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

3. Actinide Electronic Structure

4. 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

5. 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

6. Ionic Radii
Trends based on ionic radii

7. 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 (
Weak spin orbit coupling
Sum spin and orbital angular momentum
J=S+L
ligand field effects are expected where 5f orbitals are involved in bonding

8. Pu absorbance spectrum

9. Oxidation states and stereochemistry

10. 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

11. Stereochemistry C.N. Geometry O.N. e.g. 4 distorted +4 U(NPh2)4 5
distorted tbp
U2(NEt2)8 6
octahedral +3
An(H2O)63+, An(acac)3
UCl62- +5
UF6-, a-UF5 +6
AnF6 +7
Li5[AnO6] (An = Np, Pu)
distorted octahedral
Li4UO5 , UO3
+5/+6
U5O8
UO2(S2CNEt2)2(ONMe3)

12. Stereochemistry 8 cubic +4 (Et4N)4[U(NCS)8], ThO2, UO2 +5 AnF83-+5
AnF83-
square antiprismatic
ThI4, U(acac)4, Cs4[U(NCS)8],
b-UF5
dodecahedral
Th(ox)44-, Th(S2CNEt2)4
bicapped trigonal prismatic +3
PuBr3
hexagonal bipyramidal +6
UO2(h2-NO3)2(H2O)2 ?
UF82- 9
tricapped trigonal prismatic
UCl3
capped square antiprismatic
Th(trop)4(H2O)

13. Stereochemistry 10 bicapped square antiprismatic +4 KTh(ox)4.4H2O 11?
fully capped trigonal prismatic? +3
UF3 12
irregular icosahedral
Th(NO3)62-
distorted cuboctahedral
An(h3-BH4)4, (Np, Pu)
14?
complex
An(h3-BH4)4, (Th, Pa, U)

14. 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 g/mL
Np g/mL
Am lighter at g/mL
Some metals glow due to activity
Ac, Cm, Cf

15. Pu metal
Plutonium a b g d d¢ e
Symmetry
monoclinic
orthorhombic
fcc
bc tetragonal
bcc
Stability
< 122°C °C °C °C °C °C
r / gcm-3
19.86
17.70
17.14
15.92
16.00
16.51
Some controversy surrounding behavior of metal

17. 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

18. 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-

19. Redox chemistry (Frost diagrams)

20. Redox chemistry

21. 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 PuO Pu3+ + 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

22. Actinide complexes

28. 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

29. 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