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1-1 Lecture 2: General Overview Presentation from typical actinide lecture from inorganic chemistry §Chapter 24, Advanced inorganic chemistry §http://www.chem.ox.a.

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Presentation on theme: "1-1 Lecture 2: General Overview Presentation from typical actinide lecture from inorganic chemistry §Chapter 24, Advanced inorganic chemistry §http://www.chem.ox.a."— Presentation transcript:

1 1-1 Lecture 2: General Overview Presentation from typical actinide lecture from inorganic chemistry §Chapter 24, Advanced inorganic chemistry §http://www.chem.ox.a c.uk/icl/heyes/LanthA ct/lanthact.htmlhttp://www.chem.ox.a c.uk/icl/heyes/LanthA ct/lanthact.html Occurrence §Ac, Th, Pa, U natural àAc and Pa daughters of Th and U §Traces of 244 Pu in Ce ores Properties based on filling 5f orbitals

2 1-2 Electronic structure Electronic Configurations of Actinides are not always easy to confirm §atomic spectra of heavy elements are very difficult to interpret in terms of configuration Competition between 5f n 7s 2 and 5f n-1 6d7s 2 configurations §for early actinides promotion 5f 6d occurs to provide more bonding electrons much easier than corresponding 4f 5d promotion in lanthanides §second half of actinide series resemble lanthanides more closely àSimilarities for trivalent lanthanides and actinides 5f orbitals have greater extension with respect to 7s and 7p than do 4f relative to 6s and 6p orbitals §The 5 f electrons can become involved in bonding àESR evidence for bonding contribution in UF 3, but not in NdF 3 *Actinide f covalent bond contribution to ionic bond *Lanthanide 4f occupy inner orbits that are not accessable Basis for chemical differences between lanthanides and actinides

3 1-3 Electronic Structure 5f / 6d / 7s / 7p orbitals are of comparable energies over a range of atomic numbers §especially U - Am àBonding can include any orbitals since energetically similar àExplains tendency towards variable valency greater tendency towards (covalent) complex formation than for lanthanides §Lanthanide complexes tend to be primarily ionic Actinide complexes complexation with -bonding ligands Hybrid bonds involving f electrons Since 5f / 6d / 7s / 7p orbital energies are similar orbital shifts may be on the order of chemical binding energies §Electronic structure of element in given oxidation state may vary with ligand §Difficult to state which orbitals are involved in bonding

4 1-4 Ionic Radii and trends Trends based on ionic radii Actinide contraction

5 1-5 Absorption Spectra and Magnetic Properties Electronic Spectra §5f n transitions ànarrow bands (compared to transition metal spectra) àrelatively uninfluenced by ligand field effects àintensities are ca. 10x those of lanthanide bands àcomplex to interpret Magnetic Properties §hard to interpret §spin-orbit coupling is large àRussell-Saunders (L.S) Coupling scheme doesn't work, lower values than those calculated *LS (http://hyperphysics.phy- astr.gsu.edu/hbase/atomic/lcoup. html)http://hyperphysics.phy- astr.gsu.edu/hbase/atomic/lcoup. html *Weak spin orbit coupling ØSum spin and orbital angular momentum ØJ=S+L §ligand field effects are expected where 5f orbitals are involved in bonding http://www.sciencedirect.com/science/article/pii/S002016 9300924873#

6 1-6 Example: Pu absorbance spectrum Ability to distinguish between Pu oxidation states §Variation in molar absorptivity Determine speciation of Pu by spectroscopy f electrons and hybrid orbitals Various orbital combinations similar to sp or d orbital mixing §Linear: sf §Tetrahedral: sf 3 §Square: sf 2 d §Octahedral: d 2 sf 3 àA number of orbital sets could be energetically accessible General geometries §Trivalent: octahedral §Tetravalent: 8 coordination Pentavalent and hexavalent actinides have double bonded oxygens §O=U=O 2+

7 1-7 Redox chemistry actinides are electropositive §From 2+ to 7+ Pa - Pu show significant redox chemistry §all 4 oxidation states of Pu can co-exist in appropriate conditions stability of high oxidation states peaks at U (Np) redox potentials show strong dependence on pH (data for Ac - Cm) §high oxidation states are more stable in basic conditions §even at low pH hydrolysis occurs §tendency to disproportionation is particularly dependent on pH § at high pH 3Pu 4+ + 2H 2 O PuO 2 2+ + 2Pu 3+ + 4H + early actinides have a tendency to form complexes §complex formation influences reduction potentials àAm 4+ (aq) exists when complexed by fluoride (15 M NH 4 F(aq)) radiation-induced solvent decomposition produces H and OH radicals §lead to reduction of higher oxidation states e.g. PuV/VI, AmIV/VI

