alpha-K4R6I14Os (R = La, Pr): A novel iodine-rich structure with a layerednetwork of electron-poor clusters. An incipient delocalized system with metal-like conductivity

The title compounds are obtained from reactions of KI, RI3, R, and Os at 75 0-850 degrees C in sealed Nb or Ta containers. The structure was establishe d by single-crystal X-ray diffraction studies (tetragonal, P4/mnc, Z = 2; a = 10.044(4), 9.879(3) Angstrom and c = 21.825(4), 21.86(1) Angstrom for La , Pr, respectively) for three samples, La and Pr at 23 degrees C and La at -100 degrees C. This particular combination of large R, interstitial Z, and halogen results in the highest X:R ratio known among rare-earth-metal clus ter halides and an unprecedented structure type. The cluster units are inte rconnected according to the pattern [(R6Os)(I8I4/2i-aI4/2a-iI2a)-I-i](4-) i n which four R in the waist of each cluster, the Os, and four bridging I-i- a,I-a-i pairs generate a planar 2D network of geometrically nearly ideal, 1 6-electron R6Os octahedra. This contrasts with the tetragonal compression f ound for many other 16-e(-) examples with the parallel disappearance of Cur ie-Weiss paramagnetism. Powdered alpha-K4La6I14Os exhibits an exceptionally low resistivity for a cluster halide (similar to 120 mu Omega.cm) over 110 -280 K and a small paramagnetism over about 35 to similar to 250 K associat ed with a probable Mott-Hubbard state that changes to a temperature-indepen dent (presumably Pauli) paramagnetism above about 300 K. EHTB band calculat ions show that a significant broadening of the HOMO (similar to t(1u)) stat es takes place in the plane of the intercluster bridging. The unique struct ure, properties, and band picture for the compound indicate a transition to a delocalized (band) state.