Meet the Lanthanides 2 Flashcards
What are the magnetic properties of any ions the combination of contributions from
- Spin and orbital angular momenta
What happens to orbital angular momentum in first row transition metals
- Orbital angular momentum often removed or partially quenched by interaction of the ligands with the partially occupied d orbitals
- Often (not always) correspond to spin only formula
What is the spin only formula
- Ueff= root(n(n+2))
- n=number of unpaired electrons
What is the electronic structure of lanthanide ions governed by
- Very large spin-orbit coupling characteristic of high Z
- means you can’t treat spin and momentum separately - Very small crystal-field splitting (contracted f orbitals)
- no partial quenching from ligands
What is the Russell-Saunders coupling scheme
- Because 4f orbitals have limited radial extension and are core like
- They have little role in bonding- the orbital angular momentum is not quenched
- Magnetic properties of the Ln3+ ions are very well described from the coupling of spin and orbital angular momenta
What does the Russell-Saunders coupling scheme assume
- Spins of the electron couple - total spin angular momentum (S)
- Orbital momenta also couple- total orbital angular momentum (L)
- Totall angular momentum, J, of the system is given by coupling of S and L
What is the typical Ln3+ spin orbit coupling in comparison to ligand field effects and what is the result of that
- Typically large (1000cm^-1)
- In comparison to ligand field effects (100cm^-1)
- As a result only the ground J-state is populated
- magnetism is essentially independent of environment.
- Thus we only need to know the ground state of an Ln3+ ion to predict and understand its magnetic and spectroscopic behaviour.
How are atomic or ionic energy levels defined
- By a term symbol (2S + 1)LJ
- Ground state term symbol contains all the information needed to calculate U for a Ln3+ complex
Describe the magnetism of Ln3+ ions
- arises from presence of unpaired 4f electrons
- Magnetic moments of Ln3+ complexes are little affected by the ligand environment- so similar to those calculated for the free ions
- Inherently very high magnetic moments
What is the Russell-Saunders approach to magnetism
- Spin-orbit coupling splits the atomic or ionic term into a series of J levels
- Spin-orbit coupling is so large that the excited levels are inaccessible and the magnetic moment is determined by the ground state of the ion
- The magnetic moment of a J-state is expressed by the Lande Formula
What is the Lande Formula
- Uj= g root(J(J+1)) where:
- g=3/2 + S(S+1)-L(L+1)/2J(J+1)
Why is there no observed magnetic moment for Pm
- It too radioactive
Do the calculated magnetic moment show a good agreement with experimental
- Yes
- Except Sm3+ and Eu3+
Why do Sm3+ and Eu3+ magnetic moment values not show good agreement with experimental
- Both ions have excited states close in energy to the ground term to be thermally accessible and contribute to the observed magnetic moment
- Low lying energetic states- different magnetic moments contributing to magnetic moment
What does Sm3+ and Eu3+ magnetic moments mean for dependence on other factors
- Low lying energetic states leads to temperature dependence
- The observed moments at RT decreases as the temperature is lowered
- Thermally populated as governed by the Boltzmann distribution
- Excited states are closer than KT (around 200cm^-1 which is RT)
Why is Eu expected to not have magnetic moment
- Ground state term- 7F0
- 0 in J state
- Orbital and spin angular momentum should cancel out
What are the 3 most common ores that the lanthanides are found in
- Monazite E.G lanthanide orthophosphates (LnPO4)
- Xenotime E.G lanthanide orthophosphates (LnPO4)
- Bastnaesite e.g. fluorocarbonate (LnCO3F)
Which of the natural ores use alkali digestion and which use acid dissolution
- Monazite/ xenotime- alkali digestion
- Bastnaesite- acid digestion
Describe the process of extraction from monazite/ xenotime
- Mineral dressing- crushing, grinding and hot filtration
- Digestion- 140 degrees/ 65% NaOH, Extract PO43- with H2O
- Produces slurry of impure hydrous oxides of Ln
- Add HCl with pH 3
- Produces impure solution of LnCl3
Describe the process of extraction from bastnaesite
- Mineral dressing
- Produces CO2
- Add H2SO4, 200 degrees
