Lecture 6: Uranium Chemistry
• From: Chemistry of actinides
 Nuclear properties
 U purification
 Free atom and ion
property
 Metallic state
 Compounds
 Chemical bonding
 Structure and
coordination chemistry
 Solution chemistry
 Organometallic and
biochemistry
 Analytical Chemistry
6-1
Nuclear properties
• Fission properties of
uranium
 Defined importance
of element and future
investigations
 Identified by Hahn in
1937
 200 MeV/fission
 2.5 neutrons
• Natural isotopes
 234,235,238U
 Ratios of isotopes
established
 234: 0.005±0.001
 235: 0.720±0.001
 238: 99.275±0.002
• 233U from 232Th
6-2
Uranium Minerals
•
•
200 minerals contain uranium

Bulk are U(VI) minerals
 U(IV) as oxides,
phosphates, silicates

Classification based on
polymerization of coordination
polyhedra

Mineral deposits based on
major anion

Secondary phases may be
important for waste forms
 Incorporation of higher
actinides
Pyrochlore

A1-2B2O6X0-1
 A=Na, Ca, Mn, Fe2+, Sr,Sb,
Cs, Ba, Ln, Bi, Th, U
 B= Ti, Nb, Ta
 U(V) may be present when
synthesized under reducing
conditions
* XANES spectroscopy
* Goes to B site
6-3
Polyhedra classification
U(VI) minerals
•
•
•
Linkage over equatorial position

Bipyramidal polyhedra

Oxygens on uranyl forms
peaks on pyramid
 Different bond lengths
for axial and equatorial
O coordinated to U
Method for classification

Remove anions not bound
by 2 cations, not equatorial
anion on bipyramid
 Associated cation
removed

Connect anions to form
polyhedra
 Defines anion topology
Chains defined by shapes

P (pentagons), R (rhombs),
H (hexagons), U (up
arrowhead chain), D (down
arrowhead chain)
6-4
Uranium purification from ores
• Common steps

Preconcentration of ore •
 Based on density of
ore

Leaching to extract
uranium into aqueous
phase
 Calcination prior to
leaching
* Removal of
carbonaceous or
sulfur
compounds
* Destruction of
hydrated species
(clay minerals)

Removal or uranium
from aqueous phase
 Ion exchange
 Solvent extraction
 Precipitation
Leaching with acid or alkaline solutions

Acid solution methods
 Addition of acid provides best
results
* Sulfuric or HCl (pH 1.5)
 U(VI) soluble in sulfuric
 Oxidizing conditions
may be needed
 MnO2 , chlorate, O2,
chlorine
 Generated in situ by bacteria
 High pressure oxidation of sulfur,
sulfides, and Fe(II)
* sulfuric acid and Fe(III)

Carbonate leaching
 Formation of soluble anionic
carbonate species
 Somewhat specific for uranium
 Use of O2 as oxidant
 Bicarbonate prevents precipitation
6-5
of Na2U2O7
* OH-+HCO3-CO32- + H2O
Recovery of uranium from solutions
•
•
•
•
Ion exchange

U(VI) anions in sulfate and
carbonate solution
 UO2(CO3)34 UO2(SO4)34
Load onto anion exchange,
elute with acid or NaCl
Solvent extraction

Continuous process

Not well suited for
carbonate solutions

Extraction with alkyl
phosphoric acid,
secondary and tertiary
alkylamines
 Chemistry similar to
ion exchange
conditions
Chemical precipitation

Older method
 Addition of base
 Peroxide
* Ultimate
formation of
(NH4)2U2O7
(ammonium
diuranate), then
heating to form
U3O8 or UO3
• TBP extraction
Contaminates depend upon
mineral

Based on formation of nitrate species

V, Mo

UO2(NO3)x2-x + (2-x)NO3- + 2TBP
UO2(NO3)2(TBP)2
6-6
Uranium atomic properties
• Ground state electron configuration
 [Rn]5f36d17s2
• Term symbol
 5L6
6-7
cm-1
6-8
6-9
Metallic Uranium
• Three different phase

a, b, g phases
 Dominate at
different
temperatures
• Uranium is strongly
electropositive

Cannot be prepared
through H2 reduction
• Metallic uranium
preparation

UF4 or UCl4 with Ca
or Mg

UO2 with Ca

Electrodeposition
from molten salt
baths
6-10
Metallic Uranium phases
 a-phase

Room temperature to 942 K

Orthorhombic

U-U distance 2.80 Å

Unique structure type
 b-phase

Exists between 668 and 775 ºC
a‐phase U-U distances in layer

Tetragonal unit cell
(2.80±0.05) Å and between layers
 g-phase
3.26 Å

Formed above 775 ºC

bcc structure
• Metal has plastic character

Gamma phase soft, difficult fabrication

Beta phase brittle and hard
• Paramagnetic
• Temperature dependence of resistivity
b-phase
6-11
Resistivity–temperature curve for a-U along
the [010] axis
6-12
Intermetallic compounds
• Wide range of intermetallic compounds and solid solutions in alpha and
beta uranium

Hard and brittle transition metal compounds
 U6X, X=Mn, Fe, Co, Ni

Noble metal compounds
 Ru, Rh, Pd
* Of interests for reprocessing

Solid solutions with:
 Mo, Ti, Zr, Nb, and Pu
6-13
Uranium-Titanium Phase
Diagram.
Uranium-Aluminum Phase
Diagram.
6-14
Chemical properties of uranium metal and
alloys
• Reacts with most elements on
periodic table
 Corrosion by O2, air,
water vapor, CO, CO2
• Dissolves in HCl
 Also forms hydrated
UO2 during dissolution
• Non-oxidizing acid results in
slow dissolution
 Sulfuric, phosphoric,
HF
• Exothermic reaction with
powered U metal and nitric
• Dissolves in base with
addition of peroxide
 peroxyuranates
6-15
Uranium compounds
• Uranium-hydrogen

b-UH3 from
H2 at 250 ºC

a-UH3
prepared at 80 ºC from H2
at 250
6-16
Uranium hydride compounds
• Uranium borohydride
• UF4 +
2Al(BH4)3U(BH4)4 +
2Al(BH4)F2
 U(BH)4 is
tetragonal
 U(BH4)3 forms
during U(BH4)4
synthesis
 Vapor pressure
 log p (mmHg)
=13.354-4265T1
• UXAlHy compounds
 UXAl absorbs
hydrogen upon
heating
 X=Ni, Co, Mn
 y = 2.5 to 2.74
 TGA analysis
evaluates
hydrogenation
6-17
Uranium-oxygen
• UO

