f-block Flashcards
Why can free ion model be used for f-block elements?
Spectra is almost same in the gas, solid and liquid phase
How are f->f transitions present on an absorption spectra?
As the e- are not in bonds they are sharp transitions
Weak transitions as parity forbidden ( u->u)
What causes intense transitions for lanthanides in absorption spectra?
From allowed transitions:
u -> g
4f -> 5d
How does spin orbit coupling change across lanthanides?
Increases
Causes larger gaps at the same L
Why is lanthanide luminescence long-lived?
Forbidden nature of f-f transitions
What is the energy gap law?
Emissive state is state with largest energy gap to next state lower in E
What is the crystal effect like on lanthanides?
Crystal field lifts degen to give different mj states
What is the free ion approx?
4f orbitals don’t overlap sig with ligand orbitals
Ligand field splittings insig wrt kT
4f orbitals in Ln3+ are degen and I=3 for 4f
When does the spin-only formula apply to a lanthanide?
4f7 as equal occuption of all orbitals
How can the magnetic moment (μeff) be calculated?
μeff = gJx Sqrt[J(J+1)] x μb
In units of μb
How does the spin a.m. contribute to magnetic moment?
μeff = gs x Sqrt[s(s+1)] x μb
gs = 2
s = total spin q.n.
Only works for Gd but works for d-block
How does the half-life of actinides change across the series?
Half-life decreases across the series
How does the energy of f-orbitals change across the f-block?
Start: nf orbitals comparable to (n+2)s and (n+1)d
Across period then all drop in energy (drop is slower for actinides due to radial node in 5f)
How can lanthanides be described wrt ionic/covalent and why?
Ionic
Due to limited penetration of 4f
LFSE v. small
How can actinides be described wrt ionic/covalent and why?
More covalent than lactinides
Due to core e- in actinide contracting due to relativistic effect - mass so large than e- experience relativistic contraction
How do e- fill lanthanide orbitals?
General: 4fn6s2
Exceptions:
Ce, Gd, and Lu
Configuration fo electrons in Ce, Gd, and Lu?
Ce (low Zeff for 4f early in the series): 4f15d16s2
Gd (e- repulsion in 4f8): 4f75d16s2
Lu:
4f145d16s2
How does the 4f change across the period?
4f drops rapidly
Max in rdf moves inside 5p core sub-shell so core-like and no sig overlap with ligand orbitals
What are the results of charge-sensitivity of 4f Lanth electrons?
3+ ions have config 4fn
Bonding in 2+ or higher oxn states are highly ionic
Low oxn states are metallic but orbital overlap involves 5d and 6s orbtials (not 4f)
What is the trend of third ionisation energies?
Decrease from Sc->Y->La as n2 increases faster than Zeff2
General increase as Zeff2 increases faster and exchange energy increases to Eu
Dip at Gd & Lu as removing e- from higher energy 5d orbital
What causes deviations from the ionisation energy trend?
1/4 & 3/4 shell effect
What is the 1/4 & 3/4 shell effect?
Ionisation of 4f e- leads to decrease of L
Causes I3 to increase
1/4 includes: Pr, Nd, Pm
3/4 includes: Dy, Ho, Er
Effect larger in 3/4 as stronger e- repulsion
How does atomisation energy change across the period?
General decrease from La -> Eu and Gd -> Yb
Metallic bonding requires promotion from 4f -> 5d/6s band, and 4f drops in E more than 5d/6s so more E required to form metallic bonds and atomisation energy decreases
Increase from Eu->Gd and Yb->Lu as Gd/Lu have 5d e- so stronger metallic bonding
2nd period higher than expected as reduces E of spin pairing, so promoting e- to regain this is easier
How does the radius M3+ change across the lanthanides?
Radii decreases gradually due to poor shielding of 4f
Means Zeff increases gradually and LFSE small
Ions have higher charge density over period & attracts H2O more strongly so more energy released from Coulomb interaction
What oxn states are present in lanthanides?
I3 compensated by high lattice & hydration enthalpies
I4 high so needs F- or O2- and either Ce, Tb, and Pr
I2 unstable wrt disproportionation other than Eu2+ & Yb2+, which has high I3
How does the 5f change across the actinides?
