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6-1 Lecture 6: Uranium Chemistry From: Chemistry of actinides §Nuclear properties §U purification §Free atom and ion property §Metallic state §Compounds.

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Presentation on theme: "6-1 Lecture 6: Uranium Chemistry From: Chemistry of actinides §Nuclear properties §U purification §Free atom and ion property §Metallic state §Compounds."— Presentation transcript:

1 6-1 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

2 6-2 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 § 234,235,238 U §Ratios of isotopes established à234: 0.005±0.001 à235: 0.720±0.001 à238: 99.275±0.002 233 U from 232 Th

3 6-3 Uranium Minerals 200 minerals contain uranium §Bulk are U(VI) minerals àU(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 àIncorporation of higher actinides Pyrochlore §A 1-2 B 2 O 6 X 0-1 àA=Na, Ca, Mn, Fe 2+, Sr,Sb, Cs, Ba, Ln, Bi, Th, U àB= Ti, Nb, Ta àU(V) may be present when synthesized under reducing conditions *XANES spectroscopy *Goes to B site

4 6-4 Polyhedra classification U(VI) minerals Linkage over equatorial position §Bipyramidal polyhedra §Oxygens on uranyl forms peaks on pyramid àDifferent 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 àAssociated cation removed §Connect anions to form polyhedra àDefines anion topology Chains defined by shapes §P (pentagons), R (rhombs), H (hexagons), U (up arrowhead chain), D (down arrowhead chain)

5 6-5 Uranium purification from ores Common steps §Preconcentration of ore àBased on density of ore §Leaching to extract uranium into aqueous phase àCalcination prior to leaching *Removal of carbonaceous or sulfur compounds *Destruction of hydrated species (clay minerals) §Removal or uranium from aqueous phase àIon exchange àSolvent extraction àPrecipitation Leaching with acid or alkaline solutions §Acid solution methods àAddition of acid provides best results *Sulfuric or HCl (pH 1.5) ØU(VI) soluble in sulfuric ØOxidizing conditions may be needed ØMnO 2, chlorate, O 2, chlorine àGenerated in situ by bacteria àHigh pressure oxidation of sulfur, sulfides, and Fe(II) *sulfuric acid and Fe(III) §Carbonate leaching àFormation of soluble anionic carbonate species àSomewhat specific for uranium àUse of O 2 as oxidant àBicarbonate prevents precipitation of Na 2 U 2 O 7 *OH - +HCO 3 -  CO 3 2- + H 2 O

6 6-6 Recovery of uranium from solutions Ion exchange §U(VI) anions in sulfate and carbonate solution àUO 2 (CO 3 ) 3 4- àUO 2 (SO 4 ) 3 4- §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 àChemistry similar to ion exchange conditions Chemical precipitation §Older method àAddition of base àPeroxide *Ultimate formation of (NH 4 ) 2 U 2 O 7 (ammonium diuranate), then heating to form U 3 O 8 or UO 3 Contaminates depend upon mineral §V, Mo TBP extraction §Based on formation of nitrate species §UO 2 (NO 3 ) x 2-x + (2-x)NO 3 - + 2TBP  UO 2 (NO 3 ) 2 (TBP) 2

7 6-7 Uranium atomic properties Ground state electron configuration §[Rn]5f 3 6d 1 7s 2 Term symbol § 5 L 6

8 6-8 cm -1

9 6-9

10 6-10 Metallic Uranium Three different phase   phases àDominate at different temperatures Uranium is strongly electropositive §Cannot be prepared through H 2 reduction Metallic uranium preparation §UF 4 or UCl 4 with Ca or Mg §UO 2 with Ca §Electrodeposition from molten salt baths

11 6-11 Metallic Uranium phases  -phase §Room temperature to 942 K §Orthorhombic §U-U distance 2.80 Å §Unique structure type  -phase §Exists between 668 and 775 ºC §Tetragonal unit cell  -phase §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  -phase  ‐ phase U-U distances in layer (2.80±0.05) Å and between layers 3.26 Å

12 6-12 Resistivity–temperature curve for  -U along the [010] axis

13 6-13 Intermetallic compounds Wide range of intermetallic compounds and solid solutions in alpha and beta uranium §Hard and brittle transition metal compounds àU 6 X, X=Mn, Fe, Co, Ni §Noble metal compounds àRu, Rh, Pd *Of interests for reprocessing §Solid solutions with: àMo, Ti, Zr, Nb, and Pu

14 6-14 Uranium-Aluminum Phase Diagram. Uranium-Titanium Phase Diagram.