Solid UO unstable, NaCl structure

From UO2 heated with U metal
 Carbon promotes reaction, formation of UC
• UO2

Reduction of UO3 or U3O8 with H2 from 800 ºC to 1100 ºC
 CO, C, CH4, or C2H5OH can be used as reductants

O2 presence responsible for UO2+x formation

Large scale preparation
 UO4, (NH4)2U2O7, or (NH4)4UO2(CO3)3
 Calcination in air at 400-500 ºC
 H2 at 650-800 ºC
 UO2has high surface area
6-18
Uranium-oxygen
•
•
•
U4O9

UO2 and U3O8
 5 UO2+ U3O8 2 U4O9
 Placed in evacuated ampoule
 Heated to 1000 ºC for 2 weeks
* Three phases
 a-U4O9 up to 350 K
 b-U4O9 350 K to 850 K
 g-U4O9 above 850 K
 Rearrangement of U4+ and U5+ forces disordering of O
U3O7

Prepared by oxidizing UO2 below 160 ºC
 30 % of the oxygens change locations to new positions during oxidation

Three phases
 b phase prepared by heating at 200 ºC
U2O5

High pressure synthesis, three phases

a-phase
 UO2 and U3O8 at 30 kbar and 400 ºC for 8 hours
 Also prepared at 15 kbar and 500 ºC

b-phase forms at 40-50 kbar above 800 ºC

g-phase sometimes prepared above 800 ºC at 60 kbar
6-19
Uranium-oxygen
• U3O8
 From oxidation of UO2 in air at 800 ºC
 a phase uranium coordinated to oxygen in
pentagonal bipyrimid
 b phase results from the heating of the a phase above
1350 ºC
 Slow cooling
6-20
Uranium-oxygen
• UO3

Seven phases can be prepared
• A phase (amorphous)
 Heating in air at 400 ºC
* UO4.2H2O, UO2C2O4.3H2O, or (HN4)4UO2(CO3)3
 Prefer to use compounds without N or C
 a-phase

Crystallization of A-phase at 485 ºC at 4 days

O-U-O-U-O chain with U surrounded by 6 O in a plane to the
chain

Contains UO22+
 b-phase

Ammonium diuranate or uranyl nitrate heated rapidly in air
at 400-500 ºC
 g-phase prepared under O2 6-10 atmosphere at 400-500 ºC
6-21
Uranium-oxygen
• UO3 hydrates

6 different hydrated
UO3 compounds
• UO3.2H2O

Anhydrous UO3
exposed to water from
25-70 ºC

Heating resulting
compound in air to 100
ºC forms a-UO3.0.8
H2 O

a-UO2(OH)2 [aUO3.H2O] forms in
hydrothermal
experiments
 b-UO3.H2O also
forms
6-22
6-23
Uranium-oxygen single crystals
•
•
•
UO2 from the melt of UO2 powder

Arc melter used
•

Vapor deposition
2.0 ≤ U/O ≤ 2.375

Fluorite structure
Uranium oxides show range of structures

Some variation due to existence of UO22+ in structure

Some layer structures
UO2 to UO3 system

Range of liquid and solid phases from O/U 1.2
to 3.5

Hypostoichiometric UO2+x forms up to O/U
2.2
 Mixed with U3O8 at higher temperature

Large range of species from O/U 2.2 to 2.6
6-24
UO2 Heat Capacity
• High temperature heat
capacity studied for nuclear
fuel
 Room temperature to
1000 K
 Increase in heat
capacity due to
harmonic lattice
vibrations
* Small
contribution to
thermal
excitation of U4+
localized
electrons in
crystal field


1000-1500 K
 Thermal expansion
induces anharmonic
lattice vibration
1500-2670 K
 Lattice and electronic
defects
6-25
Oxygen potential
• Equilibrium oxygen partial
pressure over uranium oxides

In 2 phase region of solid
oxides
 ΔG(O2)=RTln pO2
* Partial pressure
related to O2
• Large increase above O/U = 2

Increase in ΔG(O2)
decreases with
increasing ratio

Increase ΔG(O2) with
increasing T
• Entropy essentially
independent of temperature

ΔS(O2)= -dΔG(O2)/dT
• Enthalpy related to Gibbs and
entropy through normal
relationship

Large peak at UO2+x, x is
very small
6-26
6-27
Vaporization of UO2
• Above and below the melting
point
• Number of gaseous species
observed
 U, UO, UO2, UO3, O, and
O2
 Use of mass
spectrometer to
determine partial
pressure for each
species
 For
hypostiochiometric
UO2, partial pressure
of UO increases to
levels comparable to
UO2
 O2 increases
dramatically at O/U
above 2
6-28
Uranium-oxides: Oxygen diffusion
• Vacancy based diffusion in hypostoichiometric UO2
 Based on diffusion into vacancy, vacancy
concentration, migration enthalpy of vacancy
 Enthalpy 52 kJ mol-1
• For stiochiometric UO2 diffusion temperature dependent
 Thermal oxygen vacancies at lower T
 Interstitial oxygen at higher T
 Equal around 1400 ºC
• For UO2+x diffusion dominated by interstitial oxygen
 Migration enthalpy 96 kJ mol-1
6-29
Uranium-oxide: Electrical conductivity
• UO2 and UO2+x

Mobility of holes in lattice
 0.0015 to 0.021 cm2V-1s-1
* Semiconductor around 1 cm2V-1s-1