Start: similar to 6d and 7s
Along the series 5f drops below but slower than 4f due to radial node in 5f
What are the electron config of actinides?
5fn7s2 and 5fn-17s2 compete
5f -> 6d for more bonding e- much easier than 4f -> 5d for lanthanides
How covalent are the actinides?
5f have greater radial extension than 4f so more covalent
So more complex forms, even with neutral/pi-bonding ligands
What are the oxidation states of actinides?
Early actinides - high oxn formed easily
Radii contracts across the group
Up to +4 then can treat actinide ion as spherical
+5 and above then linear dioxo ions
What is the structure of PrO2 and TbO2?
Fluorite structure
What is the trend of disproportionation of +2 lanthanides?
Trend dominated by I3 and if large then stabilises
Includes:
Max -> Eu2+ (4f7) and Yb2+ (4f14)
Near max -> Sm2+ and Tm2+
What is the coordination sphere of lanthanides?
Irregular as ionic bonding is non-directional and coordination geometry determined by ligand packing/repulsion
As ionic radii decreases across series there is a tendency for coordination number to drop from 8-9 (and 12 in complexes) to 6-8, especially when larger anions
What structure do lanthanide halides have?
9 coordinate from La to Eu and 8 coordinate from Gd to Lu
Switch occurs earlier when heavier halide as more repulsion due to smaller cation
How acidic are the aqueous lanthanides?
Hydrolysis increases across series as the ionic radius decreases
What redox chemistry occurs in lanthanides?
Little redox chem except Ce4+/Ce3+ couple
What is the stability of lanth complexes with neutal and anionic N/O ligands?
Neutral ligands - generally unstable, requires non-aq solution
Anionic ligands - form readily with hard acids
Thermo driving force is entropy gained when charges are neutralised on complex formation & water molecules solvation released from metal ion and anionic ligand
How are mono/polydentate ligands favoured by lanthanides?
Unidentate - weakly coordinate, sometimes solvent coordinates in addition to poly
Polydentate - chelates more stable
Do lanthanides form carbonyl complexes?
No as the 4f e- are not available for π-donation
Unlike carbonyl complexes
How stable are organometallic lanthanide complexes?
Ionic bonding
Air and moisture sensitive -> hydrolysed instantly by water
Highly electrophillic as few anionic ligands to satisfy electroneutrality principle, so attract e- density from weakest donors (N2, alkenes)
What are lanthanide complex with carbanionic ligands?
Not sterically hindered - dimeric/oligomeric/polymeric and insoluble in organic solvents
Sterically hindered - monomeric soluble compounds, and prevents ligand exchange
Why are lanthanides good catalysts?
Strongly electrophilic catalysts so attract e- density from ligands
Weak interaction results in them activated for attack
What actinides are stable in the 2+ oxn state?
Common for only heaviest actinides (Cf -> Lr)
Due to largest Zeff
Similar properties to Ba2+
What actinides are stable in the +3 oxn state?
Most common, most stable after Am
Low IE for early one causes this
Similar properties to lanthanides
What actinides are stable in the +4 oxn state?
Common for Th->Pu
Many dioxides (fluorite structure), oxy halides and tetrahalides
Th4+ similar chem to Zr4+ and Hf4+
V.oxidising after Am
Hydrolysed aquo ions, rich coordination and redox chem
What actinides are stable in the +5 oxn state?
Found up to Am - only as F, oxy halides, oxides, and oxy anion salts
Aqueous: AnO2+ for U, Np, Pu, and Am
Principal oxidation state for Pa
Multiple bonding involves d and f
What actinides are stable in the +6 oxn state?
Important oxn for uranium -> uranyl ion UO22+
Same ions also found for Np, Pu, Am
What actinides are stable in the +7 oxn state?
Highly oxidising
NpO53- and PuOsub>5</sub>3-
How is uranium isolated?
Uranium ore -> UO22+
1st step: ion exchange chromatography then tri-n-butylphosphate
Then:
UO22+ + heat -> UO3
UO3 + H2 + heat -> UO2
UO2 + HF + heat -> UF4
UF4 + Mg + heat -> U
