Transcript Lecture 1: RDCH 710 Introduction

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.html
• 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
1-1
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
1-2
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
1-3
Ionic Radii and
trends
Trends based on ionic radii
Actinide contraction
1-4
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.phyastr.gsu.edu/hbase/atomic/lcoup.
html)
* 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
1-5
http://www.sciencedirect.com/science/article/pii/S002016
9300924873#
Example: Pu absorbance spectrum
• Ability to distinguish
between Pu oxidation
states
5
6+
Pu (835 nm)
4+
Pu (489 nm)
Absorbance
4
Normal
Heavy
Light
3

2
1
0
400
500
600
700
800
Variation in molar
absorptivity
• Determine speciation of
Pu by spectroscopy
Wavelength (nm)
•
•
•
f electrons and 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
Pentavalent and hexavalent actinides have double bonded oxygens

O=U=O2+
1-6
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  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
1-7
Redox chemistry (Frost diagrams)
1-8
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)
+6
Li4UO5 , UO3
+5/+6
U5O8
+6
UO2(S2CNEt2)2(ONMe3)
+4
(Et4N)4[U(NCS)8], ThO2, UO2
+5
AnF83-
+4
ThI4, U(acac)4, Cs4[U(NCS)8],
+5
b-UF5
distorted octahedral
8
cubic
square antiprismatic
1-9
Stereochemistry
dodecahedral
+4
Th(ox)44-, Th(S2CNEt2)4
bicapped trigonal prismatic
+3
PuBr3
hexagonal bipyramidal
+6
UO2(h2-NO3)2(H2O)2
?
+6
UF82-
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-
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)
8
9
14? complex
+4
An(h3-BH
4)4,
(Th, Pa, U)
1-10
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
1-11
Pu metal
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
• Some controversy
surrounding
behavior of metal
http://www.fas.org/s
gp/othergov/doe/lan
l/pubs/00818030.pdf
1-12
1-13
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
1-14
•
•
•
•
•
•
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
1-15
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
1-16
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?
1-17
Pop Quiz
• List 3 pentavalent actinides.
1-18