present. However, the distorted octahedral geometry of each Mo(VI) is completed by oxygen atoms originating from a fully deprotonated citrate ligand, adopting a tridentate coordination mode. The mononuclear complex 3, with two terminal oxo ligands and four oxygen atoms originating from two dianionic, bidentately coordinated citrate ligands positioned in a distorted octahedral geometry around W(VI), shows the presence of unique icosameric water clusters trapped within the crystal lattice.

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