Holes move in oxide structure along with local distortion within
lattice

Holes and electrons localized on individual atoms
 Holes U5+ and electrons form U3+

From 500 to 1400 ºC for UO2+x
 Decrease in conductivity with decrease in x when x<0.1
• U3O8-z

Similar to UO2+x

Phase transition at 723 K results in change of temperature
dependence
6-30
Uranium oxide chemical properties
• Oxides dissolve in strong mineral acids
 Valence does not change in HCl, H2SO4, and H3PO4
 Sintered pellets dissolve slowly in HNO3
 Rate increases with addition of NH4F, H2O2, or
carbonates
* H2O2 reaction
 UO2+ at surface oxidized to UO22+
6-31
Group 1 and 2 uranates
•
•
Wide series of compounds

M2UnO3n+1 for M+

MUnO3n+1 for M2+
 Other compounds known
* M4+UO5, M22+UO5,
M32+UO6, and
M22+U3O11
Crystal structures

Layered structures and UO22+ in
the crystals

Monouranates (n=1)
 Layered planes, O atom
coordinate to U on the
plane
* Some slight spacing
around plane

Ba and Mg UO4
 Deformed ochahedron
* Secondary O bridges
adjacent U atoms
 Shared
corners
 Shared edges

M4UO5 (M=Li, Na)
 No uranyl group
 4 orthogonal planar U-O
bonds
•
Preparation

Carbonates, nitrates or chlorides of group 1
or 2 elements mixed with U3O8 or UO3

Heat in air 500-1000 ºC
 Lower temperature for Cs and Rb

Different phases of some compounds
6-32
Group 1 and 2 uranates
•
•
•
Physicochemical properties

Hydroscopic

Colored
 Yellow to orange

Heavier group 1 species volatile

IR active
 Asymmetric stretch of UO22+
 600-900 cm-1
* Frequency varies based on other O coordinated to uranyl group

Diamagnetic compounds
 Can be examined by U NMR
* Some weak paramagnetism observed
 Covalency in uranyl group
Uranates (V) and (IV)

MUO3 (M=Li, Na, K, Rb)

M3UO4 (M=Li, Na)

MU2O6 (M=Mg, Ca, Sr, Ba)

MUO3 (M=Ca, Sr, Ba), tetravalent U
Synthesis

Pentavalent uranates
 Tetravalent and hexavalent uranium species mixed in 1:1 ratio
* Heated in evacuated sealed ampoule
 UO2 + Li2UO4 2 LiUO3
 Hydrogen reduction of hexavalent uranates
 at elevated temperatures tetravalent uranates form
6-33
Group 1 and 2 uranates
• Crystal structure

No uranyl present, lacks layered structure
 Perovskite type structure is common
• Physicochemical properties

Brown or black in color

Dissolves in mineral acids, nitric faster dissolution rates

Oxidize to hexavalent state when heated in air

Electronic spectra measured

Magnetic paramagnetic properties measured
 5f1 from U5+
 Oh crystal field
* Some tetragonal distortions
• Non-stoichiometry

Removal of oxide
 Formation of xNa2O from Na2U2O7 forms Na2-2x+U2O7-x

Non-stoichiometric dissolution of metal in UO2
 NaxUO3 (x≤0.14)

Oxygen non-stoichiometry
6-34
 Na2U2O7-x (x≤0.5)
Transition metal uranates
• Wide range of compounds
• Preparation method

heating oxides in air with UO3 or U3O8
 Changing stoichiometry can result in different
compounds
* U/M = 3, MU3O10 (M=Mn, Co, Ni, Cu, Zn)

Uranyl nitrate as starting material
 Metal nitrates, temperatures below 600 ºC
 MxUO4
• Crystal structures

Chain of edge sharing of oxygen

Some influence of metal on uranyl oxygen bond length

Lanthanide oxides form solid solutions
 Can form Ln6UO12
6-35
Solid solutions with UO2
• Solid solutions formed with
group 2 elements,
lanthanides, actinides, and
some transition elements
(Mn, Zr, Nb, Cd)
 Distribution of metals
on UO2 fluorite-type
cubic crystals based on
stoichiometry
• Prepared by heating oxide
mixture under reducing
conditions from 1000 ºC to
2000 ºC
 Powders mixed by coprecipitation or
mechanical mixing of
powders
• Written as MyU1-yO2+x
 x is positive and
negative
6-36
Solid solutions with UO2
• Lattice parameter change in
solid solution

Changes nearly linearly
with increase in y and x
 MyU1-yO2+x
 Evaluate by change
of lattice parameter
with change in y
* δa/δy
 a is lattice
parameter
in Å
 Can have
both
negative
and
positive
values

δa/δy is large for metals
with large ionic radii

δa/δx terms negative and
between -0.11 to -0.3
 Varied if x is positive
or negative
6-37
Solid solutions of UO2
• Tetravalent MyU1-yO2+x
 Zr solid solutions
 Large range of systems
 y=0.35 highest value
 Metastable at lower temperature
 Th solid solution
 Continuous solid solutions for 0≤y≤1 and x=0
 For x>0, upper limit on solubility
* y=0.45 at 1100 ºC to y=0.36 at 1500 ºC
 Also has variation with O2 partial pressure
* At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at
1500 ºC
6-38
Solid solutions of UO2
•
•
Tri and tetravalent MyU1-yO2+x

Cerium solid solutions
 Continuous for y=0 to y=1
 For x<0, solid solution restricted to y≤0.35
* Two phases (Ce,U)O2 and (Ce,U)O2-x
 x<-0.04, y=0.1 to x<-0.24, y=0.7
 0≤x≤0.18, solid solution y<0.5
 Air oxidized hyperstoichiometric
* y 0.56 to 1 at 1100 ºC
* y 0.26-1.0 1550 ºC
Tri and divalent

Reducing atmosphere
 x is negative
 fcc
 Solid solution form when y is above 0
 Maximum values vary with metal ion

Oxidizing atmosphere
 Solid solution can prevent formation of U3O8
 Some systematics in trends
* For Nd, when y is between 0.3 and 0.5, x = 0.5-y
6-39
6-40
6-41
Solid solution UO2
• Oxygen potential