15 6-15 Chemical properties of uranium metal and alloys Reacts with most elements on periodic table §Corrosion by O 2, air, water vapor, CO, CO 2 Dissolves in HCl §Also forms hydrated UO 2 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

16 6-16 Uranium compounds Uranium-hydrogen   -UH 3 from H 2 at 250 ºC   -UH 3 prepared at - 80 ºC from H 2 at 250

17 6-17 Uranium hydride compounds Uranium borohydride UF 4 + 2Al(BH 4 ) 3  U(BH 4 ) 4 + 2Al(BH 4 )F 2 §U(BH) 4 is tetragonal àU(BH 4 ) 3 forms during U(BH 4 ) 4 synthesis §Vapor pressure àlog p (mmHg) =13.354-4265T - 1 UXAlH y compounds §UXAl absorbs hydrogen upon heating àX=Ni, Co, Mn ày = 2.5 to 2.74 §TGA analysis evaluates hydrogenation

18 6-18 Uranium-oxygen UO §Solid UO unstable, NaCl structure §From UO 2 heated with U metal àCarbon promotes reaction, formation of UC UO 2 §Reduction of UO 3 or U 3 O 8 with H 2 from 800 ºC to 1100 ºC àCO, C, CH 4, or C 2 H 5 OH can be used as reductants §O 2 presence responsible for UO 2+x formation §Large scale preparation àUO 4, (NH 4 ) 2 U 2 O 7, or (NH 4 ) 4 UO 2 (CO 3 ) 3 àCalcination in air at 400-500 ºC àH 2 at 650-800 ºC àUO 2 has high surface area

19 6-19 Uranium-oxygen U 4 O 9 §UO 2 and U 3 O 8 à5 UO 2 + U 3 O 8  2 U 4 O 9 àPlaced in evacuated ampoule àHeated to 1000 ºC for 2 weeks *Three phases   U 4 O 9 up to 350 K   U 4 O 9 350 K to 850 K   U 4 O 9 above 850 K ØRearrangement of U 4+ and U 5+ forces disordering of O U 3 O 7 §Prepared by oxidizing UO 2 below 160 ºC à30 % of the oxygens change locations to new positions during oxidation §Three phases à  phase prepared by heating at 200 ºC U 2 O 5 §High pressure synthesis, three phases §  -phase àUO 2 and U 3 O 8 at 30 kbar and 400 ºC for 8 hours àAlso prepared at 15 kbar and 500 ºC §  -phase forms at 40-50 kbar above 800 ºC §  -phase sometimes prepared above 800 ºC at 60 kbar

20 6-20 Uranium-oxygen U 3 O 8 §From oxidation of UO 2 in air at 800 ºC   phase uranium coordinated to oxygen in pentagonal bipyrimid §  phase results from the heating of the  phase above 1350 ºC àSlow cooling

21 6-21 Uranium-oxygen UO 3 §Seven phases can be prepared A phase (amorphous) àHeating in air at 400 ºC *UO 4. 2H 2 O, UO 2 C 2 O 4. 3H 2 O, or (HN 4 ) 4 UO 2 (CO 3 ) 3 ØPrefer to use compounds without N or C  -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 UO 2 2+  -phase §Ammonium diuranate or uranyl nitrate heated rapidly in air at 400-500 ºC  -phase prepared under O 2 6-10 atmosphere at 400-500 ºC

22 6-22 Uranium-oxygen UO 3 hydrates §6 different hydrated UO 3 compounds UO 3. 2H 2 O §Anhydrous UO 3 exposed to water from 25-70 ºC  Heating resulting compound in air to 100 ºC forms  -UO 3. 0.8 H 2 O   -UO 2 (OH) 2 [  - UO 3. H 2 O] forms in hydrothermal experiments   -UO 3. H 2 O also forms

23 6-23

24 6-24 Uranium-oxygen single crystals UO 2 from the melt of UO 2 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 UO 2 2+ in structure §Some layer structures UO 2 to UO 3 system §Range of liquid and solid phases from O/U 1.2 to 3.5 §Hypostoichiometric UO 2+x forms up to O/U 2.2 àMixed with U 3 O 8 at higher temperature §Large range of species from O/U 2.2 to 2.6

25 6-25 UO 2 Heat Capacity High temperature heat capacity studied for nuclear fuel §Room temperature to 1000 K àIncrease in heat capacity due to harmonic lattice vibrations *Small contribution to thermal excitation of U 4+ localized electrons in crystal field §1000-1500 K àThermal expansion induces anharmonic lattice vibration §1500-2670 K àLattice and electronic defects

26 6-26 Oxygen potential Equilibrium oxygen partial pressure over uranium oxides §In 2 phase region of solid oxides àΔG(O 2 )=RTln pO 2 *Partial pressure related to O 2 Large increase above O/U = 2 §Increase in ΔG(O 2 ) decreases with increasing ratio §Increase ΔG(O 2 ) with increasing T Entropy essentially independent of temperature §ΔS(O 2 )= -dΔG(O 2 )/dT Enthalpy related to Gibbs and entropy through normal relationship §Large peak at UO 2+x, x is very small

27 6-27

28 6-28 Vaporization of UO 2 Above and below the melting point Number of gaseous species observed §U, UO, UO 2, UO 3, O, and O 2 àUse of mass spectrometer to determine partial pressure for each species àFor hypostiochiometric UO 2, partial pressure of UO increases to levels comparable to UO 2 àO 2 increases dramatically at O/U above 2

29 6-29 Uranium-oxides: Oxygen diffusion Vacancy based diffusion in hypostoichiometric UO 2 §Based on diffusion into vacancy, vacancy concentration, migration enthalpy of vacancy àEnthalpy 52 kJ mol -1 For stiochiometric UO 2 diffusion temperature dependent §Thermal oxygen vacancies at lower T §Interstitial oxygen at higher T àEqual around 1400 ºC For UO 2+x diffusion dominated by interstitial oxygen §Migration enthalpy 96 kJ mol -1

