Synthesis, Physical Properties, and Theoretical Study of R₁₆Mo₂₁O₅₆ Compounds (R = La, Ce, Pr, and Nd) Containing Bioctahedral Mo₁₀ Clusters and Single Mo Atoms

Polycrystalline samples and single crystals of R₁₆Mo₂₁O₅₆ compounds (R = La, Ce, Pr, and Nd) were synthesized by solid-state reactions at high temperature in sealed Mo crucibles. The structure of La₁₆Mo₂₁O₅₆ was determined by single-crystal X-ray diffraction methods. It crystallizes in a monoclinic lattice, P2₁/c, with a = 13.532(2) Å, b = 13.464(1) Å, c = 13.447(1) Å, β = 99.681(7)°, and Z = 2. It is isostructural to Ce₁₆Mo₂₁O₅₆ and Nd₁₆Mo₂₁O₅₆ previously reported. The R₁₆Mo₂₁O₅₆ compounds contain bioctahedral Mo₁₀ clusters and MoO₆ octahedral units linked together via oxygen bridges. Magnetic susceptibility measurements indicate that the oxidation state of the single Mo atoms is +4 and that of the Ce and Pr atoms is +3, affording 30 metallic valence electrons to the Mo₁₀ nonmagnetic clusters. Such an electron count per Mo₁₀ cluster is confirmed by molecular calculations performed on an isolated Mo₁₀ cluster unit. Resistivity measurements and extented Hückel tight-binding calculations show that the R₁₆Mo₂₁O₅₆ compounds are insulating materials.