The novel quaternary reduced molybdenum oxides LaR4Mo36O52 (R = Dy, Er, Yb, and Y) have been synthesized with solid-state reactions at 1400 degrees C for 48 h in sealed molybdenum crucibles. The crystal structure was determined on a single crystal of LaEr4Mo36O52 by X-ray diffraction. LaEr(4)Mo(36)o(52) crystallizes in the tetragonal space group I (4) over bar with two formula units per cell and the following lattice parameters: a = 19.8348(2) and c = 5.6594(1) angstrom. The Mo network is dominated by infinite chains of trans-edge-shared Mo-6 octahedra, which coexist with Mo-2 pairs and rectangular Mo-4 clusters. The Mo-Mo distances within the infinite chains range from 2.5967(7) to 2.8529(8) angstrom and from 2.239(3) to 2.667(2) angstrom in the Mo-2 pairs and rectangular Mo-4 clusters, respectively. The Mo-O distances are comprised between 1.993(7) and 2.149(7) angstrom, as usually observed in these types of compound. The La3+ and Er3+ ions are in a square-prismatic [LaO8] and a tricapped trigonal-prismatic [ErO9] environment of oxygen atoms, respectively. The La-O distances range from 2.555(6) to 2.719(6) angstrom and the Er-O ones from 2.260(6) to 2.469(5) A. Theoretical calculations allow the determination of the optimal electron count of both motifs in the title compound. Weak interactions occur between neighboring dimetallic and tetrametallic clusters and between trans-edge-sharing infinite chains and dimers and tetramers. The presence of rectangular clusters is favored on the basis of theoretical considerations. Single-crystal resistivity measurements show that LaEr4Mo36O52 is metallic between 4.2 and 300 K, in agreement with the band structure calculations. Magnetic susceptibility measurements indicate that the oxidation state of the magnetic rare earths is +3, and there is an absence of localized moments on the Mo network.