Zr solid solution
 Lower than the UO2+x system
* x=0.05, y=0.3
 -270 kJ/mol for solid solution
 -210 kJ/mol for UO2+x

Th solid solution
 Increase in DG with increasing y
 Compared to UO2 difference is small at y less than 0.1

Ce solid solution
 Wide changes over y range due to different oxidation
states
 Shape of the curve is similar to Pu system, but values
differ
* Higher DG for CeO2-x compared to PuO2-x
6-42
Solid solution UO2
• Trivalent
 Oxygen potential
increases with
increasing x
 Inflection point
at x=0
 For lanthanides La
has highest DG due
to larger ionic radius
• Divalent
 Higher oxygen
potential than
trivalent system
 Configuration
change
 Formation of
pentavalent U
 At low O2 partial
pressures cannot
dissolve high levels
of Mg
6-43
Borides, carbides, silicides
• UB2, UB4, UB12 are known
compounds
• Prepared by mixing elements at
high temperature
• Other reactions

UCl4+2MgB2UB4 +
2MgCl2
• UB and UB4 form in gas phase
• Inert species

Potential waste forms

UB12 more inert
• Large amount of ternary
systems

U5Mo10B24, UNi4B
 Sheets with 6 and 8
member rings
A view down the c‐axis of
the structure of UB4
6-44
Uranium carbides
•
•
•
•
•
Three known phases

UC, UC2, and U2C3
UC and UC2 are completely miscible at
higher temperature

At lower temperatures limited

Synthesized by mixture of
elements at high temperature
U2C3 prepared by heating UC and UC2
in vacuo from 1250-1800 °C

Once formed stable at room
temperature
Alkanes produced by arc-melting

Oxalic acid produced by
carbide dissolution in nitric
acid

Ternary carbides
 Melting elements in
carbon crucible
* U2Al3C4
UC2 reacts slowly in air

With N2 at 1100 °C to form UN
6-45
•
•
•
Uranium-silicon
Compounds

U3Si, U3Si2, USi, U3Si5, USi1.88,
and USi3
Complicated phase diagram

Number of low temperature
points
Forms ternary compounds with Al

U(Al, Si)3
 Formed in U in contact
with Al

Cu, Nb, and Ru ternary phases
 U2Nb3Si4 ferromagnetic
below 35 K

URu2Si2
 Heavy fermion material
* metallic materials
having large electronic
mass enhancement
 antiferromagnetic
interaction
between
conduction
electrons and
local magnetic
moments (d- or felectron)
6-46
N, P, As, Sb, and Bi uranium
•
•
Monopnictides

UN, UP, UAs
 Cubic NaCl structure
U-nitrides

UN, U2N3, UN2

UN prepared by uranium metal
with nitriding agents
 N2, NH3
 Thermal decomposition of
higher nitrides
* Higher nitride unstable
with respect to UN
 Mixture of higher nitrides
with uranium metal
* Treat surface with
HNO3 and washed with
organics
 Remove traces of
oxides and
carbides

UN easily oxidized by air,
unstable in water
6-47
6-48
6-49
P, As, Sb, Bi-uranium
• UX, U3X4, and UX2
 X=P, As, Sb, Bi
 UX is cubic except b-UBi
 U3X4 is body centered cubic
 UX2 is tetragonal
• Preparation
 Synthesis from the elements in an autoclave
 2U + P42UP2
 Uranium hydride with phosphine or arsine
 UH3+PH3UP+3H2
6-50
S, Se, Te-uranium
• Uranium-sulfur

US, US2, U2S3, U3S5
 Preparation
* Heating U
metal or UH3
with H2S
* Heating
elements in
sealed tube
* Decomposition
of higher
sulfides in heat
under vacuum
* UCl4 with Li2X
(X=S, Se, Te)

U3S5 mixed U valence
structure
 U3+ and U4+
• Se and Te prepared as the
sulfur complexes

UTe2 contains Te-Te
bonds and mixed
valence states
 U3+ and Te1-,2-
6-51
6-52
Uranium halides
• Thoroughly studied uranium compound
 Isotope separations
 Molten salt systems and reactors
 Preparation of uranium metal
• Tetravalent and hexavalent oxidation state compounds
• Covalent halide compounds have 5f electron interaction
 Ionic property highest with higher U oxidation
state and more electronegative halides
 Exception UF3 move covalent than UCl3
6-53
Trivalent uranium halides
• Sensitive to oxidation
• Stability decreases with increasing atomic number of
halide
• Hydroscopic
• Stable in deoxygenated solvents
 Soluble in polar solvents
• Range of colors
• Synthesis
 Oxygen free
 Temperature 600 ºC
 Ta or Mo tubes to avoid reaction with Si
6-54
Trivalent uranium halides
•
•
Electronic properties

5f3
4I

9/2 ground state configuration

Crystal field analysis of low temperature compounds
 Large range of compounds evaluated for free ion and crystal field
parameters
Absorption spectra for U3+ halides examined

Strong f-d bands
 Mixing of electrons from different quantum levels
* Laporte rule
 First f-d transition at 23000 cm-1 for CsUCl4.3H2O
* 5f35f26d1
* Shifted toward IR region for NH4UCl4.4H2O by 5000 cm-1
 27000 cm-1=370 nm, 15000 cm-1=666.7 nm
* For substitution of U3+ substitution with halides
 Increase in covalence properties related to red shift in fd band
6-55
Trivalent uranium halides
• Preparation of UF3
 Reduction of UF4 by Al metal
 With Al, place in graphite crucible and heat to
900 ºC
 With UN or U2N3 at 900 ºC
• Stable in air at room temperature
• Insoluble in water, dissolved in nitric-boric acid
• Structure is capped trigonal prism
• Hydrate species also forms, but oxidizes in air
 U3+ in 1 M HCl and precipitation with NH4F
6-56
•
•
•
•
•
•
Trivalent uranium halides
UCl3

