Chemical Properties
white fine crystalline powder
Physical properties
White monoclinic crystal; sublimes at 317°C; melts at 432°C at 33 atm (triple point); critical temperature 452.5°C; critical pressure 53.49 atm; critical volume 314 cm3/mol; hydrolyzes in water.
Uses
Hafnium tetrachloride is an important intermediate in production of hafnium metal. It also is used to prepare many hafnium compounds.
Preparation
Hafnium tetrachloride can be prepared (i) by chlorination of hafnium dioxide in the presence of carbon: HfO2 + 2Cl2 + C → HfCl4 + 2CO It also may be prepared by several other methods, such as (ii) reaction of carbon tetrachloride with hafnium dioxide above 450°C; (iii) heating a mixture of hafnium dioxide and carbon above 700°C; and (iv) reaction of chlorine with hafnium at elevated temperatures.
Reactions
Hafnium(IV) chloride hydrolyzes, evolving hydrogen chloride:
HfCl4 + H2O→ HfOCl2+ 2 HCl
Aged samples thus often are contaminated with oxychlorides, which are also colourless.
THF forms a monomeric 2:1 complex:
HfCl4+ 2 OC4H8→HfCl4(OC4H8)2
Flammability and Explosibility
Nonflammable
Solubility in organics
Soluble in methanol, acetone, reacts strongly with water, highly hygroscopic, hydrolyzes in water.
Structure and conformation
This group 4 halide contains hafnium in the +4 oxidation state. Solid Hafnium(IV) chloride is a polymer with octahedral Hf centers. Of the six chloride ligands surrounding each Hf centre, two chloride ligands are terminal and four bridge to another Hf centre.