30 6-30 Uranium-oxide: Electrical conductivity UO 2 and UO 2+x §Mobility of holes in lattice à0.0015 to 0.021 cm 2 V -1 s -1 *Semiconductor around 1 cm 2 V -1 s -1 §Holes move in oxide structure along with local distortion within lattice §Holes and electrons localized on individual atoms àHoles U 5+ and electrons form U 3+ §From 500 to 1400 ºC for UO 2+x àDecrease in conductivity with decrease in x when x<0.1 U 3 O 8-z §Similar to UO 2+x §Phase transition at 723 K results in change of temperature dependence

31 6-31 Uranium oxide chemical properties Oxides dissolve in strong mineral acids §Valence does not change in HCl, H 2 SO 4, and H 3 PO 4 §Sintered pellets dissolve slowly in HNO 3 àRate increases with addition of NH 4 F, H 2 O 2, or carbonates *H 2 O 2 reaction ØUO 2 + at surface oxidized to UO 2 2+

32 6-32 Group 1 and 2 uranates Wide series of compounds §M 2 U n O 3n+1 for M + §MU n O 3n+1 for M 2+ àOther compounds known *M 4 + UO 5, M 2 2+ UO 5, M 3 2+ UO 6, and M 2 2+ U 3 O 11 Crystal structures §Layered structures and UO 2 2+ in the crystals §Monouranates (n=1) àLayered planes, O atom coordinate to U on the plane *Some slight spacing around plane §Ba and Mg UO 4 àDeformed ochahedron *Secondary O bridges adjacent U atoms ØShared corners ØShared edges §M 4 UO 5 (M=Li, Na) àNo uranyl group à4 orthogonal planar U-O bonds Preparation §Carbonates, nitrates or chlorides of group 1 or 2 elements mixed with U 3 O 8 or UO 3 §Heat in air 500-1000 ºC àLower temperature for Cs and Rb §Different phases of some compounds

33 6-33 Group 1 and 2 uranates Physicochemical properties §Hydroscopic §Colored àYellow to orange §Heavier group 1 species volatile §IR active àAsymmetric stretch of UO 2 2+ à600-900 cm -1 *Frequency varies based on other O coordinated to uranyl group §Diamagnetic compounds àCan be examined by U NMR *Some weak paramagnetism observed ØCovalency in uranyl group Uranates (V) and (IV) §MUO 3 (M=Li, Na, K, Rb) §M 3 UO 4 (M=Li, Na) §MU 2 O 6 (M=Mg, Ca, Sr, Ba) §MUO 3 (M=Ca, Sr, Ba), tetravalent U Synthesis §Pentavalent uranates àTetravalent and hexavalent uranium species mixed in 1:1 ratio *Heated in evacuated sealed ampoule ØUO 2 + Li 2 UO 4  2 LiUO 3 àHydrogen reduction of hexavalent uranates àat elevated temperatures tetravalent uranates form

34 6-34 Group 1 and 2 uranates Crystal structure §No uranyl present, lacks layered structure àPerovskite 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 à5f 1 from U 5+ àO h crystal field *Some tetragonal distortions Non-stoichiometry §Removal of oxide àFormation of xNa 2 O from Na 2 U 2 O 7 forms Na 2-2x+ U 2 O 7-x §Non-stoichiometric dissolution of metal in UO 2 àNa x UO 3 (x≤0.14) §Oxygen non-stoichiometry àNa 2 U 2 O 7-x (x≤0.5)

35 6-35 Transition metal uranates Wide range of compounds Preparation method §heating oxides in air with UO 3 or U 3 O 8 àChanging stoichiometry can result in different compounds *U/M = 3, MU 3 O 10 (M=Mn, Co, Ni, Cu, Zn) §Uranyl nitrate as starting material àMetal nitrates, temperatures below 600 ºC àM x UO 4 Crystal structures §Chain of edge sharing of oxygen §Some influence of metal on uranyl oxygen bond length §Lanthanide oxides form solid solutions àCan form Ln 6 UO 12

36 6-36 Solid solutions with UO 2 Solid solutions formed with group 2 elements, lanthanides, actinides, and some transition elements (Mn, Zr, Nb, Cd) §Distribution of metals on UO 2 fluorite-type cubic crystals based on stoichiometry Prepared by heating oxide mixture under reducing conditions from 1000 ºC to 2000 ºC §Powders mixed by co- precipitation or mechanical mixing of powders Written as M y U 1-y O 2+x §x is positive and negative

37 6-37 Solid solutions with UO 2 Lattice parameter change in solid solution §Changes nearly linearly with increase in y and x àM y U 1-y O 2+x àEvaluate by change of lattice parameter with change in y *δa/δy Øa is lattice parameter in Å ØCan 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 àVaried if x is positive or negative

38 6-38 Solid solutions of UO 2 Tetravalent M y U 1-y O 2+x §Zr solid solutions àLarge range of systems ày=0.35 highest value àMetastable at lower temperature §Th solid solution àContinuous solid solutions for 0≤y≤1 and x=0 àFor x>0, upper limit on solubility *y=0.45 at 1100 ºC to y=0.36 at 1500 ºC àAlso has variation with O 2 partial pressure *At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at 1500 ºC