8 1-8 Redox chemistry (Frost diagrams)

9 1-9 Stereochemistry C.N.GeometryO.N.e.g. 4distorted+4U(NPh 2 ) 4 5distorted tbp+4U 2 (NEt 2 ) 8 6octahedral+3An(H 2 O) 6 3+, An(acac) 3 +4UCl 6 2- +5 UF 6 -, -UF 5 +6AnF 6 +7Li 5 [AnO 6 ] (An = Np, Pu) distorted octahedral+6Li 4 UO 5, UO 3 +5/+6U5O8U5O8 +6UO 2 (S 2 CNEt 2 ) 2 (ONMe 3 ) 8cubic+4(Et 4 N) 4 [U(NCS) 8 ], ThO 2, UO 2 +5AnF 8 3- square antiprismatic+4ThI 4, U(acac) 4, Cs 4 [U(NCS) 8 ], +5 -UF 5

10 1-10 Stereochemistry 8dodecahedral+4Th(ox) 4 4-, Th(S 2 CNEt 2 ) 4 bicapped trigonal prismatic+3PuBr 3 hexagonal bipyramidal+6 UO 2 ( 2 -NO 3 ) 2 (H 2 O) 2 ?+6UF 8 2- 9tricapped trigonal prismatic+3UCl 3 capped square antiprismatic+4Th(trop) 4 (H 2 O) dodecahedral+4Th(ox) 4 4-, Th(S 2 CNEt 2 ) 4 bicapped trigonal prismatic+3PuBr 3 hexagonal bipyramidal+6 UO 2 ( 2 -NO 3 ) 2 (H 2 O) 2 ?+6UF 8 2- 10 bicapped square antiprismatic +4KTh(ox) 4.4H 2 O 11? fully capped trigonal prismatic? +3UF 3 12irregular icosahedral+4Th(NO 3 ) 6 2- distorted cuboctahedral+4 An( 3 -BH 4 ) 4, (Np, Pu) 14?complex+4 An( 3 -BH 4 ) 4, (Th, Pa, U)

11 1-11 Actinide metals Preparation of actinide metals §Reduction of AnF 3 or AnF 4 with vapors of Li, Mg, Ca or Ba at 1100 – 1400 °C §Other redox methods are possible àThermal decomposition of iodine species àAm from Am 2 O 3 with La *Am volatility provides method of separation Metals tend to be very dense §U 19.07 g/mL §Np 20.45 g/mL §Am lighter at 13.7 g/mL Some metals glow due to activity §Ac, Cm, Cf

12 1-12 Pu metal Some controversy surrounding behavior of metal http://www.fas.org/s gp/othergov/doe/lan l/pubs/00818030.pdf Plutonium Symmetrymonoclinic orthorhombicfccbc tetragonalbcc Stability< 122°C122-207°C207-315°C315-457°C457-479°C479-640°C / gcm -3 19.8617.7017.1415.9216.0016.51

13 1-13

14 1-14 Organometallic Organometallic chemistry of actinides is relatively recent §Interest is expanding but still focused on U Similar to lanthanides in range of cyclopentadienides / cyclooctatetraenides / alkyls Cyclopentadienides are -bonded to actinides

15 1-15 Uranocene Paramagnetic Pyrophoric Stable to hydrolysis Planar 'sandwich' Eclipsed D 8h conformation UV-PES studies show that bonding in uranocene has 5f & 6d contributions e 2 u symmetry interaction shown can only occur via f-orbitals

16 1-16 Overview Radius trends for ions and metals of the actinides General trends in actinide electronic structure Electronic and magnetic spectroscopy §Variations in the actinides Actinide stereochemistry Range of oxidation states for the actinides Role of organometallic chemistry for understanding f-electrons

17 1-17 Questions What is the trend in for the ionic radii of actinides? Which electrons are more likely to be involved in bonding, 4f or 5f? Why? What is the spectroscopic nature of 5f electrons and how is this observed? What are examples of f electron hybridization? What is the relationship between molecular geometry and coordination number? Describe a method for the preparation of actinide metals? How many phases of Pu metal exist under normal pressure? What drives the change in phases?

18 1-18 Pop Quiz List 3 pentavalent actinides.


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