SYNTHESES, CRYSTAL-STRUCTURES, AND MAGNETIC-PROPERTIES OF SC(19)BR(28)Z(4) COMPOUNDS WITH Z = MN, FE, RU, OR OS - STRUCTURAL AND BONDING TRENDS IN R(16)X(20)Z(4)-TYPE OLIGOMERS

The title compounds are obtained from reactions of the appropriate pro portions of Sc, ScBr3, and elemental Z in sealed Nb tubing at 800--900 degrees C. The structures with Z = Mn, Ru, Os were established by sin gle crystal means to be Sc-deficient examples of the Gd4I84+(Gd16I20Mn 44-) structure (cubic P(4) over bar3$ m, Z = 1, a = 10.941(9), 10.9897 (5), 11.0032(3) Angstrom) with 3.6(1), 3.0(1), and 2.9(1) scandium ato ms in the tetrahedral cation portion, respectively. The cation deficie ncy with Z = Ru, Os affords 61-electron Sc(16)Br(20)Z(4) oligomeric un its relative to the closed-shell expectation of 60 e(-). The dimension s, proportions, and bond orders in the Sc(16)Br(20)Z(4) oligomers (Sq symmetry) are compared with those in the four yttrium and one gadolini um examples previously known in this or other structure types. The Sc- Mn combination again displays the longest Sc-Z distances. A systematic breathing distortion of R(16)X(20)Z(4) clusters that increases in the order Y-I, Y-Br, Sc-Br(Ru,Os,Ir) to Sc-Br-Mn and Gd-I-Mn parallels ma ny other comparisons. Extended Huckel MO calculations show these varia tions depend not only on relative atom sizes but also on appreciable v ariations in orbital energies and the resulting R-Z mixing in the fron tier cluster orbitals (Kockerling and Martin, unpublished). Unusual va riations in the magnetic susceptibilities of the Ru, Os examples over the range of 6-300 K are shown to be fit very well by a model based on a Boltzmann distribution of the odd electron between close-lying fron tier al and tl orbitals with a temperature-dependent energy separation .