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, P2(1)/c, with a = 13.532(2) Angstrom, b = 13.463(1) Angstrom, c = 13.447(1) Angstrom, beta = 99.681(7)degrees and Z = 2. It is isostructural to Ce16Mo21O56 and Nd-16- Mo21O56 previously reported. The R16Mo21O56 compounds contain bioctahedral (Mo10O18O8a)-O-i 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 Moro nonmagnetic clusters. Such an electron count per Moro cluster is confirmed by molecular calculations performed on an isolated (Mo10O18O8a)-O-i cluster unit. Resistivity measurements and extented Huckel tight-binding calculations show that the R16Mo21O56 compounds are insulating materials.