2 CLOSELY-RELATED STRUCTURE TYPES WITH UNPRECEDENTED BIOCTAHEDRAL RARE-EARTH-METAL CLUSTERS CENTERED BY TRANSITION-METALS - A(2)R(10)I(17)Z(2) (A=RB,CS, R=LA,CE,PR, Z=CO,NI,RU,OS) AND LA10I15OS2

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 interbridge d 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 obtain ed 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 Cs2La10I17Co 2 (monoclinic, C2/m, Z = 2, R(F)/R(w) = 4.0/4.4%), and the structure o f 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 bi clusters in both structures are extensively interconnected into three- dimensional arrays through bridging iodine atoms. The structural inter conversion between Cs2La10I17Os2 and La10I15Os2 + 2CsI may be easily v isualized in terms of changes in iodide bridging modes and accommodati on of cesium cations. All the biclusters contain an odd number of clus ter-bonding electrons, and this feature has been confirmed by magnetic susceptibility studies. EHMO cluster and band calculations on biclust ers and their structures are in agreement with the localized bonding p roperties and ranges of electron counts.