3 TYPES OF CONDENSED CLUSTER PHASES OF RARE-EARTH-METAL IODIDES WITH TRANSITION-METAL INTERSTITIALS - PR4I5NI, PR3I3OS, PR2INI2, AND LA(2)IZ(2)

Syntheses of the title compounds result from appropriate reactions of the elements and RI3 (R = La, Pr) in sealed Nb containers at, variousl y, 800-990 degrees C. The structures of three were detailed by single crystal X-ray diffraction: Pr4I5Ni, Pmmn, Z=2, R(F)/R-w=3.6/5.3%; Pr3I 3Os, P2(1)/m, Z=2, R/R-w 2.4/2.8%; Pr2INi2, P6(3)/mmc, Z=2, R/R-w=2.2/ 3.6. The new compounds La(2)IZ(2), Z=Fe, Co, Ru, Os, were shown to be isostructural with Pr2INi2 (Gd2IFe2) by Guinier powder diffraction. Pr 4I5Ni, the first orthorhombic example among the R(4)I(5)Z series, cons ists of R(6)Z octahedra condensed into chains. The new example contain s the most extreme R:Z size proportions and clearly lacks the strong d pi-d pi bonding between Z and apical R seen within other members. Pr3 I3Os previously known only with a cubic Gd3CI3C-(Ca3I3P-)-type Structu re, also occurs as a relatively undistorted member of the monoclinic P r3I3Ru (double chain) family. Apical Pr-Os pi-bonding appears signific ant, with Pr-Os distances that are 0.24 Angstrom shorter than the aver age within the waist of the parent octahedra. The R(2)IZ(2) phases exp and the number of examples of the unusual Gd2IFe2 structure, which con tains AlB2-like slabs; namely, graphite-like Z(2) nets between eclipse d pairs of R layers that are in turn separated by single iodine layers . Notable distances in Pr2INi2 are Ni-Ni, 2.36 Angstrom, and Pr-Ni, 2. 98 Angstrom. Charge-consistent, extended Huckel band calculations in L a2IFe2 (in contrast with results reported for Gd2IFe2) demonstrate a s trong mixing of La-Fe and Fe-Fe bonding in a partially filled band, th e Fe states on balance falling only slightly below those of La. (C) 19 97 Academic Press.