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 R(16)Mo(21)O(56) compounds (R = La, Ce, Pr, and Nd) were synthesized by solid-state reactions at high temperature in sealed Mo crucibles. The structure of La(16)Mo(21)O(56) was determined by single-crystal X-ray diffraction methods. It crystallizes in a monoclinic lattice, P2(1)/c, with a = 13.532(2) Å, b = 13.464(1) Å, c = 13.447(1) Å, beta = 99.681(7) degrees, and Z = 2. It is isostructural to Ce(16)Mo(21)O(56) and Nd(16)Mo(21)O(56) previously reported. The R(16)Mo(21)O(56) compounds contain bioctahedral Mo(10) clusters and MoO(6) 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 Mo(10) nonmagnetic clusters. Such an electron count per Mo(10) cluster is confirmed by molecular calculations performed on an isolated Mo(10) cluster unit. Resistivity measurements and extented Hückel tight-binding calculations show that the R(16)Mo(21)O(56) compounds are insulating materials.