Lanthanides & Actinides Flashcards
1
Q
4f Orbitals
A
n=4, l=3, m_l = ±3, ±2, ±1, 0 no radial nodes 3 angular nodes ungerade symmetry core-like: penetrate the [Xe] core.
2
Q
Chemical nature of 4f orbitals
A
Due to the contracted nature of the orbitals, the lanthanides are very unreactive, as the 4f orbitals are so contracted they cannot interact with any ligand environment.
3
Q
Lanthanum (La) ground state configuration
A
[Xe] 5d1 6s2
since this is the beginning of the series, the 4f orbitals are not yet incredibly contracted, and 5d is higher in energy than 4f.
4
Q
Cerium (Ce) ground state configurations
A
[Xe] 4f1 5d1 6s2
since this is the beginning of the series, the 4f orbitals are not yet incredibly contracted, and 5d is higher in energy than 4f. After Ce, the effective nuclear charge increases so 4f is lower in energy than 5d.
5
Q
Praseodymium (Pr) ground state configuration
A
[Xe] 4f3 6s2
At this point the 4f orbitals are very contracted and poorly shielded, and are now lower in energy than 5d orbitals.
6
Q
Neodymium (Nd) ground state configuration
A
[Xe] 4f4 6s2
7
Q
Promethium (Pm) ground state configuration
A
[Xe] 4f5 6s2
8
Q
Samarium (Sm) ground state configuration
A
[Xe] 4f6 6s2
9
Q
Europium (Eu) ground state configuration
A
[Xe] 4f7 6s2
10
Q
Gadolinium (Gd) ground state configuration
A
[Xe] 4f7 5d1 6s2
This is the ground state configuration as it is energetically more favorable to have a half-filled f orbital and one electron in the d orbital rather than 8 electrons in the f orbital/
11
Q
Terbium (Tb) ground state configuration
A
[Xe] 4f9 6s2
12
Q
Dysprosium (Dy) ground state configuration
A
[Xe] 4f10 6s2
13
Q
Holmium (Ho) ground state configuration
A
[Xe] 4f11 6s2
14
Q
Erbium (Er) ground state configuration
A
[Xe] 4f12 6s2
15
Q
Thulium (Tm) ground state configuration
A
[Xe] 4f13 6s2
16
Q
Ytterbium (Yb) ground state configuration
A
[Xe] 4f14 6s2
17
Q
Lutetium (Lu) ground state configuration
A
[Xe] 4f14 5d1 6s2
18
Q
Typical oxidation state of the lanthanides
A
+3 is the most common ox state of the lanthanides, as for virtually all of the elements, the ionization energy to reach the +4 oxidation state is significantly higher than the previous ionization energies. This is due to the highly contracted nature of the 4f orbitals, which are stabilized much more than any other orbitals by the removal of an electron. Hence the lanthanide ions tend to obtain [Xe] 4fn-1 electron configurations, where n is the number in the ground state.
19
Q
+2 Oxidation state lanthanides
A
Eu and Yb have maxima for IEs for the third ionization. This is because they have full or half-full 4f orbitals, and are thus especially stable. Therefore Eu and Yb easily form +2 oxidation states rather than +3 ox states. Sm, since it’s close to Eu, also can have +2 ox. state. This means that Sm 2+, Eu 2+, and Yb 2+ have a more diverse chemistry than the other +3 lanthanides. This impacts metallic radius and chemical behaviour. Eu and Yb behave like heavier group 2 metals.
20
Q
+4 oxidation state lanthanides
A
Ce, Tb, Pr, Nd, and Dy all show some tetravalent chemistry as there are minima in the I4 energy. Ce4+ is accessible due to the high energy of the 4f orbitals early in the series, and has extensive chemistry. The other lanthanides are limited to fluorides and dioxides.
21
Q
Lanthanide Contraction
A
The metallic and ionic radii decrease linearly through the series, as 4f orbitals do not screen the valence electrons from the nuclear charge. For metals the valence electrons can be considered 6s or 5d, while the ions have 5s or 5p valence orbitals.
The exceptions are Eu and Yb due in the metallic radius as they tend to be in +2 state rather than +3 state.
