The new compound Rb10Mo39S43 has been synthesized as single crystals by a solid-state reaction in a sealed molybdenum crucible at 1500 degrees C. It constitutes the fifth member of the homologous series of the trigonal (space group R (3) over barc) compounds Rb-2n(Mo9S11)(Mo6nS6n+2). Consequently, its crystal structure is based on an equal mixture of Mo9S11 and Mo30S32 cluster units interlinked through Mo-S bonds with the Rb+ cations localized in the voids between the Mo9S11 and Mo30S32 units. The coexistence of the two high-nuclearity Mo-9 and Mo-30 clusters in the crystal structure leads to an unusual c parameter of 163.96(1) angstrom, and thus, Rb10Mo39S43 is the first ternary and multinary compound in solid-state chemistry to have such a large parameter. Contrary to the first four members, Rb10Mo39S43 was not found to be superconducting down to 2 K. First-principles calculations showed that the electronic structures of this series of compounds can be determined from those of the Mo-9 and Mo(6n )clusters and that fewer interactions between clusters in Rb-2n(Mo9S11)(Mo6nS6n+2) occur when n increases.