Abstract The geometry parameters of [FeII(OH)6Mo6O18]4− and [CoIII(OH)6Mo6O18]3− as protonated form of Anderson-type anions have been investigated at the level of density functional theory, using B3LYP and ab initio Hartree–Fock (HF) methods. The effect of basis sets has been investigated using four basis sets. Based on optimized structures, vibrational frequencies calculations have been done. The agreement between reported experimental data and theoretical values is, in general, good. Also electronic properties of protonated form have been investigated for [AlIII(OH)6Mo6O18]3− with the other title anions which suggested the [(X(OH)6)n−@M6O18] general formula to describe the anionic subunit X(OH)6 surrounded by M6O18 neutral ring.

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