22
Q
5f orbitals
A
The 5f orbitals have 3 angular nodes and 1 radial node. As they are one larger than the 4f orbitals, they have greater extension beyond the 6s and 6p orbitals than 4f has beyond the 5 equivalents. As a result there are more covalent interactions in bonding in the early members of the series. EPR data suggests that there is covalent bonding in UF3 but not NdF3.
The greater extension of the 5f orbitals is due to relativistic effects.
23
Q
Relativistic effects on Actinides
A
Due to the greater nuclear mass of the actinides relative to the mass of an electron, the electrons orbiting the nuclei are travelling very fast, close to the speed of light. Since that speed barrier cannot be broken, the electron mass increases. This causes the s and p orbitals to contract and stabilise, while the d and f orbitals experience an expansion/destabilisation due to the increases shielding. This is in part the cause of the increased radial extension of the 5f orbitals.
24
Q
Electronic configuration of the actinides
A
Up to and including Np, the electron configurations incorporate d and s orbitals as they are lower in energy than the 5f orbitals. At Pu, the 5f orbitals are more stabilised and there are only f and s electrons in the configuration. Cm and Lr are exceptions with half filled and filled shells and a d electron.
25
Oxidation States of the Actinides
The early actinides do NOT prefer +3 ox state- Th is amost exclusively in the +4 state and U 3+ is only obtained by the reduction of higher valent species. These behave more like TMs and therefore have appreciable chemistry. Up to Uranium, the most common ox states are dictated by valence electrons.
Later actinides favor the +3 state- they act like the lanthanides and do not have appreciable chemistry.
26
Ionic Radii of the Actinides
There is a clear actinide contraction as you move along the period. As long as the ox state is the same, the trend is linearly decreasing.
27
Metallic Radii of the Actinides
There is no discernable pattern to the metallic radii of the actinides, probably due to the variation in possible oxidation states.
28
Nuclear Fission
```
When a large nucleus splits into two smaller nuclei, which may collide and create more fission.
Nuclear energy (and other uses) relies on the enrichment of U-235.
```
29
Uranium Enrichment Isotope Separation
Gaseous Diffusion
Gas Centrifuge
Electromagnetic Separation
Laser separation
30
Gaseous Diffusion
UF6 vapour diffuse through barriers that are resistant to F2 eg Al or Ni, with pores 10-25 nm at 70-80˚C.
31
Gas Centrifuge
Low tech approach. Centrifuge UF6 vapour, use the fact that the heavier U-238 is found at the edges which the lighter U-235 is in the center.
32
Electromagnetic Separation
Ionized UCl4 separated in cyclotron-like system.
33
Laser Separation
Selectively ionize U-235 with a laser.
34
Actinide-Halide complexes
The actinides up to U can form halides based on the number of valence electrons. After U all of the actinides for AnX3 compounds.
UF6 is the most important actinide halide, which is used in enriching U
35
Synthesis of UF6
HF + UO2 --> UF4 + 2 H20
| +F2 --> UF6
36
Separation of the Lanthanides
Monazite and xenotime treated under alkaline conditions, then acidified to generate LnCl3 solution. ThO2 removed by precipitation.
Oxidizing roast, then bastnaesite is treated with H2SO4 to generate Ln2(SO4)3 and extract CeO2. After this point the lanthanides may be separated by fractional crystallization, chemical separation using multiple ox states of Ce and Eu, Ion-exchange chromatography, and solvent extraction.
37
Ion-exchange chromatography
In a column with resin, heavier smaller lanthanides, which are stronger Lewis acids bind more strongly to a chelating eluent like EDTA or citric acid, and are removed in order of their stability constant, highest first, so the heaviest elements come out first.
38
Production of Lanthanide Metals (general)
Metallothermic reduction of anhydrous fluorides or chlorides with Ca.
LnX3 --(Ca, 145˚C)--> Ln/Ca + CaX3
The reaction takes place under Ar, and the Ln/Ca alloy requires distillation of the Ca for the pure metal to be produced.
