Linear Trimeric Hafnium Clusters in Hf0.86(1)I3

The reduction of hafnium tetraiodide, HfI4, with aluminum at 600 degrees C or 850 degrees 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.