39 6-39 Solid solutions of UO 2 Tri and tetravalent M y U 1-y O 2+x §Cerium solid solutions àContinuous for y=0 to y=1 àFor x<0, solid solution restricted to y≤0.35 *Two phases (Ce,U)O 2 and (Ce,U)O 2-x àx<-0.04, y=0.1 to x<-0.24, y=0.7 à0≤x≤0.18, solid solution y<0.5 àAir oxidized hyperstoichiometric *y 0.56 to 1 at 1100 ºC *y 0.26-1.0 1550 ºC Tri and divalent §Reducing atmosphere àx is negative àfcc àSolid solution form when y is above 0 àMaximum values vary with metal ion §Oxidizing atmosphere àSolid solution can prevent formation of U 3 O 8 àSome systematics in trends *For Nd, when y is between 0.3 and 0.5, x = 0.5-y

40 6-40

41 6-41

42 6-42 Solid solution UO 2 Oxygen potential §Zr solid solution àLower than the UO 2+x system *x=0.05, y=0.3 Ø-270 kJ/mol for solid solution Ø-210 kJ/mol for UO 2+x §Th solid solution  Increase in  G with increasing y àCompared to UO 2 difference is small at y less than 0.1 §Ce solid solution àWide changes over y range due to different oxidation states àShape of the curve is similar to Pu system, but values differ *Higher  G for CeO 2-x compared to PuO 2-x

43 6-43 Solid solution UO 2 Trivalent §Oxygen potential increases with increasing x àInflection point at x=0  For lanthanides La has highest  G due to larger ionic radius Divalent §Higher oxygen potential than trivalent system §Configuration change àFormation of pentavalent U §At low O 2 partial pressures cannot dissolve high levels of Mg

44 6-44 Borides, carbides, silicides UB 2, UB 4, UB 12 are known compounds Prepared by mixing elements at high temperature Other reactions §UCl 4 +2MgB 2  UB 4 + 2MgCl 2 UB and UB 4 form in gas phase Inert species §Potential waste forms §UB 12 more inert Large amount of ternary systems §U 5 Mo 10 B 24, UNi 4 B àSheets with 6 and 8 member rings A view down the c ‐ axis of the structure of UB 4

45 6-45 Uranium carbides Three known phases §UC, UC 2, and U 2 C 3 UC and UC 2 are completely miscible at higher temperature §At lower temperatures limited §Synthesized by mixture of elements at high temperature U 2 C 3 prepared by heating UC and UC 2 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 àMelting elements in carbon crucible *U 2 Al 3 C 4 UC 2 reacts slowly in air §With N 2 at 1100 °C to form UN

46 6-46 Uranium-silicon Compounds §U 3 Si, U 3 Si 2, USi, U 3 Si 5, USi 1.88, and USi 3 Complicated phase diagram §Number of low temperature points Forms ternary compounds with Al §U(Al, Si) 3 àFormed in U in contact with Al §Cu, Nb, and Ru ternary phases àU 2 Nb 3 Si 4 ferromagnetic below 35 K §URu 2 Si 2 àHeavy fermion material *metallic materials having large electronic mass enhancement Øantiferromagnetic interaction between conduction electrons and local magnetic moments (d- or f- electron)

47 6-47 N, P, As, Sb, and Bi uranium Monopnictides §UN, UP, UAs àCubic NaCl structure U-nitrides §UN, U 2 N 3, UN 2 §UN prepared by uranium metal with nitriding agents àN 2, NH 3 àThermal decomposition of higher nitrides *Higher nitride unstable with respect to UN àMixture of higher nitrides with uranium metal *Treat surface with HNO 3 and washed with organics ØRemove traces of oxides and carbides §UN easily oxidized by air, unstable in water

48 6-48

49 6-49

50 6-50 P, As, Sb, Bi-uranium UX, U 3 X 4, and UX 2 §X=P, As, Sb, Bi  UX is cubic except  -UBi àU 3 X 4 is body centered cubic àUX 2 is tetragonal Preparation §Synthesis from the elements in an autoclave à2U + P 4  2UP 2 §Uranium hydride with phosphine or arsine àUH 3 +PH 3  UP+3H 2

51 6-51 S, Se, Te-uranium Uranium-sulfur §US, US 2, U 2 S 3, U 3 S 5 àPreparation *Heating U metal or UH 3 with H 2 S *Heating elements in sealed tube *Decomposition of higher sulfides in heat under vacuum *UCl 4 with Li 2 X (X=S, Se, Te) §U 3 S 5 mixed U valence structure àU 3+ and U 4+ Se and Te prepared as the sulfur complexes §UTe 2 contains Te-Te bonds and mixed valence states àU 3+ and Te 1-,2-

52 6-52

53 6-53 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 àException UF 3 move covalent than UCl 3

54 6-54 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

55 6-55 Trivalent uranium halides Electronic properties §5f 3 § 4 I 9/2 ground state configuration §Crystal field analysis of low temperature compounds àLarge range of compounds evaluated for free ion and crystal field parameters Absorption spectra for U 3+ halides examined §Strong f-d bands àMixing of electrons from different quantum levels *Laporte rule àFirst f-d transition at 23000 cm -1 for CsUCl 4. 3H 2 O *5f 3  5f 2 6d 1 *Shifted toward IR region for NH 4 UCl 4. 4H 2 O by 5000 cm -1 Ø27000 cm -1 =370 nm, 15000 cm -1 =666.7 nm *For substitution of U 3+ substitution with halides ØIncrease in covalence properties related to red shift in f- d band

