Lanthanides Flashcards
why are lanthanide salts colourless/pale compared to d block salts?
They are very low intensity as d/d and f/f trs are laporte forbidden but these can become allowed if mixing occurs. F orbitals do not feel changes in geometry or or molecular vibrations, and remain pure f-orbitals with no mixing. So f-f transitions remain very weak. Unlike d orbitals where d/p mixing occurs.
why are f/f trs absorption peaks narrow by d/d absorptions peaks are broad?
d-d transitions are broad because vibrations of ligands affect d-orbital energies. As ligands vibrate the bond lenght changes and ∆oct changes. f orbital energies are unaffected by ligand vibrations so f-f
absorptions are fixed in energy and peaks are very narrow.
why do spin forbidden transitions occur easily for f/f trs but nor for d block elements?
High spin-orbit coupling for heavy atoms means that pure ‘spin’ quantum number is no longer valid (S and L mixed up in J) so spin selection rule is evaded.
Why do lanthanides photoluminescene?
Normally excited states lose their energy to mol vibrations. But weak interactions between f-orbitals and vibrating ligands mean that likelihood of f-f excited state energy being converted to molecular vibrations is low, so energy of excited state is emitted as a photon: luminescence.
when is luminescence strong?
when there is big energy gap between excited
state and ground state manifold (as here) because the energy gap is too large for any one vibration to take the energy.
true or false: The triplet excited state of the ligand needs to be higher in energy than the luminscent f/f level?
True
How do fluorescent lightbulbs work?
Red, green, blue solid state materials combine to give white light. excited state generated by UV light absorption from Hg vapour discharge. UV light re-absorbed by Ln3+ ions to give f/f excited states which emit a photon.
How do OLED`s work
stable and volatile diketonate complexes which are deposited across a thin film. High potential across device which injects eletrons and hole pairs at opposite. The energy released from their recombination induced lanthanide exited state - luminescence.
How do Nd(III) - YAG lasers work
Y3Al5O12 doped with ≈1% Nd3+ ions in Y3+ sites
irradiation of Nd3+ ions promotes ions from ground state (4I9/2) to manifold of levels state from 4F3/2 upwards
4F3/2 state is long-lived so end up with more Nd3+ ions in excited state than ground state (‘population inversion’)
these can be stimulated to collapse simultaneously resulting in intense burst photons with burst duration of ≈ 10 ns
Repopulation of excited state takes milliseconds: result is series of short pulses of laser light
How does luminescent imaging of cells work?
strong luminescent complexes can be used to light up cell interiors. Can selectivity light up different parts of the cell using molecules that localise in specific parts of the cell. More intense luminescence due to localisation of dye in protien rich areas due to interactions of proteins with dye.
why is the spin only formula applicable to d orbitals but not f orbitals.
this ignores contribution of orbital angular momentum. The movement of d electrons between orbitals generates orbital angular momentum but when the orbitals split electron is prevented from moving between orbitals and removes angular orbital momentum BUT In Ln3+ ions, orbital angular moment is (i) inherently larger (because L = 3), and (ii) not quenched by ligand-field splitting, so cannot be ignored.
how to get high µ values?
Large numbers of unpaired electrons (up to 7 for Gd3+) and high values of J
Why don`t Eu and Sm show good agreement of their calculated and observed values of µ?
spacings of energy levels close to ground state are small such that excited states with larger values of J are thermally accessible at RT:
Eu3+ has µ of zero despite being 7F0?
less than half filled. so L and S cancel each other
µ tends to 0 at low temperatures
when only 7F0 is populated why?
at low temperature these higher levels become depopulated: ( increase temp and higher levels populate - Appreciable population)
what does ∆δ mean?
weighted average of ‘normal’ and ‘shifted’ δ value depending what proportion of its time the substrate is ‘free’ or bound to Eu3+
How can lanthanides be NMR shift reagents?
Tris diketonates are good lewis acids and e rich organic molecules coordinate to them. The amount they are shifted depends on their proximity to the paramagnertic centre. Close they are they are shifted more
How can lanthanides be used to distinguish enatiomers?
Chiral shift reagent binds to one enantiomer of substrate more strongly than the other, giving differential shifts allowing enantiomers to be resolved eg Eu(facam)3
What affects the magnitude of ∆δ?
depends on the affinity of the lewis acid to the metal.stronger binding means that substrate spends more time bound to Eu3+ and is shifted more
(RNH2 > ROH > R2O > R2C=O > CO2R ≈ R2S > R-CN) – roughly correlates with ‘hardness’ of electron donor.
How are lanthanides used as a diagnostic tool for soft tissue?
Creates a 2D image by using HNMR signals of H in water. Uses a gradient magnetic field which varies at every point such that resonant frequency of 1H nucleus
depends on its spatial location.
• Intensity of signal depends on relaxation time (T1) of protons after excitation: a nucleus cannot absorb another photon until it has relaxed
How is Gd3+ used in imaging?
Highly paramagnetic Gd3+ complexes (4f7) have molecular vibrations which produce oscillating magnetic fields which stimulate excited 1H nuclei to relax faster. Effect proportional to 1/r6 so most effective on water molecules coordinated to Gd3+.
How do they make Gd3+ less toxic when used in soft tissue imagine?
Needs to be wrapped up in strongly binding ligand to give highly stable complex which does not liberate Gd3+ ions, Need a vacant site for water.
what type of ligands are used to bind to Gd3+ in soft tissue imaging?
anionic amino/carboxylate polydentate or ones with Hard N/O– donor set + strong chelate effect gives very stable complexes (log K > 20)
Ligands 7- or 8-dentate leaving space for 1 or 2 H2O molecules to bind to Gd3+
The distributions of Gd3+ in the body depends on what?
charge; hydrophobic or hydrophilic; ability to cross lipid membranes etc.) so different agents are used to visualise different parts of the body
How are tumors detected by Gd3+ complexes?
Magnevist’ is hydrophilic; does not cross cell membranes so accumulates in extra-cellular space. Tumours have more extracellular space than normal tissue so hydrophilic MRI agents will accumulate in cancerous areas and selectively enhance signals from those regions
How are gastro-intestinal tract / stomach illnesses imaged?
use [Gd(H2O)9]Cl3 trapped in pores of a zeolite: this suspension can be swallowed. Water molecules diffuse in and out of cavity to be relaxed by Gd(III)
what are the two differences in organometallic lanthanide(III) compounds compared to d-block organometallic compounds?
18 e rule not applicable- steric/electric effects dominate e count and coordination number.
No overlap with f orbitals and ligands means π back-bonding is not significant
what is the general route for σ-bonded organometallic lanthanide(III) complexes?
LnCl3 + 3 LiR → LnR3 + 3 LiCl
Ln(OR’)3 + 3 LiR → LnR3 + 3 Li(OR’)
the precipitation of stable Li salt is the driving force. in presence of excess LiR, reaction can go further if R is not too bulky
True or flase: are σ-bonded organometallic lanthanide(III) complexes water- and O2-sensitive?
True - they are very reactive
how are σ-bonded organometallic lanthanide(III) complexes stabilized?
stabilised by association with Li+ cations via bridging CH3 groups
Presence of additional coordinating groups to block metal sites can stabilise Ln(III)–alkyl complexes based on less bulky alkyl / aryl substituents
