Page 22 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19

230 Actinide Elements

The wide variety of oxidation states known for the ac- AnX 6 , and some representative data for these are given in
tinides is reflected in the stoichiometry of their binary ox- Table XII. The thermal stability of the halides toward re-
ides; however, the highest attainable oxidation state may duction of higher oxidation state actinides decreases with
not be observed. The largest O/M ratio for an f -element increasing atomic number of the halogen.
binary oxide is achieved in UO 3 . Truly divalent actinide halides are known only for
All the solid actinide monoxides which have been americium and californium. AnX 2 species for Es have
reported are now believed to have been oxynitrides, been identified by their absorption spectra. For Fm, Md,
oxycarbides, or hydrides. The highest potential for exis- and No, AnX 2 halides should be possible if sufficient
tence would have the monoxides for the divalent actinide amounts of these metals could be obtained. ThI 2 is also
metals einsteinium through nobelium. Only the gaseous known, but crystallographic studies of this compound re-
−
−
monoxides are well-established species. All actinides are veal the true formulation to be Th(IV), 2I , and 2e . This
known or expected to form gaseous monoxides. compound has some metallic character, including its luster
The sesquioxide is known for actinium and all the ac- and electrical conductivity.
tinides from plutonium through einsteinium and is proba- The actinide trihalides behave similarly to the lan-
bly the highest binary oxide that could be formed for the thanide trihalides. The trifluorides through berkelium tri-
heaviest actinides with nobelium as an exception, which fluoride crystallize at room temperature with the LaF 3
may only form a solid monoxide. Oxides of the heaviest hexagonal structure. Nine fluorine atoms are arranged
actinides beyond einsteinium have not been prepared or around the actinide in a heptagonal bipyramid geometry.
studied experimentally. The sesquioxides of Pu, Am, and CfF 3 and a second form of BkF 3 have the orthorhombic
Bk are readily oxidized to their dioxides, whereas those YF 3 structure, where nine fluorines form an approximate
˚
of Cm, Cf, and Es are resistant to air oxidation. tricapped prism with one fluorine 0.3 A farther from the
The dioxide is known for all the actinides from thorium metal. All of the trifluorides are high-melting solids, in-
through californium. Attempts to prepare einsteinium soluble in water, and only slowly oxidized in air.
dioxide have not been successful. All the dioxides crys- The actinide trichlorides are hygroscopic and water sol-
tallize with the fluorite face-centered cubic structure. Ac- uble and melt between 1030 and 1110 K. They can be
tinides that form both a dioxide and a sesquioxide may obtained by reaction of the metal hydride with HCl at
form complex intermediate oxides, which have O/M ra- elevated temperatures or by the reaction of CCl 4 with
tios between 1.5 and 2.0. An(OH) 3 . With the larger actinide(III) ions, the crystal
Binary oxides with higher oxygen stoichiometries have structures of the trichlorides show nine chlorine atoms
been confirmed only for the elements Pa, U, and Np. Nu- arranged in a tricapped trigonal prismatic geometry. As
merous phases in the composition range UO 2 to UO 3 have the atomic number increases, the three actinide to face-
been observed.Thereported formation ofnonstoichiomet- capping-chlorine distances increase relative to the other
ric PuO 2+x has to be confirmed. Only UO 3 is known for six chlorines. At californium, a second form of CfCl 3 has
the anhydrous actinide trioxides and is prepared by de- eight coordination.
composing uranyl nitrate or a hydrated uranyl hydroxide AnBr 3 compounds can be prepared by reaction of HBr
+
◦
containing NH at 350 C. There are seven crystal modi- with the proper actinide hydride, hydroxide, oxalate hex-
4
fications of UO 3 . Many of these contain oxygen-bridged ahydrate, or oxide. Structures similar to the trichlorides
structures, with uranyl present. δ-UO 3 with its cubic ReO 3 are observed with the structural change from nine coordi-
structure consists of linked UO 6 octahedra. nation to eight coordination occurring with ß-neptunium
Actinide sulfides, selenides, and tellurides are also tribromide. The triiodides to α-americium triiodide have
known. The sulfides and selenides are generally isostruc- the same eight-coordinate structure found for the heavier
tural, but not with the analogous tellurides. The thermal bromides and chlorides. From ß-americium triiodide on,
stability of these compounds decreases in the order sul- the metals are six coordinate. ThI 3 is best formulated as
−
−
fides > selenides > tellurides. These compounds are usu- Th(IV), 3I , and 1e .
ally prepared via direct reaction of finely divided actinide The best known actinide halides are the tetrahalides,
metal powder with the chalcogen at about 400–600 C. the fluorides being known through californium. All of the
◦
Semimetallic behavior and nonstoichiometry are observed AnF 4 species are monoclinic, the metal being eight co-
for these compounds. ordinate with antiprismatic geometry. These compounds,
prepared by heating HF with the dioxides, are insol-
uble in water. The remaining tetrahalides can be pre-
3. Halides
pared by heating the actinide dioxides in CCl 4 (ThCl 4 to
A wealth of information has been accumulated on actinide NpCl 4 ), Cl 2 /SOCl 2 (BkCl 4 ), or from the elements (ThBr 4
halides. The known binary halides range from AnX 2 to to NpBr 4 and ThI 4 to UI 4 ). The tetrachlorides are eight