- Leach H2O
- Produce solution of Ln2(SO4)3
What are the problems with extraction from native ores using alkali/acidic methods
- Enormous amount of waste
What is history of separation and isolation of Lanthanide
- Until 1950 most separations involved hundreds of tedious fractional crystallisations of compounds
Which two Lns were easier to extract
- Cerium and Europium
- Exploit availability of 4+ and 2+ OS respectively
Give an example of extraction of Ce
- Oxidise only Ce to M4+ using HClO or KMnO4, KIO3
- Ce3+ –> Ce4+–> Ce(IO3)4 precipitates
Give an example of extraction of Eu
- On action of Zn/Hg, only Eu forms a stable M2+ that does not reduce H2O
- Then isolate by precipitation as EuSO4
- Eu3+ –> Eu2+ –> EuSO4 precipitates
What are ion exchange methods
- Spin off from the Manhattan Project- makes use of the lanthanide contraction
- The mixed lanthanides are loaded on to a cation-exchange resin and then eluted with a suitable complexing agent like EDTA or citric acid
How does the ion exchange method separate the Lns
- Heavier, smaller lanthanides are better Lewis acids so form stronger EDTA complexes
- Removed from the resin first in the order of their stability constants e.g. decreasing atomic number
Describe the process of solvent extraction
- Favoured process since 1960s
- Aqueous solution of the mixed lanthanide(3+) cations extracted into a non-polar organic liquid (kerosene)- contains around 10% of DEHPA or TBP
- As in chromatographic process, smallest/heaviest lanthanides form the most stable TBPO complexes and are extracted most efficiently in a series of continuous counter-current extractions
What methods are used to extract Ln
- Extraction from natural ores
- Ion exchange
- Solvent extraction
What is a new way that could be used to extract Ln
1.Using magnetic moments information and big magnetic field
How can you produce elemental metals
- Individual lanthanides prepared by metallothermic reduction of the anhydrous lanthanide fluorides or chlorides with calcium meal
- LnX3 –> Ln
What are the conditions for the metallothermic reduction to produce Ln elements
- Performed under argon and the product is a Ca/Ln alloy from which the Ca is removed by distillation as has lower BP
- 300 degrees
When can metallothermic reduction not be used to produce elemental metal
- Metals with tendency to form a divalent state (Sm, Eu, Yb)
- Only reduce to the 2+ state
How do you produce elemental metals when metallothermic reduction cannot be used
- By reduction of their oxides with lanthanum
- 2La + M2O3 –> La2O3 + 2M
- M= Sm, Eu, Yb
- The divalent lanthanides are more volatile and can be removed by distillation
Describe the properties of the Ln metals
- Rather soft
- Silvery white
- Later Ln are harder
- Tarnish in air due to their highly electropositive character
Describe the reactivity of the Ln metals
- Simple Ln compounds are strongly ionic
- Burn easily in air to form sesquioxides, M2O3
- React with N2 slowly to form ionic nitrides LnN
- Undergo exothermic reaction with H2 to yield ionic hydrides MHn (n=2,3)
What is produced when Ln + H2O
- M2O3 or M(OH)3 + H2 - slow if cold, rapid if heated
What is produced if Ln + H+ (Dilute acid)
- Ln3+ + H2 - rapid at RT
What are uses of metals
- Mischmetall - 50%Ce, 25%La, 25% other light Lns
- Pyrophoric- allowed with 30% Fe it is used in lighter flints
- <1% Mischmetall or Ln silicides improves the strength and workability of low alloy steels for plate and pipes
Comment on values of magnetic moment and why Sm is different
- The values should be correct due to the magnitude of spin-orbit coupling as Sm is a heavy atom and the limited interaction of the 4f valence orbitals with the ligand field hence no quenching
- means there is no quenching of L (orbital angular momentum)
- but Sm shows deviation from expected moment
due to low lying excited J states
Describe why all simple salts of Ln have the same colours
- The colours in trivalent lanthanides originate from f-f transitions.
- These originate from disparate energies of the S, L and J-states i.e. repositioning the electrons in different orbitals, pairing of electrons, or changes in spin orbit coupling, not crystal field
- and are thus not dependent on environment
- Dependent on identity of the element as the 4f orbitals are core-like and do not interact with the ligands
