f-block and nuclear chemistry Flashcards
What ore was uranium discovered in and when (roughly)?
Pitchblend, 1789 (Late 18th century)
Name 4 ways that some of the f-block elements were discovered
Neutron bombardment
2H (deuterium) bombardment
Alpha-particle bombardment
Thermonuclear detonation
Describe the general abundance of the f-block elements, with some key examples
Called the rare earth metals, but aren’t that rare.
La/Ce/Nd more common than Pb.
Nd (used in lasers) is more abundant than Au.
Tm is the least abundant but there’s more in the earth’s crust than iodine.
What are the names of the 4f and 5f elements
4f= lanthanides 5f= actinides
Why is Promeithium (Pm) absent from the earth’s crust?
All isotopes of Pm have short half-lives.
Describe and explain the pattern of the lanthanides abundance in the earth’s crust
Even-odd alternation of abundance with atomic number. Even nuclei are more stable hence the nuclear stability follows this pattern.
How is Pa found?
Found only as a decay product in Uranium (U) supplies.
What is a key difference between the oxidation properties of the lanthanides/actinides?
None of the lanthanides change oxidation state, whereas can change the oxidation states of the actinides.
How do you separate a solution containing a mixture of lanthanides (all in a single oxidation state)?
Use an ion exchange column (eg colloidal Prussian blue).
What is the structure of ‘hex’?
UF6
Describe the structure of f-orbitals compared to d-orbitals
f-orbitals are like d-orbitals but have more angular nodes (more lumpy).
f-orbitals are well shielded by other electrons, but they stabilise very quickly upon oxidation (especially impo with 4f orbitals).
How does the size of the lanthanides change as you progress from the left—> right of the periodic table? Explain why this happens
The size of the ions decreases- lanthanide contraction. Similar to other trends in periodic table (increasing Zeff), was thought that this was solely due to increasing Zeff, but it has been shown that 30% of the contraction was due to the RELATIVISTIC EFFECT.
What is Zeff?
Effective nuclear charge
What is the relativistic effect and how does it contribute towards the lanthanide contraction?
As the electron in the orbital approaches the nucleus, its speed increases significantly, approaching the speed of light (~70% c). Because of the relativity, the mass of the electron gets bigger (by about 35%). This effect is especially true for highly charged nuclei. 4f electrons spend their time nearer to the nucleus hence are CORE LIKE.
How many f-orbitals are there?
7
What are the 2 sets of f-orbitals and why is there more than 1 set?
General and cubic set. Due to different solutions to the Schrodinger wave equation for f-orbitals.
What is the symmetry of the f-orbitals?
Ungerade
Describe and explain the ligand interaction with (4) f-orbitals
Weak ligand interaction (similar to 3d orbitals) due to high angular nodality and the ‘core-like’ nature of the 4f orbitals.
How many nodes are there in the radial distribution function of the 4f and 5f orbitals?
4f= none 5f= 1
What do nodes prevent?
Penetration towards the nucleus.
Why is there little difference in the ionic radii of the 4d and 5d transition elements?
Lanthanide contraction (due to increasing Zeff and relativistic effects).
Describe the effects of the relativistic effect on radial distribution functions
Orbitals have a radius which is dependent of 1/mass of the electron in that orbital; the relativistic effect decreases the radius of the orbital. Due to screening caused by the relativistic contraction of the s-orbitals, the f-orbitals undergo an INDIRECT relativistic expansion.
What orbital is particularly affected by the relativistic contraction?
s-orbital
Compare the indirect relativistic expansion between lanthanides an actinides and the consequences of this.
Lanthanides= small
Actinides= significant, meaning that the s f orbitals can overlap with the ligands.
Therefore lanthanides and actinides display very different chemical properties.
Compare the bonding between lanthanides and ligands with actinides and ligands
Lanthanides and ligands= covalent
Actinides and ligands= ionic
According to what principle do the f-block orbitals fill and where are the anomalies?
The Aufbau principle; anomalies at half-filled (f7) and filled f-orbitals (f14).
Explain the significance of the half-filled and filled f-orbitals and how this affects ionisation
They have unusual stability hence ionisation will nearly always result in half-filled/filled f-orbitals (ionisation may occur out of the 5/6d orbitals).