Reaction of gaseous HCl with UH3 at 350 ºC

Reduction of UCl4 with Zn or Al at 400 ºC

Thermal vacuum decomposition of NH4UCl4

Disproportionates to U and UCl4 at 837 ºC
Olive green powder or dark-red crystals
Soluble in polar organic solvents
Easily oxidized
Hexagonal symmetry
Forms hexa- and heptahydrate

Water in inner coordination sphere

Heptahydrate built from separate [U2Cl2(H2O)14]4+ units
and Cl- ions
 Uraniums connected over bridging Cl

A number of hydrated complexes prepared
 MUCl4
* From U3+ in 11 M HCl with MCl
* Tri- and tetrahydrates show 5f35f26d1 at
21500 cm-1 and 16000 cm-1
* Red shift indicates covalent character of water
interaction
 Bond lengths based on inner sphere
complexes
6-57
Trivalent uranium
halides
•
•
UCl3 with neutral ligands

Ammonia adducts, UCl3.7NH3
 From UCl3 heated in ammonia
under pressure

UCl3(THF)x

Wide range of crown ether
complexes
 Prepared from ligand and UCl4
reduced with Zn
 Intense f-d transitions in visible
and UV region
* IR needed to identify
ligand coordination
 Compounds hydroscopic and
oxidized in air
UBr3 species

Prepared by reaction of UH3 with
HBr at 300 ºC

Reduction of UBr4 by Zn at 600 ºC
 UBr3 reacts with quartz at
room temperature, need to
prepare in sealed Ta or Mo
vessel

Hydroscopic and oxidizes more
readily than UCl3

Isostructural with UCl3

Hydrate species formed by reaction
of UBr3 with oxygen free water
vapor

M2UBr5 and M3UBr6
 Melting points are high and
increase with M mass
6-58
Trivalent uranium halides
• UI3

Prepared from I2 on U metal at 525 ºC

UI4 with Zn

Vacuum decomposition of UI4

UH3 with methyl iodide
• Hydroscopic and attacks glass
• Dissolves in aqueous solution, methanol, ethanol, acetic acid

Forms unstable U3+
• 5f35f26d1 at 13400 cm-1

Shift from 23000 cm-1 for UF3
• Synthesis of neutral donor complexes with solvent, U metal and I2
at 0 ºC
• Mixed oxide species prepared

UOX (X=Cl, Br, I)
 Heating stoichiometric mixtures of UO2X2, UO2, and U
or UX4, U3O8 and U at 700 ºC for 24 hours
6-59
Tetravalent uranium halides
•
•
•
•
•
•
•
•
UF4 stable upon exposure to air

Lattice energy responsible for enhanced stability over other tetravalent
halides
All expect UF4 soluble in polar solvents

U4+ can be stabilized in solution
Different structures for solids

UF4: square antiprism

UCl4: dodecahedron

UBr4: pentagonal bipyramid
Ground State electronic configuration 5f2 (3H4)

Compounds have 5f25f2 transitions

f-d transitions begin 40000 -50000 cm-1 (UV-region)
 Higher energies than U3+
Absorption data collected at low temperature for transition assignment
Evidence of 5f17p1 for Cs2UBr6
Over 60 5f25f2 transitions identified

U4+ doped in BaY2Cl7
 Absorption, excitation, luminescence spectra
 Crystal field strength for U4+ dominated by symmetry of central ion
rather than ligand
* Lower symmetry results in lower crystal field
4+
U has strong anti-stokes emission
6-60
Tetravalent uranium halides
• Complexes with inversion symmetry (UCl62-) used to determine
electronic transitions

Low temperature

Evaluation of side bands
• Low temperature UF4 absorbance identified 91 ff transitions
6-61
Tetravalent uranium halides
•
•
•
•
•
UF4 exploited in nuclear fuel production

Conversion to UF6
 Based on chemical stability and insolubility in solution
Formed by a number of reactions

Uranium oxides with HF (UO2, U3O8)
 U3O8+ 8 HF2UO2F2 + UF4 + 4 H2O if no H2 in system
 UO3 with ammonia-hydrogen fluoride mixtures
* UO2 and heating with same compounds

Can also be prepared by the reduction of UF6
Dissolves in the presence of reagents that can form fluoride complexes

Fe3+, Al3+, boric acid
Fitting of UF4 spectra resulted in assignment of 69 crystal field levels
Hydrates formed from aqueous fluoride solution

nH2O (0.5<n<2.5)

n=2.5 most stable

Water completely removed at 550 ºC
6-62
Tetravalent uranium halides
•
•
•
Complex uranium fluorides

Metal fluoride uranium fused salts
 Fuels and reactors

LiF-BeF2-UF4 and NaF-BeF2-UF4

MgUF6 and CaUF6 for uranium
metal production
Produced in a number of reactions

Solid state reaction between metal
fluorides in inert atmosphere

U oxides with metal fluorides or
carbonates in HF or HF-O2

Reduction of UF6 with metal
fluorides

Controlled decomposition of
higher fluoro complexes
 (NH4)4UF8
Structures of compounds known

UF62-: octahedral

UF73-: pentagonal bipyramid

UF84-: bicapped triangular prism
 Some complexes differ
* Chains tricapped trigonal
prisms for b-K2UF2
6-63
Tetravalent uranium halides
• Uranium oxide- and nitride fluorides
 Melting UO2 (or other oxides) and UF4
Mono- and dihydrate precipitates
Mixed oxidation states of U found
* 5+ and 6+
* 4+ and 5+
 UN1.33 and UF4
Compounds between UNF and
UN0.9F1.2)
6-64
Tetravalent uranium halides
•
•
•
•
•
Uranium tetrachloride

Starting material for a range of uranium compounds
 Ease of preparation
 Solubility in polar organic solvents

Synthesis
 Chlorination of UO2
 Need reactive form of UO2
 Converts to U3O8 in air at 600 ºC

Isostructural with other actinide tetrachlorides
 Tetragonal symmetry
Range of complex chlorides

