Metal-metal repulsion and bonding in confacial bioctahedra. Crystal structures of Cs3Y2I9 and Cs3Zr2I9 and comparison with related phases

The title compounds were synthesized by reaction of CsI3 and Y or CsI, Zr, and ZrI4 at 750 to 8500C in welded Ta containers. Both were shown to have the Cs3Cr2Cl9-type structure, space group P63/mmc. Positional and thermal parameters were established by full-matrix least-squares refinement of single-crystal diffractometer data. Data for the structures and the refinements of Cs3Y2I9 and Cs3Zr2I9 are respectively a = 8.406 (1) and 8.269 (1) A, c = 21.280 (5) and 19.908 (3) A, R = 0.050 and 0.082, and R/sub w/ = 0.082 and 0.077 for 328 and 610 independent reflections with 20 less than or equal to 50 and 600 collected with Mo Kα radiation. The transition from the yttrium to the zirconium compound is accompanied by a decrease of the metal-metal distance in the M2I93- confacial bioctahedra from 4.052 (9) to 3.129 (4) A owing to the formation of a Zr-Zr bond. Correspondingly large changes occur in the internal angles in the M-I-M bridges. Parameters of five M2X9 groups in the Cs3Cr2Cl9-type structure and of CsCdCl3, all of which lack M-M bonds, are considered in terms of a model which recognizes the existence of both longer M-X bridge bonds and comprise bridge angles. Nonbonding repulsion between M atoms is concluded to not be a significant factor in these structures. The effects of packing and metal-metal bonding are also considered