39
Production of Lanthanide Metals (2+ ox. state)
Sm, Eu, and Yb cannot be made using metallothermic reduction, but their oxides may be reduced with lanthanum to produce the pure metal
2La + M2O3 (M=Sm, Eu, Yb) ----> La2O3 + 2M
40
Properties of the Lanthanide Metals
Soft metals - later ones are harder.
Silvery-white
Highly electropositive so they tarnish in air
Simple Ln compounds are highly ionic
41
Lanthanide Sesquioxides Ln2O3
May be made by heating the Ln metals in air or heating oxygen-containing compounds (nitrates or carbonate)
Ln + 3 O2 ----> 2Ln2O3
4Ln(NO3)3 --∆--> 2Ln2O3 + 12 NO2 +3 O2
Ce, Pr, and Tb form LnO2 due to ability to form 4+ oxidation states but can be reduced to the sesquioxides with H2.
42
Lanthanide halides LnX3
May be prepared y thermal dehydration of the hydrated salt. Cl, Br, I require dehydrating agents present or they form oxyhalides.
Compounds are ionic, crystalline and have high melting points. Other than trifluorides, they are highly deliquescent (they absorb moisture from the air then dissolve in the absorbed liquid).
Structure indicative of the lanthanide contraction as the coordination number decreases along the series.
The early Ln have a "tysonite" structure that is capped trigonal prismatic.
43
Ternary and complex oxides (e.g. Perovskites)
Due to the stable and large nature of lanthanide cations, they can take on one or more of the cation positions of ternary and complex oxides eg. perovskites ABO3 with Ln on the A cation site. (Superconductors?)
44
Ln (+4) Oxides and Halides Structures
Ce(IV), Pr(IV), and Tb(IV) form fluorides, but only the Ce complex is thermally stable.
LnO2 adopts fluorite crystal structure.
Pure CeO2 is colorless, but normally is pale yellow due to the presence of 3+ ions.
Useful in catalytic converters and self-cleaning ovens.
45
Magnetism of 4f orbitals
From spin and orbital angular momenta. Ln(III) the core-like and shielded 4f orbitals are not influenced by the surrounding ligand field, unlike TMs. The orbital angular momentum contribution cannot be considered to be quenched.
46
Russell-Saunders Coupling
Due to the core-like nature of the f orbitals, orbital contributions play a significant role.
S= sum of the orbital angular momenta (±1/2)
sum all the angular momenta to give L ***blah blah ***
This approach is highly ionic.
47
Landé formula
```
µ(J)= g√J(J+1)
g= 3/2 + {S(S+1)-L(L+1)}/{2J(J+1)}
```
48
Agreement of experimental and calculated Ln magnetism
Good agreement except for Eu and Sm due to the presence of excited states close enough to the ground state that they may be populates with thermal energy fluctuations.
49
Magnetic Properties of the Actinides
Very complex, with strong sim-orbit coupling (2000-4000 cm^-1).
Now there are ligand effects- quantum number can no longer be assumed to be J.
µ(eff) varies with temperature and is generally lower than the corresponding lanthanides.
50
NdFeB
The strongest known permanent magnet. Unit is Nd2 Fe14 B - this contains 4 unit cells.
The magnetic moments of individual ions align, creating very strong bulk magnetism. Nd is not the main workhorse here- Fe plays a crucial role.
51
Electronic Spectra of Ln(III)
f-f transitions are forbidden by Laporte/parity selection rule.
Contracted 4f orbitals do not interact with ligand field, so the surrounding ligands (at first approximation) do not impact the electronic spectra. There is no vibronic coupling so there is like no chance for there to be f-f transitions through destruction of symmetry.
Less intense colors, spectra almost independent of environment (i.e. phase, type of ligand.
52
Absorption Spectra of Ln (III)
Sharp peaks that represent f-f transitions, between ground to excited states- basically the electrons moving to different configurations in the orbitals. The peaks are sharp due to the lack of vibronic coupling.
53
Intense colors of Ln(3+)
Some lanthanides show intense colors due to 4f-5d electron promotion.
MsI2 [Xe] 4f6 --> [Xe] 4f5 5d1
Ce(3+) - since promotion leads to empty sub shell
Tb(3+) - since promotion leads to half filled shell.