Explain how the 4f orbitals change upon ionisation and how this prevents further ionisation past the +3 ox state
Because there are no nodes in the 4f orbitals, they are sensitive to the nuclear charge and drop quickly in energy upon ionisation. As we reach the +3 ox state, they are so low that we can’t easily ionise further.
What is the general trend in lanthanide metallic radii and the exceptions?
Across the row there is a decrease in the metallic radii due to an increase in Zeff and the ion getting smaller. Exceptions= Eu and Yb due to the unusual effects of a half-filled/filled f-level.
What is the general trend in actinide metallic radii and how do they compare to lanthanide metallic radii?
No distinct general trend, quite weird. Metallic radius is smaller (and therefore denser) than the lanthanides due to covalency (draws atoms together).
Why are Eu2+/Yb2+ stable and other Ln2+ ions not and where else is this anomalous behavious seen?
Eu and Yb have higher 3rd ionisation energy valyes than other lanthanides due to the stability of their half-filled/filled shells. Also seen in their enthalpies of atomisation- easier to atomise Eu/Yb.
What oxidation state is stable for the most lanthanides?
+3
What lanthanide is stable in the +4 oxidation state?
Ce (Ce4+ is a commmon oxidising agent)
What is the main difference between lanthanide/actinide oxidation states?
Actinides have a much wider range of oxidation states available- much more complex; is because the s f orbitals are spatially extended.
What are the 3 energy terms?
e-e repulsion, ligand field and spin-orbit coupling
What are the dominant energy terms in the lanthanides and what is the result of this?
e-e repulsion and spin-orbit coupling; means the lanthanides behave as if they’re free, gaseous ions and term symbols for free ions become excellent representations of the energy of the electron.
How are energy states determined?
By Hund’s rules and e-e repulsion.
How can we account for spin-orbit coupling?
There are lots of individual magnetic fields created by the electrons. Both spin and orbital angular momenta can couple in multiple different ways (too complex to easily comprehend).
Out of all the coupling mechanisms spin-orbit coupling is the largest so can take, for each electron, its spin (s) and its orbital angular momentum (l) and add them to get the spin-orbit coupling quantum number= j.
Can use RUSSELL-SAUNDERS COUPLING (S + L) to estimate the qunatum state of spin-orbit coupled systems.
How does the Russell-Saunders coupling reproduce the experimental spin-orbit coupling?
Takes all the individual spins: s1 + s2 + s3…. = S
and all the orbital angular momenta: l1 + l2 + l3… = L
S + L have an excellent match with experimental data.
What is the name for the rules used to predict the grouund state term, and what is defined as the ground state (using these rules)?
Hund’s rules; ground state is the one with max multiplicity. If ground states have same multiplicity, then the state with the max angular momentum is the ground state. When S=L, the state with the min J for half-filled shells or less (f7 or less) is the ground state and the state with the max value of J is the ground state for > half filled shells.
What is a special property of the lanthanides?
Rare earth magnets
What is the magnetism of the 3d TMs mainly due to?
Electron spin
Compare the calculated (predicted) and experimental magnetic moments of the 4f orbitals- what does this show?
Very good match between experiment and theory- shows that the energy of the electrons in 4f orbitals is very well emulated by the Russell-Saunders coupling scheme.
What 4f element doesn’t fit well with the Russell-Saunders coupling scheme (poor match between theory/experimental magnetic moments) and why is this?
Eu; because of the unusual stability of the +2 ox state, the ground state has nearby excited states which mix with the ground state term and make it much less simple- wave function mixing.
What formula is used when spin-orbit coupling is much greater than the ligand field?
Lande formula
How does actinide magnetism differ to lanthanide magnetism?
Much more complicated, here the R-S coupling scheme breaks down so need to use j-j coupling. greater spatial extension of the 5f orbitals means that the ligand field is very important. Due to the covalency, we can no longer describe the ground state with a simple term symbol.
How are the energy levels defined in the electronic absorption spectra of the lanthanides?
Completely defined by the R-S term.
Describe the electronic absorption spectra of the lanthanides and the selection rules
Sharp and narrow bands (transitions); so well-defined that these compounds/solutions are used to reference spectrometers.
Delta S= 0, delta l = +/- 1 this is allowed between different R-S states. However the transitions are also subject to the Laporte selection rule; all f-orbitals are ‘u’ symmetry so transitions between them are forbidden by the Laporte selection rule. Since the 4f orbitals are so core-like, their symmetry is hardly affected by the ligands (shows lack of ligand interaction with the orbital) so the Laporte selection rule is strongly held.
