Structural identification of two differently coordinated heptamolybdate ligands in a hexamagnesium compound

Abstract

Dissolution of freshly prepared molybdenum trioxide in hot aqueous magnesium carbonate followed by crystallization results in the formation of a hexamganesium-bis(heptamolybdate) compound viz. [Mg(H₂O)₆]₃[Mg(H₂O)₅(Mo₇O₂₄)][(H₂O)₅Mg(µ_2-Mo₇O₂₄)Mg(H₂O)₅]∙6H₂O 1. Herein we report the crystal structure, spectral characteristics, thermal and electrochemical properties of 1. The all-inorganic compound 1, which crystallizes in the acentric polar space group Cc, contains six unique Mg(II) ions. The two crystallographically independent heptamolybdate anions function as a monodentate (h¹) and µ_2-bridging bidentate ligand respectively forming the anionic [Mg(H₂O)₅(Mo₇O₂₄)]^4- and [(H₂O)₅Mg(µ_2-Mo₇O₂₄)Mg(H₂O)₅]^2- species, which are charge balanced by three unique hexaaquamagnesium(II) cations. The electrochemical and conductivity studies of 1 reveal the presence of [Mg(H₂O)₆]²⁺ cations and uncoordinated (Mo₇O₂₄)^6- anions in solution. Thermal decomposition of 1 leads to the formation of Mg₂Mo₃O₁₁ via Mg₆Mo₁₄O₄₈ and 3Mg₂Mo₃O₁₁∙5MoO₃.