Holmium perchlorate is an inorganic compound of holmium and the perchlorate ion, with the formula Ho(ClO4)3. It is known in anhydrous form and as several hydrates, including octa- and nonahydrates. In dilute aqueous solution, Ho3+ occurs principally as the octaaqua ion [Ho(H2O)8]3+.
Preparation Hydrated holmium perchlorate can be prepared by dissolving holmium(III) oxide in aqueous perchloric acid:
Ho2O3 + 6 HClO4 → 2 Ho(ClO4)3 + 3 H2O
A modern spectroscopic study prepared aqueous Ho(ClO4)3 by dissolving Ho2O3 in 6 mol/L perchloric acid and subsequently diluting the resulting stock solution.
Anhydrous Ho(ClO4)3 has been prepared from hydrated material using chlorine oxide dehydrating agents under reduced pressure. Because anhydrous rare-earth perchlorates are strong oxidizers and can be sensitive to shock or contact with oxidizable substances, such preparations require careful handling.
Properties Anhydrous structure Anhydrous Ho(ClO4)3 crystallizes in the hexagonal crystal system, space group P63/m (No. 176), with two formula units per unit cell.
Its lattice parameters are a = 9.218(1) Å, c = 5.618(1) Å. It belongs to the common structural family of anhydrous lanthanide perchlorates extending from lanthanum through the low-temperature form of thulium.
In this structure the lanthanide ions are nine-coordinate. The oxygen atoms belong to tridentate perchlorate groups, producing a channel-containing three-dimensional structure.
The progressive decrease in the lattice parameters from La(ClO4)3 to the later lanthanides reflects the lanthanide contraction.
Hydrates Several hydrated forms of holmium perchlorate have been reported, including the octahydrate Ho(ClO4)3·8H2O and nonahydrate Ho(ClO4)3·9H2O.
Hydrated rare-earth perchlorates are readily obtained from aqueous perchloric-acid solutions, while controlled dehydration gives lower hydrates and ultimately the anhydrous compound.
Aqueous solution Raman spectroscopy shows that in dilute aqueous Ho(ClO4)3, the holmium ion is predominantly present as the octaaqua complex [Ho(H2O)8]3+.
A broad, strongly polarized Raman band at approximately 387 cm−1 has been assigned to the symmetric Ho–O breathing vibration of this ion. Among the heavy rare-earth ions studied, the Ho–O distance is the largest, decreasing progressively through Er, Tm, Yb and Lu as a consequence of the lanthanide contraction.
In dilute solutions, perchlorate behaves mainly as a weakly coordinating counterion. At higher concentrations, outer-sphere and contact ion pairing become increasingly important in heavy rare-earth perchlorate solutions.
Coordination chemistry Holmium perchlorate is often used as a soluble source of Ho3+ for the synthesis of coordination compounds.
For example, reaction with silver nitrate, isonicotinic acid and 2,2′-biphenyldicarboxylic acid under hydrothermal conditions has been used to prepare a heterometallic Ho(III)–Ag(I) coordination framework.
Safety Holmium perchlorate is an oxidizing perchlorate salt. Anhydrous lanthanide perchlorates in particular can react strongly with reducing or combustible materials and should be handled accordingly.
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