56 6-56 Trivalent uranium halides Preparation of UF 3 §Reduction of UF 4 by Al metal àWith Al, place in graphite crucible and heat to 900 ºC §With UN or U 2 N 3 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 §U 3+ in 1 M HCl and precipitation with NH 4 F

57 6-57 Trivalent uranium halides UCl 3 §Reaction of gaseous HCl with UH 3 at 350 ºC §Reduction of UCl 4 with Zn or Al at 400 ºC §Thermal vacuum decomposition of NH 4 UCl 4 §Disproportionates to U and UCl 4 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 [U 2 Cl 2 (H 2 O) 14 ] 4+ units and Cl - ions àUraniums connected over bridging Cl §A number of hydrated complexes prepared àMUCl 4 *From U 3+ in 11 M HCl with MCl *Tri- and tetrahydrates show 5f 3  5f 2 6d 1 at 21500 cm -1 and 16000 cm -1 *Red shift indicates covalent character of water interaction ØBond lengths based on inner sphere complexes

58 6-58 Trivalent uranium halides UCl 3 with neutral ligands §Ammonia adducts, UCl 3. 7NH 3 àFrom UCl 3 heated in ammonia under pressure §UCl 3 (THF) x §Wide range of crown ether complexes àPrepared from ligand and UCl 4 reduced with Zn àIntense f-d transitions in visible and UV region *IR needed to identify ligand coordination àCompounds hydroscopic and oxidized in air UBr 3 species §Prepared by reaction of UH 3 with HBr at 300 ºC §Reduction of UBr 4 by Zn at 600 ºC àUBr 3 reacts with quartz at room temperature, need to prepare in sealed Ta or Mo vessel §Hydroscopic and oxidizes more readily than UCl 3 §Isostructural with UCl 3 §Hydrate species formed by reaction of UBr 3 with oxygen free water vapor §M 2 UBr 5 and M 3 UBr 6 àMelting points are high and increase with M mass

59 6-59 Trivalent uranium halides UI 3 §Prepared from I 2 on U metal at 525 ºC §UI 4 with Zn §Vacuum decomposition of UI 4 §UH 3 with methyl iodide Hydroscopic and attacks glass Dissolves in aqueous solution, methanol, ethanol, acetic acid §Forms unstable U 3+ 5f 3  5f 2 6d 1 at 13400 cm -1 §Shift from 23000 cm -1 for UF 3 Synthesis of neutral donor complexes with solvent, U metal and I 2 at 0 ºC Mixed oxide species prepared §UOX (X=Cl, Br, I) àHeating stoichiometric mixtures of UO 2 X 2, UO 2, and U or UX 4, U 3 O 8 and U at 700 ºC for 24 hours

60 6-60 Tetravalent uranium halides UF 4 stable upon exposure to air §Lattice energy responsible for enhanced stability over other tetravalent halides All expect UF 4 soluble in polar solvents §U 4+ can be stabilized in solution Different structures for solids §UF 4 : square antiprism §UCl 4 : dodecahedron §UBr 4 : pentagonal bipyramid Ground State electronic configuration 5f 2 ( 3 H 4 ) §Compounds have 5f 2  5f 2 transitions §f-d transitions begin 40000 -50000 cm -1 (UV-region) àHigher energies than U 3+ Absorption data collected at low temperature for transition assignment Evidence of 5f 1 7p 1 for Cs 2 UBr 6 Over 60 5f 2  5f 2 transitions identified §U 4+ doped in BaY 2 Cl 7 àAbsorption, excitation, luminescence spectra àCrystal field strength for U 4+ dominated by symmetry of central ion rather than ligand *Lower symmetry results in lower crystal field U 4+ has strong anti-stokes emission

61 6-61 Tetravalent uranium halides Complexes with inversion symmetry (UCl 6 2- ) used to determine electronic transitions §Low temperature §Evaluation of side bands Low temperature UF 4 absorbance identified 91 f  f transitions

62 6-62 Tetravalent uranium halides UF 4 exploited in nuclear fuel production §Conversion to UF 6 àBased on chemical stability and insolubility in solution Formed by a number of reactions §Uranium oxides with HF (UO 2, U 3 O 8 ) àU 3 O 8 + 8 HF  2UO 2 F 2 + UF 4 + 4 H 2 O if no H 2 in system àUO 3 with ammonia-hydrogen fluoride mixtures *UO 2 and heating with same compounds §Can also be prepared by the reduction of UF 6 Dissolves in the presence of reagents that can form fluoride complexes §Fe 3+, Al 3+, boric acid Fitting of UF 4 spectra resulted in assignment of 69 crystal field levels Hydrates formed from aqueous fluoride solution §nH 2 O (0.5 { "@context": "", "@type": "ImageObject", "contentUrl": "", "name": "6-62 Tetravalent uranium halides UF 4 exploited in nuclear fuel production §Conversion to UF 6 àBased on chemical stability and insolubility in solution Formed by a number of reactions §Uranium oxides with HF (UO 2, U 3 O 8 ) àU 3 O 8 + 8 HF  2UO 2 F 2 + UF 4 + 4 H 2 O if no H 2 in system àUO 3 with ammonia-hydrogen fluoride mixtures *UO 2 and heating with same compounds §Can also be prepared by the reduction of UF 6 Dissolves in the presence of reagents that can form fluoride complexes §Fe 3+, Al 3+, boric acid Fitting of UF 4 spectra resulted in assignment of 69 crystal field levels Hydrates formed from aqueous fluoride solution §nH 2 O (0.5

