Linear Trimeric Hafnium Clusters in Hf 0
The reduction of hafnium tetraiodide, HfI4, with aluminum at 600 °C or 850 °C in the presence of a NaI flux resulted in black single crystals of Hf0.86(1)I3. This composition corresponds well to the upper end of the non-stoichiometry range 0.89 ≤ x ≤ 1.00 previously reported for HfxI3. The crystal structure (a = 1250.3(2), c = 1999.6(3) pm, R-3m, Z = 18) is made up of hexagonal closest packed layers of iodide ions. One third of the octahedral holes would be filled as in TiI3 or ZrI3 if it were Hf1.00I3. In Hf0.86(1)I3, one out of six octahedral holes along [001] are, however, only occupied by 16%. In contrast to TiI3-I and ZrI3, one striking structural feature is in the formation of linear hafnium trimers with identical Hf―Hf distances of 318.3(2) pm rather than the formation of dimers. These may be associated with Hf―Hf bonding although only 2.64 electrons are available for one Hf5.16I18 column.
Revision of the structure of zirconium triiodide. The presence of metal dimers
Single-crystal x-ray studies on stoichiometric ZrI{sub 3} by film and diffractometer means show that a considerable number of very weak diffraction intensities require an orthorhombic cell and space group Pmmm (Z = 4, RuBr{sub 3}-type), while the stronger {approximately}75% of the reflection data are consistent with the hexagonal {beta}-TiCl{sub 3}-type structure (P6{sub 3}/mcm, Z = 2) commonly assigned to many trihalides. The compound contains infinite chains of confacial (ZrI{sub 6}) octahedra that are distorted through pairing of the zirconium(III) atoms; d(Zr-Zr) = 3.172 (2) and 3.507 (2) {angstrom}. Data sets collected from the same crystal in the hexagonal setting were also refined in space groups P6{sub 3}/mcm and P{bar 6}2m; the latter provides nominally satisfactory residuals but an incorrect result with less tightly bound dimers. A more complex structure (which we did not study) applies to the compound near the oxidized limit ZrI{sub 3.4-3.5}. Other structural and property studies on ZrI{sub 3} and related phases are considered relative to these findings. A smaller pairing distortion in ZrCl{sub 3} seems probable because of the less restrictive chlorine matrix. A second-order phase transition is possible between the P6{sub 3}/mcm and Pmmm structures for these trihalides, and a continuous distortion from the equallymore » spaced metals atoms of the former model can be expected, especially as a function of temperature. 33 refs., 4 tabs.« less
Die Kristallstruktur von Praseodymdiiodid (Modifikation V)
PrI2 tritt in mindestens funf verschiedenen Modifikationen auf; vier davon konnen den bekannten Schichtstruktur-Typen CuTi2, 2H1-MoS2, 3R-MoS2 und CdCl2 zugeordnet werden. Die funfte Modifikation enthalt cubanartige Cluster [Pr4I4]4+, die im Sinne der Formulierung [Pr4Ii4]Ia–a12/3 uber I−-Ionen miteinander verbruckt sind. Die Anordnung kann anschaulich aus der Spinellstruktur AB2X4 abgeleitet werden. Hierzu denkt man sich die A-Platze unbesetzt, die B-Platze mit Pr und die X-Platze mit I belegt. Die Pr-Atome und die Halfte der I-Atome sind allerdings erheblich aus den Ideallagen verschoben. Die beobachtete Verzerrung last sich wie bei den strukturell recht ahnlichen Substanzen MoSBr und GaMo4S8 auf Metall-Metall-Bindungen zuruckfuhren. Die Kristallstruktur von PrI2V wurde mit rontgenographischen Einkristallmethoden bestimmt: Raumgruppe F43m, a = 1236,0(5) pm, Diffraktometerdaten (MoKα-Strahlung), R = 4,1% bei 92 unabhangigen Reflexen. Die Kristalldaten der and ren vier Modifikationen von PrI2 sowie entsprechende Daten von PrI3 und PrOI sind im Text angegeben. 
 
 
 
Crystal Structure of Praseodymium Diiodide (Modification V) 
 
 
 
There are at least five different modifications of PrI2, four of which can be assignee to the well-known layer structure types of CuTi2, 2H1-MoS2, 3R-MoS2, and CdCl2 respectively. Thd fifth modification is built from cubane-like [Pr4I4]4+ clusters which are linked by I− ions according to the representation [Pr4Ii4]Ia–a12/3. Alternatively this arrangement may be visualized by its relationship with the spinel structure AB2X4. Imagine the A-sites vacant, the B-sites occupied by praseodymium and the X-sites by iodine, but mind that the Pr-atoms and half of the I-atoms are noticeably shifted from their ideal positions. The observed distortion results from metal-metal bonds as is known from structurally related compounds like MoSBr or GaMo4S8. The crystal structure of PrI2-V has been determined by single crystal X-ray methods: space group F43m, a = 1236.0(5) pm, diffractometer data taken with MoKα radiation, R = 4.1% for 92 independent reflexions. Crystal data of the other four PrI2 modifications, of PrI3 and PrOI are given in the paper.