Lecture 2: General Overview

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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/
• Occurrence
• Ac, Th, Pa, U natural
• Ac and Pa daughters of Th and U
• Traces of 244Pu in Ce ores
• Properties based on filling 5f orbitals

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Actinide Electronic Structure
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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
• 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
• 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
• Bonding can include any orbitals since
  energetically similar
• 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

• Trends based on ionic radii

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Absorption Spectra and Magnetic Properties

• Electronic Spectra
• 5fn transitions
• narrow bands (compared to transition metal
  spectra)
• relatively uninfluenced by ligand field effects
• intensities are ca. 10x those of lanthanide bands
  
• complex to interpret
• Magnetic Properties
• hard to interpret
• spin-orbit coupling is large
• Russell-Saunders (L.S) Coupling scheme doesn't
  work, lower values than those calculated
• LS (http//hyperphysics.phy-astr.gsu.edu/hbase/ato
  mic/lcoup.html)
• Weak spin orbit coupling
• Sum spin and orbital angular momentum
• JSL
• ligand field effects are expected where 5f
  orbitals are involved in bonding

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Pu absorbance spectrum
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Oxidation states and stereochemistry
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Hybrid orbitals

• Various orbital combinations similar to sp or d
  orbital mixing
• Linear sf
• Tetrahedral sf3
• Square sf2d
• Octahedral d2sf3
• A number of orbital sets could be energetically
  accessible
• General geometries
• Trivalent octahedral
• Tetravalent 8 coordination

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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 Li5AnO6 (An Np, Pu)
  distorted octahedral 6 Li4UO5 , UO3
    5/6 U5O8
    6 UO2(S2CNEt2)2(ONMe3)
       
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Stereochemistry
8 cubic 4 (Et4N)4U(NCS)8, ThO2, UO2
    5 AnF83-
  square antiprismatic 4 ThI4, U(acac)4, Cs4U(NCS)8,
    5 b-UF5
  dodecahedral 4 Th(ox)44-, 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)
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Stereochemistry
10 bicapped 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
• Thermal decomposition of iodine species
• 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
Plutonium a b g d d e
Symmetry monoclinic monoclinic orthorhombic fcc bc tetragonal bcc
Stability lt 122C 122-207C 207-315C 315-457C 457-479C 479-640C
r / gcm-3 19.86 17.70 17.14 15.92 16.00 16.51

• Some controversy surrounding behavior of metal
  http//www.fas.org/sgp/othergov/doe/lanl/pubs/0081
  8030.pdf

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Oxidation states

• 2
• Unusual oxidation state
• Common only for the heaviest elements
• No2 and Md2 are more stable than Eu2
• 5f?6d promotion
• Divalent No stabilize by full 5f14
• Element Rn5f147s2
• Divalent actinides similar properties to divalent
  lanthanides and Ba2
• 3
• The most common oxidation state
• The most stable oxidation state for all
  trans-Americium elements exxept No
• Of marginal stability for early actinides Pa, U
  (But Group oxidation state for Ac)
• General properties resemble Ln3 and are
  size-dependent
• Binary Halides, MX3 easily prepared, easily
  hydrolyzed to MOX
• Binary Oxides, M2O3 known for Ac, Pu and trans-Am
  elements

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Oxidation states

• 4
• Principal oxidation state for Th
• similar to group 4
• Very important, stable state for Pa, U, Pu
• Am, Cm, Bk Cf are increasingly easily reduced -
  only stable in certain complexes e.g. Bk4 is
  more oxidizing than Ce4
• MO2 known from Th to Cf (fluorite structure)
• MF4 are isostructural with lanthanide
  tetrafluorides
• MCl4 only known for Th, Pa, U Np
• Hydrolysis / Complexation / Disproportionation
  are all important in (aq)
• 5
• Principal state for Pa (similar to group 5)
• For U, Np, Pu and Am the AnO2 ion is known
• Comparatively few other AnV species are known
• fluorides fluoro-anions, oxochlorides, uranates,
  
• 6
• AnO22 ions are important for U, Np, Pu, Am
  UO22 is the most stable
• Few other compounds e.g. AnF6 (An U, Np, Pu),
  UCl6, UOF4 etc..., U(OR)6
• 7
• Only the marginally stable oxo-anions of Np and
  Pu, e.g. AnO53-

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Redox chemistry (Frost diagrams)
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Redox chemistry
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Redox chemistry

• actinides are electropositive
• 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 PuO22 2Pu3 4H
• early actinides have a tendency to form complexes
  
• complex formation influences reduction potentials
  
• 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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Actinide complexes
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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