M2UCl6 and MUCl5
 Monovalents include NR4, PR3H compounds

Can be prepared from fused salts of UCl4 with metal chlorides
Chlorine atoms can be replaced

UCl4 in non-aqueous media with decomposition reaction
Species are paramagnetic

Temperature dependent up to 350 ºC
Oxychloride species

From UO2 in excess UCl4 followed by sublimation

Dissolves in water and aqueous nitric acid

Isostructural with Th, Pa, and Np oxychloride
6-65
Tetravalent Uranium halides
•
•
Uranium tetrabromide

Prepared from:
 Oxides with bromine
 Oxides or UOBr3 with CBr4
 UO2 and sulfur bromine mixture

insoluble in non-polar organic solvents

Soluble in polar solvents
 HBr evolved in ethanol, methanol, phenol, acetic acid, or moist air

Absorption bands 5f25f16d1 at 41400-32160 cm-1

Charge transfer at 30165 cm-1

Forms compounds with numerous ligands

Pentagonal bipyramid around U

M2UBr6 with group 1 elements
 Can coordinate with organic cations
* Soluble in water, aqueous HBr, polar non-aqueous solvents
 fcc crystals
 Oh from solution spectroscopy
* 5f25f16d1 27400 to 39000 cm-1
* Vibronic side bands
* Hydrogen bonding can distort Oh to permit ff
Oxybromides similar to oxychlorides
6-66
Tetravalent uranium halides
• UI4

Prepared by direct combination of the elements at 500 ºC

Used in preparation of UI3

M2UI6 from components in anhydrous methyl cyanide
 Hydroscopic compounds
 Used to obtained spectroscopic terms for electronic
transitions
• UOI2 from heating U3O8, U, and I2 sealed at 450 ºC
• UNI from UI4 with ammonia
• Mixed halides

Range of compounds

Higher fluoride species are more stable
 UClF3>UCl2F2

Mixed Cl-Br and Cl-I, Br-I
6-67
Pentavalent uranium halides
•
•
•
•
•
•
Strong tendency to hydrolyze and disproportionate to tetra- and hexavalent
species
Preparation

UO3 with thionyl chloride under reflux
Decomposes in CCl4, CH2Cl2
Varied coordination geometry

Octahedral (a-UF5)

Pentagonal bipyramid (b-UF5)

Edge-sharing octahedral (U2Cl10)
5f1 electronic configuration: 4F5/2 ground state
UF5

Two phases, alpha over 150 ºC

Oxidation of UF4 or reduction of UF6
 Oxidation with HF, noble gas fluorides
 Reduction with HN3, SOCl2

Water causes disproportionation

2UF5+3H2OUF4+UO2F2+4HF

Reduced to UF4 by H2 or Ni

Stable in 50 % HF solution
6-68
Pentavalent uranium halides
•
•
•
•
Structure

a-UF5 chains of UF6 octahedral
bridged by trans-fluorides
Complex compound preparation

Alkali halides in inert atmosphere at
300 ºC

Ammonia reaction

Metal halides reaction in HF

Bonds covalent
Oxide fluorides

UF4 in intermittent O2 flow at 850 ºC
creates U2OF8

Complex compounds also form
UCl5

Unstable through thermal
decomposition

Prepared by oxide treatment with
CCl4 at 80-250 ºC and UCl5 catalyst
or UO3 with SiCl4

a-Cl5 (monoclinic)from
recrystallization from CCl4

b-Cl5 (triclinic) by recrystallization
of UCl6 in CCl4 or CH2Cl2

Absorbance spectra same for both
phases
 Similar to UCl6-
6-69
Pentavalent uranium halides
•
•
•
•
•
Complex compounds

Range of compounds with ligands containing N, P, As, S, Se, and Te donor

Variety of MUCl6
 Group 1 and organic cations
oxide species and complex
 UOCl3 from MoCl5 at 200 ºC
 UCl4 and UO2Cl2 at 370ºC
 UO2Cl2 with WCl5, ReCl5 at 200 ºC
 Dissolves in anhydrous ethanol
Pentabromide

Bromination of metal or UBr4 at 55 ºC

UOBr3 from UO3 with CBr4
 UO2Br can also be prepared from thermal decomposition of UO2Br2
Intermediate uranium halides

UF4 with UF6
UF5 fluctuates between C4v and D3h

Participation of 5f orbitals in bonding

5f, 6p, and 6d
 Low population of 7s and 7p
6-70
Hexavalent uranium halides
• Stability decreases with increasing halide mass
• No simple bromine or iodine forms
• React with water to form uranyl halides

Uranyl forms weak halides except with fluoride
• Soluble in polar organic solvents
• Generally yellow compounds

UF6 colorless, UCl6 green
• 5f0: 1S0 ground state
• Spectra of UO22+ has vibrational fine structure

Coupling with O=U=O stretching modes
• UF6 has similar spectroscopic properties

Superimposed on charge transfer bands centered near 26670
cm-1 and 38460 cm-1

Coupling resulting fine structure based on transitions
t1u(s+p) to empty 5f orbitals
• Compounds show weak, temperature dependent paramagentism
6-71
Hexavalent uranium halides
• UF6
 Readily volatile uranium compound
Isotope enrichment
6-72
• Orthorhombic colorless
crystals
• Sublime at 56.5 ºC
• Liquid and gas Oh symmetry
• Temperature independent
paramagnetism
• Reactive and moisture
sensitive
• Oxidizing agent
 nUF6+MnUF5 +MFn
• 1st bond dissociation at 134
kJ/mol
 Similar to F2 (153.2
kJ/mol)
• Formation of MxUF(6+x) x=1,2
from UF6 and MF
 Based on UF6 electron
affinity and lattice energy
• Reduction from a number of
reagents or alpha decay
• Some eutectic phase with
BrF2, BrF3, BrF5
UF6
6-73
•
•
UF6 species
Tend to decompose to UF6 when heated
Oxide species