63 6-63 Tetravalent uranium halides Complex uranium fluorides §Metal fluoride uranium fused salts àFuels and reactors §LiF-BeF 2 -UF 4 and NaF-BeF 2 -UF 4 §MgUF 6 and CaUF 6 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-O 2 §Reduction of UF 6 with metal fluorides §Controlled decomposition of higher fluoro complexes à(NH 4 ) 4 UF 8 Structures of compounds known §UF 6 2- : octahedral §UF 7 3- : pentagonal bipyramid §UF 8 4- : bicapped triangular prism àSome complexes differ *Chains tricapped trigonal prisms for  -K 2 UF 2

64 6-64 Tetravalent uranium halides Uranium oxide- and nitride fluorides §Melting UO 2 (or other oxides) and UF 4 àMono- and dihydrate precipitates àMixed oxidation states of U found *5+ and 6+ *4+ and 5+ §UN 1.33 and UF 4 àCompounds between UNF and UN 0.9 F 1.2 )

65 6-65 Tetravalent uranium halides Uranium tetrachloride §Starting material for a range of uranium compounds àEase of preparation àSolubility in polar organic solvents §Synthesis àChlorination of UO 2 àNeed reactive form of UO 2 àConverts to U 3 O 8 in air at 600 ºC §Isostructural with other actinide tetrachlorides àTetragonal symmetry Range of complex chlorides §M 2 UCl 6 and MUCl 5 àMonovalents include NR 4, PR 3 H compounds §Can be prepared from fused salts of UCl 4 with metal chlorides Chlorine atoms can be replaced §UCl 4 in non-aqueous media with decomposition reaction Species are paramagnetic §Temperature dependent up to 350 ºC Oxychloride species §From UO 2 in excess UCl 4 followed by sublimation §Dissolves in water and aqueous nitric acid §Isostructural with Th, Pa, and Np oxychloride

66 6-66 Tetravalent Uranium halides Uranium tetrabromide §Prepared from: àOxides with bromine àOxides or UOBr 3 with CBr 4 àUO 2 and sulfur bromine mixture §insoluble in non-polar organic solvents §Soluble in polar solvents àHBr evolved in ethanol, methanol, phenol, acetic acid, or moist air §Absorption bands 5f 2  5f 1 6d 1 at 41400-32160 cm -1 §Charge transfer at 30165 cm -1 §Forms compounds with numerous ligands §Pentagonal bipyramid around U §M 2 UBr 6 with group 1 elements àCan coordinate with organic cations *Soluble in water, aqueous HBr, polar non-aqueous solvents àfcc crystals àO h from solution spectroscopy *5f 2  5f 1 6d 1 27400 to 39000 cm -1 *Vibronic side bands *Hydrogen bonding can distort O h to permit f  f Oxybromides similar to oxychlorides

67 6-67 Tetravalent uranium halides UI 4 §Prepared by direct combination of the elements at 500 ºC §Used in preparation of UI 3 §M 2 UI 6 from components in anhydrous methyl cyanide àHydroscopic compounds àUsed to obtained spectroscopic terms for electronic transitions UOI 2 from heating U 3 O 8, U, and I 2 sealed at 450 ºC UNI from UI 4 with ammonia Mixed halides §Range of compounds §Higher fluoride species are more stable àUClF 3 >UCl 2 F 2 §Mixed Cl-Br and Cl-I, Br-I

68 6-68 Pentavalent uranium halides Strong tendency to hydrolyze and disproportionate to tetra- and hexavalent species Preparation §UO 3 with thionyl chloride under reflux Decomposes in CCl 4, CH 2 Cl 2 Varied coordination geometry  Octahedral (  -UF 5 )  Pentagonal bipyramid (  -UF 5 ) §Edge-sharing octahedral (U 2 Cl 10 ) 5f 1 electronic configuration: 4 F 5/2 ground state UF 5 §Two phases, alpha over 150 ºC §Oxidation of UF 4 or reduction of UF 6 àOxidation with HF, noble gas fluorides àReduction with HN 3, SOCl 2 §Water causes disproportionation §2UF 5 +3H 2 O  UF 4 +UO 2 F 2 +4HF §Reduced to UF 4 by H 2 or Ni §Stable in 50 % HF solution

69 6-69 Pentavalent uranium halides Structure   -UF 5 chains of UF 6 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 §UF 4 in intermittent O 2 flow at 850 ºC creates U 2 OF 8 §Complex compounds also form UCl 5 §Unstable through thermal decomposition §Prepared by oxide treatment with CCl 4 at 80-250 ºC and UCl 5 catalyst or UO 3 with SiCl 4 §  -Cl 5 (monoclinic)from recrystallization from CCl 4 §  -Cl 5 (triclinic) by recrystallization of UCl 6 in CCl 4 or CH 2 Cl 2 §Absorbance spectra same for both phases àSimilar to UCl 6 -

