Holmium perchlorate

{{Chembox
| Name = Holmium perchlorate
| OtherNames = Holmium(III) perchlorate
|Section1={{Chembox Identifiers
| CASNo = 14017-54-0
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo2 = 14692-19-4
| CASNo2_Comment = octahydrate
| CASNo3 = 14692-16-1
| CASNo3_Comment = nonahydrate
}}
|Section2={{Chembox Properties
| Formula = Ho(ClO₄)₃
| MolarMass = 463.28 g/mol
| Appearance = Crystalline solid
| Solubility = Soluble in water
}}
|Section3={{Chembox Structure
| CrystalStruct = Hexagonal (anhydrous)
| SpaceGroup = ”P”6₃/”m”, No. 176
| LattConst_a = 0.9218 nm
| LattConst_c = 0.5618 nm
| UnitCellFormulas = 2
}}
}}

”’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⁺.<ref name=Rudolph />

== 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.<ref name=Rudolph>{{cite journal
|last1=Rudolph
|first1=Wolfram W.
|last2=Irmer
|first2=Gert
|title=On the Hydration of Heavy Rare Earth Ions: Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ and Lu³⁺—A Raman Study
|journal=Molecules
|year=2019
|volume=24
|issue=10
|article-number=1953
|doi=10.3390/molecules24101953
|pmid=31117271
|pmc=6572539
}}</ref>

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.<ref name=Pascal>{{cite journal
|last1=Pascal
|first1=Jean-Louis
|last2=Favier
|first2=Frédéric
|last3=Cunin
|first3=Frédérique
|last4=Fitch
|first4=Andrew N.
|last5=Vaughan
|first5=Gavin
|title=Crystalline and Molecular Structures of Anhydrous Lanthanide Perchlorates Ln(ClO₄)₃ with Ln = La, Ce, Pr, Sm, Eu, Ho, Er, Tm, and Lu
|journal=Journal of Solid State Chemistry
|year=1998
|volume=139
|issue=2
|pages=259–265
|doi=10.1006/jssc.1998.7838
}}</ref>

== Properties ==
=== Anhydrous structure ===
Anhydrous Ho(ClO₄)₃ crystallizes in the [[hexagonal crystal system]], space group ”P”6₃/”m” (No. 176), with two formula units per unit cell.<ref name=Pascal />

Its lattice parameters are ”a” = 9.218(1) Å, ”c” = 5.618(1) Å.<ref name=Pascal /> It belongs to the common structural family of anhydrous lanthanide perchlorates extending from lanthanum through the low-temperature form of thulium.<ref name=Pascal />

In this structure the lanthanide ions are nine-coordinate. The oxygen atoms belong to tridentate perchlorate groups, producing a channel-containing three-dimensional structure.<ref name=Pascal />

The progressive decrease in the lattice parameters from La(ClO₄)₃ to the later lanthanides reflects the [[lanthanide contraction]].<ref name=Pascal />

=== 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⁺.<ref name=Rudolph />

A broad, strongly polarized Raman band at approximately 387 cm⁻¹ has been assigned to the symmetric Ho–O breathing vibration of this ion.<ref name=Rudolph /> 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.<ref name=Rudolph />

== 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.<ref>{{cite journal
|last1=Lin
|first1=Xiao-Ming
|last2=Ding
|first2=Yu-Jia
|last3=Liang
|first3=Shui-Mei
|title=Two series of Ln(III)–Ag(I) heterometallic–organic frameworks constructed from isonicotinate and 2,2′-biphenyldicarboxylate: synthesis, structure and photoluminescence properties
|journal=CrystEngComm
|year=2015
|volume=17
|issue=20
|pages=3800–3808
|doi=10.1039/C5CE00478K
}}</ref>

== 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.<ref name=Pascal />

== References ==
{{reflist}}

{{Holmium compounds}}
{{Perchlorates}}

[[Category:Holmium compounds]]
[[Category:Perchlorates]]