What is the consequence of the Laporte selection rule for the lanthanides?
Ln3+ salts are all pretty much colourless.
How do the actinide electronic spectra differ to the lanthanide electronic spectra?
Actinide spectra more complicated; s f orbtials in the actinides are more spatially extended therefore overlap with the ligands and get vibronic coupling. This broadens the absorption and breaks down the Laporte selection rule hence can get coloured solutions. Spectra are still weak, but are more intense than for the lanthanides. Much broader peaks.
How do the extinction coefficients compare between the lanthanides/actinides?
Lanthanides= typically <10 mol-1dm3cm-1 Actinides= 30-7- mol-1dm3cm-1
Why are the lanthanides strong, permanent magnets?
Due to the high number of unpaired electrons- can get some strong magnetic fields.
How can we get a fluorescent detector using lanthanide complex luminescence?
If the energy separation of the Ln levels can be ‘tuned’ by another molecules then we have the basis of a fluorescent detector of the ‘other molecule’.
What are screen phosphors and what is an application of these?
Phosphors which give narrow-based emission give brilliant coloured species, used for anti-forgery devices in Eu banknotes.
Name an example Ln MRI contrast agent and describe how it works
Gd3+ complex (f7, max number of unpaired electrons so very strong magnet), enhances the relaxation of the proton signal
What type of bonding occurs between the 4f Ln orbitals and ligands?
Ionic
Are Ln3+ ions hard or soft?
Hard (described as oxo-phillic).
Describe the coordination geometry around the Ln3+ central ion
High CNs (chelate effect) with no stereochemical preference. Large range of CNs (up to 12).
What 2 effects are very important in Ln coordination chemistry?
Chelate and macrocyclic effect.
How can you separate the lanthanides, and what property of the Lns does this use?
Ion exchange column; difference in stability constants (Ks) of the lanthanides.
What is the trend in the stability constants of the lanthanides from left to right?
See an increase in stability of the Ln-ligand complex due to the increase in Zeff at the nucleus (like the Irving-Williams series).
What 2 Lns are very difficult to separate and why is this?
Eu/Gd- very similar stability constants, probably as there is a change in coordination number.
What does CEST stand for and how do these agents work?
CEST= chemical exchange of saturation transfer; give some control of the analyte in Ln contrast agents. The ligand chelates Lns (wraps around them), forming a cavity into which other molecules (analytes) bind, displacing the water molecule- this can be observed in the NMR spectrum hence have a detector for the analyte.
What is the key species in aq solution in actinide coordination chemistry? Give 2 examples
The actinyls; UO2 2+ , NpO2 2+
Describe the bonding in actinyls
On each oxygen, there are 3x p-orbitals, each of which can form a strong bond to the f-orbitals. Hence very stable- hard to separate out the actinide from actinyls.
If actinyls are so stable, how do we separate them during the re-processing of nuclear waste (U from Pu)?
Can use the stability of UO2 2+ because it’s difficult to reduce chemically (as is so stable), whereas Pu is easier- Pu(II) can be reduced to Pu(III) by Fe(II).
What is SmI2 used for?
Used in organic synthesis as a reducing agent.
What is Ce(IV) used for?
Powerful oxidising agent.
Why can Ln(III) salts be used as Lewis acid catalysts?
Because they are redox stable
If the bonding in lanthanides is so ionic, how can we form covalent organometallic complexes?
The ligands can also bond to the valence d-orbitals, which would normally be unoccupied by electrons (as they are very high in energy) and therefore any interaction would be useless. However, in some cases, the d-orbital energy levels are close to the occupied f-orbital energy levels so we can get promotion of f-electrons to the bonding d-levels (f–> d promotion). Then we can get d-like bonding in Lns.
What lanthanides experience f–> d promotion and hence can form covalent organometallic complexes?
Pr, Nd, Gd, Tb, Dy, Ho, Er
Describe and explain how the half lives (of the most stable isotope) of the actinides changes across the row
Stability of the nucleus decreases dramatically- range of t1/2 covers 10^16 difference in stability. Decrease in stability because the proton-proton repulsion gets greater as the number of protons increases.
Describe the change in the neutron:proton ratio on the Segre plot
Neutron:proton ratio increases as the atomic number increases because increased Coulombic repulsion in the heavier elements can’t accommodate the high number of protons.