70 6-70 Pentavalent uranium halides Complex compounds §Range of compounds with ligands containing N, P, As, S, Se, and Te donor §Variety of MUCl 6 àGroup 1 and organic cations oxide species and complex àUOCl 3 from MoCl 5 at 200 ºC àUCl 4 and UO 2 Cl 2 at 370ºC àUO 2 Cl 2 with WCl 5, ReCl 5 at 200 ºC àDissolves in anhydrous ethanol Pentabromide §Bromination of metal or UBr 4 at 55 ºC §UOBr 3 from UO 3 with CBr 4 àUO 2 Br can also be prepared from thermal decomposition of UO 2 Br 2 Intermediate uranium halides §UF 4 with UF 6 UF 5 fluctuates between C 4v and D 3h §Participation of 5f orbitals in bonding §5f, 6p, and 6d àLow population of 7s and 7p

71 6-71 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 §UF 6 colorless, UCl 6 green 5f 0 : 1 S 0 ground state Spectra of UO 2 2+ has vibrational fine structure §Coupling with O=U=O stretching modes UF 6 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 t 1u (  +  ) to empty 5f orbitals Compounds show weak, temperature dependent paramagentism

72 6-72 Hexavalent uranium halides UF 6 §Readily volatile uranium compound àIsotope enrichment

73 6-73 UF 6 Orthorhombic colorless crystals Sublime at 56.5 ºC Liquid and gas O h symmetry Temperature independent paramagnetism Reactive and moisture sensitive Oxidizing agent §nUF 6 +M  nUF 5 +MF n 1 st bond dissociation at 134 kJ/mol §Similar to F 2 (153.2 kJ/mol) Formation of M x UF (6+x) x=1,2 from UF 6 and MF §Based on UF 6 electron affinity and lattice energy Reduction from a number of reagents or alpha decay Some eutectic phase with BrF 2, BrF 3, BrF 5

74 6-74 UF 6 species Tend to decompose to UF 6 when heated Oxide species §In liquid HF à3UF 6 +SiO 2  3UOF 4 +SiF 4 à3UF 6 +B 2 O 3  3UOF 4 +2BF 3 §Orange solid, non-volatile, decomposes at 200-250 ºC §UOF 4 at 250 ºC in vacuum decomposes to UF 6 and UO 2 F 2 §UO 2 F 2 also formed from UO 3 in gaseous HF at 300 ºC àUO 2 F 2 yellow compound, slightly soluble in H 2 O, methanol and ethanol àHydrated species from recrystallization in water

75 6-75 Hexavalent uranium halides UCl 6 §From thermal decomposition of UCl 5 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 UO 2 Cl 2 §From the oxidation of UCl 4 §Insoluble in non-polar solvents §A large number of different oxychloride compounds produced Oxybromide compounds §From the reaction of O 2 with UBr 4 §UO 2 Br 2 loses Br even at room temperature §Hydrates and hydroxide species form Iodine compounds §Extremely unstable UO 2 I 2 reported §Number of moieties with organic Mixed halogen species §M 2 UO 2 Cl 2 Br 2 §X 3 I (X=Cl or Br)

76 6-76 Chemical bonding Tri- and tetravalent U mainly related to organometallic compounds §Cp 3 UCO and Cp 3 UCO + àCp=cyclopentadiene *5f CO  backbonding  Metal electrons to  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 àVO 2 +, MoO 2 2+, WO 2 2+ *Oxygen atoms are cis to maximize (p  )  M(d  ) àLinear MO 2 2+ 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 àSmaller effective charge on pentavalent U §Multiple bond characteristics, 1  and 2 with  characteristics

77 6-77 Uranyl chemical bonding Bonding molecular orbitals   g 2  u 2  g 4  u 4 àOrder of HOMO is unclear *  g <  u <  g <<  u proposed  Gap for  based on 6p orbitals interactions  5f  and 5f  LUMO §Bonding orbitals O 2p characteristics §Non bonding, antibonding 5f and 6d §Isoelectronic with UN 2 Pentavalent has electron in non-bonding orbital

78 6-78

79 6-79 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 UO 2 (H 2 O) 5 2+, 3.2 for fluoride systems àNet negative 0.43 on oxygens àLewis 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

80 6-80 Structure and coordination chemistry 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 àDistorted O h àDistorted pentagonal bipyramid àHexagonal bipyramid *MUO 2 (NO 3 ) 3, K 4 UO 2 (CO 3 ) 3 àSquare bipyrimid *Can occur in complexes with strong steric interference

81 6-81 U(VI) structure and coordination UO 2 CO 3(s) §3 oxygens for each uranium àWill not be composed of a discrete complex àOxygens 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

82 6-82

83 6-83 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 àNo large differences in bond distances or coordination geometries àAny differences based on variation in ionic radius, larger for actinides U(III) complexes have high coordination numbers §8 or 9 àDistorted trigonal prism §No structural determination of simple inorganic ligands in solution

