ABSTRACT
ZrF4 forms a solid with 8:2 coordination where each Zr atom is surrounded by 8F atoms in a square antiprism configuration (compare Fig. 15.5b). The other tetrahalides are volatile solids. In the vapour phase, the structure is monomeric and tetrahedral, while in the solid the Zr or Hf atoms are in octahedral coordination. The structure of one form of ZrCl4 is a chain of linked octahedra (ZrCl2Cl4/2) like the NbI4 structure shown in Fig. 15.8. The halides hydrolyse vigorously to the oxyhalide, which is stable to further hydrolysis. The most stable complex halides are the fluorides, and the hexa-, hepta-and octafluorides are known. The octahedral ZrF2," ion is found for example in Li2ZrF6. In the formally similar K2ZrF6, the structure involves bridging fluorines and ZrF8 coordination. In the mixed potassium-cupric hexafluorozirconate, the structure is even more complex and is formulated as K2Cu(H2. 0)6[Zr2Fi2]. The (Zr2F12)4~ anión has the unusual pentagonal bipyramidal coordination around each zirconium, and the two bipyramids share an edge-F5ZrF2/2ZrF5. The pentagonal bipyramid is also found in the sodium salts Na3ZrF7 and Na3HfF7. These contain pentagonal bipyramid MF7 groups, as in IF7, but the ammonium salt of the heptafluorides has the same structure as the isoelectric niobium and tantalum heptafluorides. This is shown in Fig. 15.1 and may be described as a trigonal prism with the seventh fluoride added beyond the centre of one of the rectangular faces. In the MF8 groups, the eight fluorines adopt the bisdisphenoid configuration shown in Fig. 15.2, and described in Section 13.6. If the cation is Cu(H20)i+, in place of the alkali metals, the antiprismatic form is found for ZrF4.", like the octafluorotantalate (Fig. 15.5b).