54
Fluorescence Emission of Ln(3+)
Irradiation with UV results in many Ln(3+) compounds fluorescing. Singlet ground state is excited to excited singlet LIGAND, which undergoes ISC to form an excited triplet state for the ligand. This then undergoes ISC to form the Ln(3+) excited state, which relaxes back to the ground state.
Most favourable for Eu and Tb as they have excited states below the typical triplet states of ligands. These find application in eg cathode ray tubes.
55
Early Actinide Electronic Spectra
5f orbitals have increased interaction with the ligand field and therefore have more vibronic coupling. The 5f-5f transitions are therefore much broader and 10x more intense than 4f-4f transitions. More intense colors.
56
Late Actinide Electronic Spectra
More closely resemble the electronic spectra of the lanthanides due to the contraction of the 5f orbitals with increasing Z(eff). Therefore they have weaker and sharper peaks than the early actinides, with far paler colors.
57
Properties of Lanthanide Coordination Complexes
Ln ions behave as hard Lewis acids
Bonds are highly ionic in nature
Large cations with high coordinations numbers. Lanthanide coordination means that the cations get smaller along the period, so coord # decreases.
Non-directional bonding means that coordination geometries are determined by size and shape of ligands.
Ligand exchange rate is very high, on the order of diffusion
58
Stability Constants (K)
K is the equilibrium constant for the formation of a complex between a Ln and ligand. logK is higher for later lanthanides, due to their smaller radii and higher charge density.
More stable complexes are formed with harder ligands due to hard-hard interactions.
59
Gadolinium Break
Looking at the logK of EDTA(2-) bound to the lanthanides, there is a change in the gradient at Gd. This is because the early lanthanides are thought to have higher coordination of water molecules than Gd and beyond (9 vs 8). This causes a decrease in ∆S as fewer water molecules are being displaced by the EDTA as it binds to the Ln(III).
60
Lanthanide Complexes with Water
Aqueous solutions of Ln are acidic since they are highly polarizing, and the ionization of the Ln coordinated to H2O becomes easier as the ions decrease in size.
Very labile complexes, lifetimes are on the order of 10^-9s
61
Chelating and Macrocyclic Complexes of Lanthanides
The chelating effect of multiple binding sites on a ligand confers kinetic stability to the complexes. The favourable exchange of one ligand liberating multiple ones leads to a favorable ∆G(rxn). The stability constants for chelating ligands is orders of magnitude higher than monodentate ligands.
62
High Coordination Numbers of the Lanthanides
Small, hard, polydentate ligands can easily lead to CNs of 9-12. The chelating ligands need a small "bite" angle, which refers to the L-M-L angle of the chelating ligand, and expresses strain of the ring. The ligands, rather than the metal, control shape.
Icosohedral geometry
63
Low Coordination Numbers of the Lanthanides
Requires incredibly bulky ligands. Trigonal prismatic geometry.
Low coordinate amides and alkoxides must be prepared in the absence of water.
Size of R groups have profound impact on nuclearity and reactivity.
64
Lanthanide NMR Shift Reagents and MRI contrast reagents
Ln (III) complexes tend to be strongly paramag. and have unusual and useless NMR peaks. Eu(III) and Pr(III) ions can be used in reagents (due to their shorter relaxation times) to induce chemical shift changes to better distinguish overlapping peaks. Nowadays they are only used to tell the difference between enantiomers , using chiral complexes. The enantiomers interact differently depending on the chirality, and enantiomeric excess can be determined by the integral of the separated peaks.
Gd(III) reduces the relaxation time of protons enhancing signals in MRIs.
65
Properties of Actinide Coordination Complexes
the 5f orbitals are more accessible than the 4f orbitals, meaning that there is some covalent character to the actinide complexes.
Early actinides behave like TMs while late actinides behave like lanthanides due to the increasing contraction of the 5f orbitals as Z(eff) increases.
66
Actinide Coordination Number
The large size of the ions results in high coordination numbers. Geometry is largely determined by the steric demands of the ligands. CNs range from 3-14. Less sterically demanding ligands result in oligomeric structures.