84 6-84 U(IV) and (V) structure and coordination U(IV) §Normal and basic salts with inorganic ligands àBasic salts due to hydrolysis or oxide formation §Large ionic radius and 8 to 10 coordination àSimilar to Ce(IV) §Carbonates form trigonal bipyramid U(V) §Few examples of structures §Hexagonal bipyramid for triscarbonate àSimilar to U(VI) species §Labile ligands in equatorial plane §Weaker complexes compared to U(VI)

85 6-85 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

86 6-86 Uranium solution chemistry Uranyl(VI) most stable in solution §Uranyl(V) and U(IV) can also be in solution àU(V) prone to disproportionation §Stability based on pH and ligands §Redox rate is limited by change in species àMaking or breaking yl oxygens *UO 2 2+ +4H + +2e -  U 4+ +2H 2 O yl oxygens have slow exchange §Half life 5E4 hr in 1 M HClO 4 §Rate of exchange catalyzed by UV light

87 6-87 Uranium solution chemistry Trivalent uranium §Dissolution of UCl 3 in water §Reduction of U(IV) or (VI) at Hg cathode àEvaluated by color change *U(III) is green §Very few studies of U(III) in solution §No structural information àComparisons with trivalent actinides and lanthanides

88 6-88 Uranium solution chemistry Tetravalent uranium §Forms in very strong acid àRequires >0.5 M acid to prevent hydrolysis àElectrolysis of U(VI) solutions *Complexation can drive oxidation §Coordination studied by XAFS àCoordination number 9±1 *Not well defined àU-O distance 2.42 Å §O exchange examined by NMR Pentavalent uranium §Extremely narrow range of existence §Prepared by reduction of UO 2 2+ with Zn or H 2 or dissolution of UCl 5 in water §UV-irradiation of 0.5 M 2-propanol-0.2 M LiClO 4 with U(VI) between pH 1.7 and 2.7 àU(V) is not stable but slowly oxidizes under suitable conditions §No experimental information on structure §Quantum mechanical predictions

89 6-89 Hexavalent uranium solution chemistry Large number of compounds prepared §Crystallization §Hydrothermal Structure examined by XAFS

90 6-90 Aqueous solution complexes Strong Lewis acid Hard electron acceptor §F - >>Cl - >Br -  I - §Same trend for O and N group à 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 §HOCH 2 COO - pKa=17, 3.6 upon complexation of UO 2 àInductive effect *Electron redistribution of coordinated ligand *Exploited in synthetic chemistry U(III) and U(V) §No data in solution àBase information on lanthanide or pentavalent actinides

91 6-91 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 ài.e., U(OH) 5 (H 2 O) n-1 - àU 4 (OH) 16 *6 coordination

92 6-92 Nanomole/L UO 2 2+ Micromole/L UO 2 2+ Millimole/L UO 2 2+ pH 6 U(VI) variation

93 6-93 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 HSiO 4 3- and SiO 4 4- difficult to study due to competition from OH - Complex structure from central U and ligand geometry §XAFS and neutron data

94 6-94 Uranium solution chemistry Organic ligands and functional groups §Carboxylic acids àAdditional 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 àU x L y (OH) z §Evaluate based on L and OH - complexation with U and steric constraints àMost ternary complexes contain OH - and F -

95 6-95 Ligand substitution reactions Most data with U focuses on rate of reaction §Mechanism of reaction are speculative àDescribes molecular details of a reaction Data available §Non-aqueous solvents §Redox §Multidentate ligands Enthalpy and entropy terms evaluated Methods §Stop-flow §NMR àProtons, 13 C, 17 O, 19 F *i.e., water change followed by 17 O Water reactions §Fast outer sphere going to rate determining inner sphere (k 2 ) §Overall rate can determined from k 2 and equilibrium constant àK obs §Associative, Dissociative, Interchange §Water exchange smaller with complexes àUO 2 (oxalate)F(H 2 O) 2 - *2E3 s -1 compared to 1.3E6 s -1

96 6-96 Experimental ΔH=26 kJ mol -1 Calculated §74 (D), 19 (A), 21 (I) §Base on similarity between experimental and calculated

97 6-97

98 6-98

99 6-99 Ligand substitution reactions NMR data for coordination §3 different fluoride ligands

100 6-100 Uranium chemistry in solution U isotopic exchange §Exchange between oxidation states and phases àIsotopic purity for a given species àSeparation and evaluation *Counting or mass spectroscopy U fluorescence §Excitation of uranyl àDifferent spectra and lifetime §Quantum yield impacted by solution chemistry àQuenching from heavy ions in solution àLow oxidation state due to electron transfer §Excited U state used in chemical reactions §No consensus on primary de- excitation mechanism  I/I o =  o  o is state without ligand, I is intensity and  is lifetime  Charge transfer characteristic due to excitations from  g and  u to empty f orbital

101 6-101 Organometallic and biochemistry Uranocene Biochemistry §RNA and DNA interactions over phosphates àPhotochemical oxidation §polysaccharides over deprotonated OH Analytical chemistry §Separation and preconcentration §Titration §Electrochemical methods §Nuclear techniques §Spectrometric àAtomic absorption, AES, XRF àIndicator dye àFluorescence àMass spectrometry

102 6-102 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

103 6-103 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 UO 2 2+

104 6-104 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.

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