In liquid HF
 3UF6+SiO23UOF4+SiF4
 3UF6+B2O33UOF4+2BF3

Orange solid, non-volatile, decomposes at 200-250 ºC

UOF4 at 250 ºC in vacuum decomposes to UF6 and UO2F2

UO2F2 also formed from UO3 in gaseous HF at 300 ºC
 UO2F2 yellow compound, slightly soluble in H2O, methanol and
ethanol
 Hydrated species from recrystallization in water
6-74
Hexavalent uranium halides
•
•
•
•
•
UCl6

From thermal decomposition of UCl5 at 120-150 °C in vacuo

Moisture sensitive

Melts at 177 °C

Reacts with water to form uranyl

Hexagonal symmetry

Charge transfer bands around 21000 cm-1
UO2Cl2

From the oxidation of UCl4

Insoluble in non-polar solvents

A large number of different oxychloride compounds produced
Oxybromide compounds

From the reaction of O2 with UBr4

UO2Br2 loses Br even at room temperature

Hydrates and hydroxide species form
Iodine compounds

Extremely unstable UO2I2 reported

Number of moieties with organic
Mixed halogen species

M2UO2Cl2Br2

X3I (X=Cl or Br)
6-75
Chemical bonding
•
•
Tri- and tetravalent U mainly related to organometallic compounds

Cp3UCO and Cp3UCO+
 Cp=cyclopentadiene
* 5f CO p backbonding
 Metal electrons to p of ligands
* Decreases upon oxidation to U(IV)

Nitrogen containing ligand (terpyridyl)shows greater backbonding
than Ce(III)
Uranyl(V) and (VI) compounds

yl ions in aqueous systems unique for actinides
 VO2+, MoO22+, WO22+
* Oxygen atoms are cis to maximize (pp)M(dp)
 Linear MO22+ known for compounds of Tc, Re, Ru, Os
* Aquo structures unknown

Short U=O bond distance of 1.75 Å for hexavalent, longer for
pentavalent
 Smaller effective charge on pentavalent U

Multiple bond characteristics, 1 s and 2 with p characteristics
6-76
•
Uranyl chemical
bonding
Bonding molecular orbitals
sg2 su2 pg4 pu4
 Order of HOMO is unclear
* pg< pu< sg<< su proposed
 Gap for s based on 6p
orbitals interactions

5fd and 5ff LUMO

Bonding orbitals O 2p characteristics

Non bonding, antibonding 5f and 6d

Isoelectronic with UN2
Pentavalent has electron in non-bonding
orbital

•
6-77
6-78
Uranyl chemical bonding
• Linear yl oxygens from 5f characteristic

6d promotes cis geometry
• yl oxygens force formal charge on U below 6

Net charge 2.43 for UO2(H2O)52+, 3.2 for fluoride systems
 Net negative 0.43 on oxygens
 Lewis bases
* Can vary with ligand in equatorial plane
* Responsible for cation-cation interaction
* O=U=O- - -M
* Pentavalent U yl oxygens more basic
• Small changes in U=O bond distance with variation in equatoral ligand
• Small changes in IR and Raman frequencies

Lower frequency for pentavalent U

Weaker bond
6-79
•
•
•
•
•
As all complexes, characterization based on
coordination geometry, coordination number and
bond distances
Relate solid state to solution structure
Large number of hexavalent uranium compounds
from aqueous solutions
O=U=O axis inert

Coordination around equatorial plane

4 to 6 coordinating ligands

Labile in solution
Uranyl(VI) compounds

Common coordination geometry pentagonal
bipyramid

Other coordination geometries
 Distorted Oh
 Distorted pentagonal bipyramid
 Hexagonal bipyramid
* MUO2(NO3)3, K4UO2(CO3)3
 Square bipyrimid
* Can occur in complexes with strong
steric interference
Structure
and
coordination
chemistry
6-80
U(VI) structure and coordination
• UO2CO3(s)
 3 oxygens for each
uranium
 Will not be
composed of a
discrete complex
 Oxygens shared by
U forming layered
structure
• Six coordination also forms
with correct ligands
• Peroxide complexation in
both solid and solution phase
 Some self-assembling
nano-clusters with
peroxide
6-81
6-82
U(III) structure and coordination
• Expected to be similar to other
trivalent actinides

U(III) does not form stable
compounds

Actinides tend to form most
stable complexes than
lanthanides
 No large differences in
bond distances or
coordination
geometries
 Any differences based
on variation in ionic
radius, larger for
actinides
• U(III) complexes have high
coordination numbers

8 or 9
 Distorted trigonal
prism

No structural
determination of simple
inorganic ligands in
solution
6-83
U(IV) and (V) structure and coordination
• U(IV)
 Normal and basic salts
with inorganic ligands
 Basic salts due to
hydrolysis or oxide
formation
 Large ionic radius and
8 to 10 coordination
 Similar to Ce(IV)
 Carbonates form
trigonal bipyramid
• U(V)
 Few examples of
structures
 Hexagonal bipyramid
for triscarbonate
 Similar to U(VI)
species
 Labile ligands in
equatorial plane
 Weaker complexes
compared to U(VI)
6-84
Uranium organic ligands
• Same trends as observed with inorganic ligands
• Organic ligands have geometric constraints
• Structural information obtained from different
methods
 EXAFS
 NMR
 Quantum calculations
• Coordination may be through limited functional groups
 Carboxyl acids
 Chelation
6-85
Uranium solution
chemistry
•
•
Uranyl(VI) most stable in
solution

Uranyl(V) and U(IV)
can also be in solution
 U(V) prone to
disproportionation

Stability based on pH
and ligands

Redox rate is limited by
change in species
 Making or
breaking yl oxygens
* UO22++4H++2eU4++2H2O
yl oxygens have slow
exchange

Half life 5E4 hr in 1 M
HClO4

Rate of exchange
catalyzed by UV light
6-86
Uranium solution chemistry
• Trivalent uranium

Dissolution of UCl3 in water

Reduction of U(IV) or (VI) at Hg cathode
 Evaluated by color change
* U(III) is green

