1-1 Lecture 2: General Overview Presentation from typical actinide lecture from inorganic chemistry §Chapter 24, Advanced inorganic chemistry §.

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Lecture 2: General Overview• Presentation from typical

actinide lecture from inorganic chemistry§ Chapter 24, Advanced

inorganic chemistry§ http

://www.chem.ox.ac.uk/icl/heyes/LanthAct/lanthact.html

• Occurrence§ Ac, Th, Pa, U natural

à Ac and Pa daughters of Th and U

§ Traces of 244Pu in Ce ores

• Properties based on filling 5f orbitals

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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 5fn7s2 and 5fn-16d7s2 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 à Similarities 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

à ESR evidence for bonding contribution in UF3, but not in NdF3

* 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

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Electronic Structure

• 5f / 6d / 7s / 7p orbitals are of comparable energies over a range of atomic numbers § especially U - Am

à Bonding can include any orbitals since energetically similar

à Explains tendency towards variable valency• greater tendency towards (covalent) complex formation than for

lanthanides§ Lanthanide complexes tend to be primarily ionic

• Actinide complexes complexation with p-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

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Ionic Radii and trends

Trends based on ionic radii

Actinide contraction

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Absorption Spectra and Magnetic Properties

• Electronic Spectra § 5fn transitions

à narrow bands (compared to transition metal spectra)

à relatively uninfluenced by ligand field effects

à intensities are ca. 10x those of lanthanide bands

à complex to interpret • Magnetic Properties

§ hard to interpret § spin-orbit coupling is large

à Russell-Saunders (L.S) Coupling scheme doesn't work, lower values than those calculated

* LS (http://hyperphysics.phy-astr.gsu.edu/hbase/atomic/lcoup.html)

* Weak spin orbit couplingØ Sum spin and orbital angular

momentum Ø J=S+L

§ ligand field effects are expected where 5f orbitals are involved in bonding

http://www.sciencedirect.com/science/article/pii/S0020169300924873#

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Example: Pu absorbance spectrum

400 500 600 700 8000

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3

4

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Ab

sorb

an

ce

Wavelength (nm)

Normal Heavy Light

Pu4+ (489 nm)

Pu6+(835 nm) • 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: sf3

§ Square: sf2d§ Octahedral: d2sf3

à A 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=O2+

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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 3Pu4+ + 2H2O PuO2

2+ + 2Pu3+ + 4H+ • early actinides have a tendency to form complexes

§ complex formation influences reduction potentials à Am4+(aq) exists when complexed by fluoride (15 M NH4F(aq))

• radiation-induced solvent decomposition produces H• and OH• radicals § lead to reduction of higher oxidation states e.g. PuV/VI, AmIV/VI

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Redox chemistry (Frost diagrams)

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Stereochemistry

C.N. Geometry O.N. e.g.

4 distorted +4 U(NPh2)4

5 distorted tbp +4 U2(NEt2)8

6 octahedral +3 An(H2O)63+, An(acac)3

    +4 UCl62-

    +5 UF6-, a-UF5

    +6 AnF6

    +7 Li5[AnO6] (An = Np, Pu)

  distorted octahedral +6 Li4UO5 , UO3

    +5/+6 U5O8

    +6 UO2(S2CNEt2)2(ONMe3)

8 cubic +4 (Et4N)4[U(NCS)8], ThO2, UO2

    +5 AnF83-

  square antiprismatic +4 ThI4, U(acac)4, Cs4[U(NCS)8],

    +5 b-UF5

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Stereochemistry 8 dodecahedral +4 Th(ox)4

4-, Th(S2CNEt2)4

  bicapped trigonal prismatic +3 PuBr3

  hexagonal bipyramidal +6 UO2(h2-NO3)2(H2O)2

  ? +6 UF82-

9 tricapped trigonal prismatic +3 UCl3

  capped square antiprismatic +4 Th(trop)4(H2O)

  dodecahedral +4 Th(ox)44-, Th(S2CNEt2)4

  bicapped trigonal prismatic +3 PuBr3

  hexagonal bipyramidal +6 UO2(h2-NO3)2(H2O)2

  ? +6 UF82-

10bicapped square antiprismatic +4 KTh(ox)4.4H2O

11?fully capped trigonal prismatic? +3 UF3

12 irregular icosahedral +4 Th(NO3)62-

  distorted cuboctahedral +4 An(h3-BH4)4, (Np, Pu)

14? complex +4 An(h3-BH4)4, (Th, Pa, U)

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Actinide metals

• Preparation of actinide metals§ Reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca or Ba

at 1100 – 1400 °C§ Other redox methods are possible

à Thermal decomposition of iodine speciesà Am from Am2O3 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

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Pu metal

• Some controversy surrounding behavior of metal http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818030.pdf

Plutonium a b g d ¢d e

Symmetry monoclinic monoclinic orthorhombic fcc bc tetragonal bcc

Stability < 122°C 122-207°C 207-315°C 315-457°C 457-479°C 479-640°C

r / gcm-3 19.86 17.70 17.14 15.92 16.00 16.51

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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 p-bonded to actinides

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Uranocene• Paramagnetic • Pyrophoric • Stable to hydrolysis • Planar 'sandwich' • Eclipsed D8h conformation • UV-PES studies show that bonding in uranocene has 5f & 6d

contributions • e2u symmetry interaction shown can only occur via f-orbitals

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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

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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?

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Pop Quiz

• List 3 pentavalent actinides.