Lanthanides Flashcards
f-orbitals
7 orbitals, ml = -3 to +3, ungerade. Some are ‘banana shaped’
Exceptions in electronic configuration
Close lying 5d and 6s orbitals:
La: [Xe]6s2 5d1
Ce: [Xe]4f1 6s2 5d1
Pr: [Xe] 4f3 6s2
Gd - half filled shell
Eu: [Xe]4f7 6s2
Gd: [Xe]4f7 6s2 5d1
Lu - filled shell
Yb: [Xe]4f14 6s2
Lu: [Xe]4f14 6s2 5d1
Oxidation states dominated by Ln3+
- f-orbitals are core-like
Exceptions: Eu2+ = 4f7; Yb2+ = 4f14; Ce4+ = 4f0; Tb4+ = 4f7 - Smaller IEs - 4th IE is more than first three combined.
- Large hydration enthalpies proportional to z:r (-ΔH for new bonds between Ln(III) and H2O).
Ln4+ > Ln3+ > Ln2+ = increased electrostatic attractions
Lanthanide contraction
Greater than expected decrease in atomic radii. Similar to 4d/5d TMs.
1. In the period, Zeff increases and inner shell e do not shield as effectively. Therefore, outer shell electrons are drawn towards the nucleus. 2. As elements get heavier, s electrons gain velocity and mass, contracting the nucleus.
Periodic trends
Coordination chemistry of Ln
- f-e in Ln3+ are core-like and do not interact with ligands. Therefore, electrostatics dominate the bonding.
- f-orbitals do not split and remain degenerate
- Ln3+ are hard Lewis acids, will react with hard Lewis bases = N, O, F - oxophilic.
- CNs = however many can fit, electrostatics + coloumbic
- ‘Non-directional’ bonding - Early Ln = Ln(H2O)9 3+, Late Ln = Ln(H2O)8 3+ - Gd break - minimum repulsion.
- Polydentate ligands take up less room than equiv. number of monodentate ligands.
Complex stability
Chelate effect
1. Enthalpy −ΔH - compare energies of broken and formed bonds.
2. Entropy +ΔS - increasing number of freely moving molecules
Macrocyclic effect - ligands are pre-organised for bonding. eg crown ethers.
Luminescence
Spontaneous emission of light at relatively cool environments.
d-block - energy is lost through vibration of ligands - excited state short-lived (vibronic coupling).
f-block - ligands have minimal effect - ES long lived.
Fluorescence - spin allowed, fast.
Phosphorescence - spin forbidden, slow.
Magnetism: J = L+S, Russel-Saunders coupling
Spin-orbit coupling
Can’t assume spin only formula, have to account for spin-orbit coupling.
Experimental exceptions for the magnetic moment calculations: Sm3+ and Eu3+. Low-lying excited states appreciably populated at room temperature. Solution:
a) measure µ at low T.
b) include population of excited states in the calculation.
LnIII + L -> LnIIIL
Complex stability increases from La to Lu (higher charge density z:r)
Higher denticity - higher stability constant
Electronic absorption spectroscopy
- 4f -> 4f
Electric dipole forbidden (Laporte selection rule), u -> u, ε < 1, SHARP - 4f -> 5d (nf -> (n+1)d)
Laporte allowed (g→u); more intense than f→f, Broader, typical ½-width = 1000 cm-1.
More common in Ln(II) due to the smaller f-gap - Ligand → 4f C.T
Laporte allowed; high intensity; Broader than f→d. Favoured if Ln is fairly Lewis acidic.
f → f - forbidden due to vibronic coupling (mixing between different electronic states as a result of small vibrations) between f-orbitals and ligands, as a result lowering the symmetry.
Consequences from electronic transition spectroscopy
- f-f transitions are less intense that d-d leading to pale colours.
Exception: Ln2+ ions = often highly coloured as 4f orbitals now closer to 5d-orbitals. f-d
transition now occurs in visible.
Easily reduced Ln3+ ions (e.g. Eu, Yb) often show high intensity LMCT bands especially if the ligand is easily oxidised - Sharp peaks assigned to specific transitions - no crystal field effects.
- Spin-orbit coupling (SOC) - spin forbidden transitions have significant intensity, leading to more lines than d metal.
- Spectrum is unaffected by changes in ligand or geometry, meaning that it is a fingerprint for specific metal.
Problems with lanthanide luminescence
- f-f is Laporte forbidden - hard to excite Ln from G.S. to E.S
- LnIII luminescence can be destroyed by (a) molecular O2; O=O stretch, the formation of singlet 1O2. (b) high frequency oscillators = ν(O-H) of H2O etc
Solution to lanthanide peak quenching - sensitised luminescence
Excite via (conjugated) organic ligand (the antenna).
- Singlet ground state.
- Singlet excited state from pi to pi*
- Intersystem crossing (ISC - one state to another of different multiplicity without the emission of radiation) - singlet to triplet.
- Energy transfer from triplet organic ligand to Ln*.
Quenching of sensitised luminescence
A) Reverse processes can occur at each step.
B) Non-radiative decay can occur from any state
