HETEROMETALLIC CONDENSED CLUSTER COMPOUNDS - PR4I5CO, PR4I5RU, PR4I5OS AND LA4I5RU - SYNTHESIS, STRUCTURE, AND BONDING

The title R4I5Z compounds are synthesized in high yield by reaction of R and RI3 with Ru, Os, or CoI2 at 875-975-degrees-C in welded Nb containers. The yields of these Ru and Os compounds vs other phases appear to be significant functions of temperature. The structure of Pr4I5Ru was refined from single-crystal data as isotypic with Y4I5C (C2/m, Z = 2, a = 19.110 (5) angstrom, b = 4.2654 (9) angstrom, c = 9.194 (3) angstrom, beta = 104.55 (2)-degrees; R,R(w) = 4.1, 6.4%). The phase consists of Ru-centered, Pr6I12-type clusters that share trans metal edges and iodine to generate quasi-infinite chains. The nominal Pr6 octahedra therein are more uniformly proportioned than previously observed in chain structures, but the apical Pr-Ru bond is shorter than the waist Pr-Ru bond by 0.327 (1) angstrom. The magnetic susceptibility of Pr4I5Ru is Curie-Weiss in character with 7.1-mu-B/formula unit while La4I5Ru behaves like a Pauli paramagnet, chi = approximately 5.0 x 10(-4) emu/ (mol Oe) after correction for core diamagnetism. The bonding in Pr4I5Ru has been considered in terms of the results of charge-consistent, extended-Huckel band calculations. Pr-I and Ru-Pr bonding is dominant, the Ru-Pr bonding states lying at the bottom of a Pr-Pr- and Pr-Ru-based conduction band crossed by E(F). Charges calculated for the two metals are similar and slightly positive, indicating the importance of covalence. The marked compression of the apical Pr-Ru bonds relative to distances in chain analogues centered by the main-group elements C or Si results from unequal pi bonding within the distorted octahedra by the d orbitals of Ru.