A family of compounds has been discovered in which pairs of octahedral rare-earth-metal clusters, each centered by a late transition metal Z, share a common edge. These R(10)Z(2) units, sheathed and interbridged by iodine atoms, occur in the quaternary phases Rb2La10I17Co2, CS(2)La(10)I(17)Z(2) (Z = CO, Ni, Ru, Os), CS2Ce10I17Os2, and Cs(2)Pr(10)I(17)Z(2) (Z = CO, RU, Os) and in the ternary La10I15Os2. All are obtained as black, air-sensitive crystals from reactions of RI(3), R, Z, and RbI or CsI as appropriate in welded Nh containers at 800-850 degrees C. The structure of Cs2Ce10I17Os2 has been refined by single-crystal X-ray diffraction methods for comparison with the isotypic Cs2La10I17Co2 (monoclinic, C2/m, Z = 2, R(F)/R(w) = 4.0/4.4%), and the structure of a unique ternary La10I15Os2 has been defined (triclinic, P (1) over bar, Z = 1, R/R(w) = 5.0/5.1%). These new bioctahedral clusters may be regarded as the missing links between discrete clusters and infinite chains of condensed octahedral clusters among rare-earth-metal cluster halides that are stabilized by interstitial transition metals. The biclusters in both structures are extensively interconnected into three-dimensional arrays through bridging iodine atoms. The structural interconversion between Cs2La10I17Os2 and La10I15Os2 + 2CsI may be easily visualized in terms of changes in iodide bridging modes and accommodation of cesium cations. All the biclusters contain an odd number of cluster-bonding electrons, and this feature has been confirmed by magnetic susceptibility studies. EHMO cluster and band calculations on biclusters and their structures are in agreement with the localized bonding properties and ranges of electron counts.