67
Aqueous chemistry of the actinides
Some of the earlier actinides can form penta- and hexavalent actinyl anions, AnO2^+/2+ (U-Am).
The later actinides behave like lanthanides.
68
Actinyl ions
Most actinides in ox states higher than +4 have actinyl anions AnO2^n+, which are linear O=An=O units. (TM dioxo units tend to be bent). This linearity is thought to be due to the 5f orbitals playing a role.
When UO2 is a unit in a compound, the Os are always, trans, and the rest of the ligands, between 4 and 6, arrange themselves in a plane 90˚ to the O=U=O axis.
69
Key points of the actinyl ion MO diagram
f orbitals are u, d orbitals are g.
no d-f mixing due to different symmetries.
d and f orbitals overlap with the p orbitals on oxygen to give sigma and π bonding MOs.
The actinyl ion fragment for UO2 is so stable because only bonding MOs are filled.
70
Chemistry of Uranyl Ion
Until recently, [UO2]2+ was considered inert until a very complex chelating macrocycle was developed that changed the redox potential of the UO2 unit, allowing it to be reduced. A metal atom is brought in to interact with a U=O bond. Modifying the TM allows a one-electron reduction to be observed, converting [O=U=O]2+ to [O=U-OR]+.
71
Lanthanide Organometallic Chemistry
Not very extensive- contracted 4f orbital.
Bonding primarily ionic.
Cannot act as π donors and therefore Ln-CO bonds are not stable
Very air and moisture sensitive due to the carbanionic nature of the organic ligands and the oxophilicity of the lanthanide cations
72
Actinide Organometallic Chemistry (overview)
Confined to U and Th mostly
Most have +4 oxidation states, but highly reactive +3 compounds are known.
Behaviour of U and Th organometallic compounds is intermediate between d block and lanthanides, meaning the symmetry and availability of the 5f orbitals matters.
73
Trivalent Ln Cp complexes (synthesis)
[Ln Cp_n X_3-n] (X=halide) (3 types for n=0,1,2)
| Synthesized by reacting anhydrous LnX3 with NaCp or KCp in the appropriate stoichiometry.
74
LnCp3 solid state structures
Vary with the lanthanide contraction:
La, Pr, coord # is 10, forms bridging Cp, one eta-5, one eta-2
Er, Tm, Yb are simply eta-5 trivalent, coord # = 9
Lu coord # 8, two Cp eta-5, bridging with two eta-1
The smaller they get the smaller the coord #
75
Bonding in Ln-Cp complexes
Primarily ionic in nature, can be seen by their use in Ferrocene synthesis instead of NaCp.
76
[Cp2LnX] Compounds
Generally form dimers using the halide as bridging (two are used to bind two centers together), though they can also form polymers using the halide atom.
Can also form base adducts with e.g. THF complexes that break up the dimer by donating electron density. These tend to have low solubility in HC solvents, and be prone to ligand redistribution processes. They also easily undergo deprotonations of the Cp ligand, which can be prevented by using Cp* instead, or SiMe3 on ortho and para sites.
77
Cp* Ln complexes
Cp*_3 Ln complexes are not accessible due to the extreme crowding around the ion, even if it is big.
Sm is the exception.
Cp*_2 Ln X can be formed from NaCp* and LnX3, and tend to exist as dimers. They are ideal starting materials for exploring organolanthanide chemistry.
78
Actinide Cp complexes
Mainly of Th(IV) and U(IV). UCP4 exists, but is a rare example of a tetra Cp complex with all rings bount eta-5. ZnCp4 experiences ring slippage due to the smaller metal center.
Cp complexes of U and Th do not react with Fe to form ferrocene, indicating that the bonding is NOT ionic.
Synthetically useful complexes are [Cp3UX] and [Cp*2ThX2].
79
Lanthanide Arene Complexes
Ln metal vapours mixed with excess bulky C6H3t-Bu3 form Ln(0) complexes, with bonding similar to the 18e complex of the arene Cr sandwich complex. The complexes that can be made this way mimic the trend of 4f --> 5d promotion energy, so an argument can be made that these complexes require this promotion, as the 4f orbitals are too contracted to actually form bound complexes. (model relies on negative results, not actually conclusive proof).
