Hafnium(IV) perchlorate is an inorganic compound of hafnium and perchlorate with the formula Hf(ClO4)4. Unlike most simple perchlorate salts, anhydrous Hf(ClO4)4 is a volatile molecular perchlorato complex in which the perchlorate groups coordinate directly to Hf(IV).
Preparation Anhydrous hafnium(IV) perchlorate can be prepared by reacting hafnium(IV) chloride with a strongly anhydrous perchlorating medium containing dichlorine heptoxide.
The use of strongly dehydrating perchlorating reagents is necessary because hydrolysis of Hf4+ otherwise favors formation of oxo- and hydroxo-containing hafnium species.
Properties Crystal and molecular structure Solid Hf(ClO4)4 is monoclinic. In the setting used in the original structure determination, its lattice parameters are a = 12.863(7) Å, b = 13.161(7) Å, c = 7.941(1) Å and γ = 107.68(4)°. The perchlorate groups coordinate to hafnium rather than behaving simply as non-coordinating counterions. Structural and vibrational studies describe them as chelating bidentate ligands, giving Hf an eight-coordinate oxygen environment.
Gas-phase electron diffraction shows that isolated Hf(ClO4)4 molecules are best described by approximately D4 symmetry. Each ClO4 group is a distorted tetrahedron and is attached to the central hafnium atom through two oxygen atoms.
Volatility and thermal decomposition Hafnium(IV) perchlorate is comparatively volatile for an inorganic hafnium compound and has been reported to sublime under reduced pressure at temperatures around 70 °C. On stronger heating it decomposes rather than boiling unchanged. Thermal decomposition initially produces hafnium oxoperchlorate, HfO(ClO4)2, in the approximate range 130–160 °C. Further heating to about 300–325 °C gives hafnium(IV) oxide, HfO2. This stepwise formation of an oxoperchlorate followed by the oxide is analogous to the thermal chemistry of zirconium and titanium perchlorates.
Thermochemistry The standard enthalpy of formation of crystalline anhydrous Hf(ClO4)4 at 298.15 K has been determined calorimetrically as −744.4 ± 2.4 kJ mol−1.
Safety Anhydrous hafnium(IV) perchlorate is a strongly oxidizing perchlorato complex. Anhydrous inorganic perchlorates can react vigorously with combustible or reducing substances and require careful handling.
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