Very few studies of U(III) in solution

No structural information
 Comparisons with trivalent actinides and lanthanides
6-87
Uranium solution chemistry
•
•
Tetravalent uranium

Forms in very strong acid
 Requires >0.5 M acid to prevent hydrolysis
 Electrolysis of U(VI) solutions
* Complexation can drive oxidation

Coordination studied by XAFS
 Coordination number 9±1
* Not well defined
 U-O distance 2.42 Å

O exchange examined by NMR
Pentavalent uranium

Extremely narrow range of existence

Prepared by reduction of UO22+ with Zn or H2 or dissolution of UCl5
in water

UV-irradiation of 0.5 M 2-propanol-0.2 M LiClO4 with U(VI) between
pH 1.7 and 2.7
 U(V) is not stable but slowly oxidizes under suitable conditions

No experimental information on structure

Quantum mechanical predictions
6-88
Hexavalent uranium solution chemistry
• Large number of compounds prepared
 Crystallization
 Hydrothermal
• Structure examined by XAFS
6-89
Aqueous solution complexes
• Strong Lewis acid
• Hard electron acceptor

F->>Cl->Br-I
Same trend for O and N group
 based on electrostatic force as dominant factor
• Hydrolysis behavior

U(IV)>U(VI)>>>U(III)>U(V)
• Uranium coordination with ligand can change protonation
behavior

HOCH2COO- pKa=17, 3.6 upon complexation of UO2
 Inductive effect
* Electron redistribution of coordinated ligand
* Exploited in synthetic chemistry
• U(III) and U(V)

No data in solution
 Base information on lanthanide or pentavalent actinides
6-90
Uranium hydrolysis
•
•
Determination of constants from spectroscopic and
titration

Determine if polymeric species form

Polynuclear species present expect at lowest
concentration
U(OH)4 structure

May form hydrated species

no evidence of anionic species formation
 i.e., U(OH)5(H2O)n-1 U4(OH)16
* 6 coordination
6-91
Nanomole/L UO22+
Micromole/L UO22+
pH 6 U(VI) variation
Millimole/L UO22+
6-92
Inorganic complexes
• Strong fluoride complexes with
U(IV) and U(VI)
• Oxygen ligand complexes increase
with charge and base of the ligand

i.e., carbonate, phosphate,
nitrate

Complexes with strong bases
HSiO43- and SiO44- difficult to
study due to competition from
OH• Complex structure from central U
and ligand geometry

XAFS and neutron data
6-93
Uranium solution chemistry
• Organic ligands and functional groups

Carboxylic acids
 Additional amino or hydroxyl group
• Aliphatic nitrogen donors are strong bases

Competition with proton prevents coordination with U below
pH 6
• Ternary uranium complexes

Addition of OH- to complex
 UxLy(OH)z

Evaluate based on L and OH- complexation with U and
steric constraints
 Most ternary complexes contain OH- and F-
6-94
Ligand substitution reactions
•
•
•
•
•
Most data with U focuses on rate of reaction

Mechanism of reaction are speculative
 Describes molecular details of a reaction
Data available

Non-aqueous solvents

Redox

Multidentate ligands
Enthalpy and entropy terms evaluated
Methods

Stop-flow

NMR
 Protons, 13C, 17O, 19F
* i.e., water change followed by 17O
Water reactions

Fast outer sphere going to rate determining inner sphere (k2)

Overall rate can determined from k2 and equilibrium constant
 Kobs

Associative, Dissociative, Interchange

Water exchange smaller with complexes
 UO2(oxalate)F(H2O)2* 2E3 s-1 compared to 1.3E6 s-1
6-95
• Experimental ΔH=26 kJ mol-1
• Calculated

74 (D), 19 (A), 21 (I)

Base on similarity between experimental and calculated
6-96
6-97
6-98
•
Ligand substitution reactions
NMR data for coordination

3 different fluoride ligands
6-99
•
•
Uranium chemistry
in
solution
U isotopic exchange

Exchange between oxidation states
and phases
 Isotopic purity for a given
species
 Separation and evaluation
* Counting or mass
spectroscopy
U fluorescence

Excitation of uranyl
 Different spectra and lifetime

Quantum yield impacted by solution
chemistry
 Quenching from heavy ions in
solution
 Low oxidation state due to
electron transfer

Excited U state used in chemical
reactions

No consensus on primary deexcitation mechanism

I/Io=t/to
 o is state without ligand, I is
intensity and t is lifetime

Charge transfer characteristic due to
excitations from sg and su to empty f
orbital
6-100
Organometallic and biochemistry
• Uranocene
• Biochemistry

RNA and DNA interactions over phosphates
 Photochemical oxidation

polysaccharides over deprotonated OH
• Analytical chemistry

Separation and preconcentration

Titration

Electrochemical methods

Nuclear techniques

Spectrometric
 Atomic absorption, AES, XRF
 Indicator dye
 Fluorescence
 Mass spectrometry
6-101
Review
•
•
•
•
•
•
•
•
•
Understand trends in Uranium nuclear properties
Range of techniques and methods for U purification
Understand the atomic properties of uranium
Techniques used in the preparation of uranium
metallic state
 Properties and phases of uranium metal
Trends and commonalties in the synthesis of uranium
compounds
Uranium compounds of importance to the nuclear fuel
cycle
Structure and coordination chemistry of uranium
compounds
 Roles of the electronic structure and oxidation
state
Solution chemistry
 Trends with oxidation state
Methods for the concentration analysis of uranium 6-102
Questions
• What are the natural isotopes of uranium
• What are some methods for the purification of
uranium ore
• How can one prepare the different phases of U
metal
• Provide 5 reactions that use U metal as a starting
reagent
• Describe the synthesis and properties of the
uranium halides
• How is the O to U ratio for uranium oxides
determined
• What are the trends in U solution chemistry
• What atomic orbitals form the molecular orbitals
for UO22+
6-103
Pop Quiz
• What low valent uranium compounds can be
synthesized? Provide an example for the
trivalent and tetravalent oxidation state.
Describe some studies that can utilize these
compounds.
6-104