80
Inverse Arene Complexes
Reducing a U(IV) precursor with potassium graphite in toluene gives an inverse sandwich complex, where the arene is bridging, bonded eta-6 to two metal centers. Simulations show that there is delta-bonding that involves 6d and 5f orbitals and the LUMO of the arene.
81
Cyclooctatetraene Dianion Complexes
For COT to be aromatic it must be COT(2-). eta-8 CTO can satisfy two metal valencies. Due to the large size, it is ideal for binding to Ln.
[(eta-8 COT)2Ln] forms the only neutral sandwich complex, all the rest form complexes with a K counterion. While Ce can readily form +4 oxidation state, computations and experiments show that Ce is best regarded as in the +3 oxidation state- [Xe]4f1 complexed by two COT (1.5-) ligands
82
Uranocene
Treatment of UCl4 with K2COT gives uranocene.
Green paramagnetic crystals.
Planar, eclipsed COT rings.
22-electron complex
In a typical 18e metal complex this would lead to the population of antibonding orbitals. Additional bonding MOs are formed by ligands interacting with 5f orbitals. (see MO diagram in separate notes).
83
Uranocene MO Interactions
COT e2g and U d(x^2-y^2) and dxy orbitals interact, as do the e2u and fz(x2-y2) and fxyz orbitals to form delta bonds. This is only possible due to the ungerade symmetry of the f orbitals, allowing for in-phase orbital interactions.
Theoretically, there are phi-bond interactions possible in the MO diagram, but it's too high in energy to be occupied.
84
Phi-bonding in U2
f-f overlap produces a quintuple bond with the remaining unpaired electrons staying on their respective U atoms.
Very strong- shown by the very short UU distance.
85
CO Activation in Ln
4f orbitals too contracted to provide any significant back bonding , the only Ln carbonyl complexes that have been synthesized have been at -40˚C with co-condensation.
86
CO Activation in Actinides
U(CO)6 exists, very unstable but IR data suggests that there is a degree of 5f-π* backbonding.
CO activation is with Th(IV) and U(IV) primarily. Some very reactive +3 CO complexes have been formed. A way to stabilize U-CO bonding is with 3 eta-5 Cp* ligands conferring kinetic stability.
87
Isocarbonyl/Carbonyl CO complex of Uranium
U(III) bound to a triazocyclonane ligand (L_(n)U) can interact with CO, which can form a bridge, µ-eta-1-eta-1 between U(III) and U(IV). IR shows a small lowering of CO vibrational frequency.
88
Reductive Coupling of CO with Uranium
Modifying ligands on U allows CO to be coupled into a series of cyclic aromatic dianions [CnOn]2-. via electron transfer.
89
CO2 activation
Examples of CO2 binding to TM are quite rare. U(III)-triazanonane complex shows eta-1 binding of CO2 through an O atom, proposed to proceed with electron transfer to generate a U(IV) complex.
90
N2 activation with Uranium
N2 can reversibly bind side-on and end-on to U(III), experiencing bond-lengthening and frequency-lowering effects each case (less with side-on). When U(III)Cp*(pentalene) is used, side-on N2 binding occurs with reduction of N2 to N2(2-) and oxidation of U(III) to U(IV), with the most dramatic bond lengthening
91
N2 activation with Lanthanide(II) metallocenes
Sm, Eu, and Yb analogues of the bent metallocene structure than is explained with the polarization model. N2 forms a reversible side-on bridging complex with the bent metallocenes that also strongly lengthens the N2 bond.
92
Thorium Cyclopentadienyl H2 activation
Can be studied with NMR by looking at the exchange between terminal and bridging hydrides and dissolved H2 gas.
Cp*2Th(Cl)2 + RLi --> (CP*)2ThR2
93
Thorium Cyclopentadienyl Alkane activation
Intramolecular is the reverse of intermolecular and vice versa.
94
Lanthanide Cp H2 activation
sigma bond metathesis is the mechanism, solid dimers form monomers in solution that then produce CH4 by breaking H2, sigma-alkyls decomposing via ß-hydride elimination of an alkene.
