Lanthanide Complexes 2

Question 1

Describe bonding in lanthanide complexes

Answer 1

1. 4f orbitals are deep-seated and hence little involved in ligands binding
2. Bonding in lanthanide complexes and solid state compounds is ionic in nature
3. Largely non-directional as this suits the sterics the best e.g. not square planar
4. Coordination chemistry is more in common with that of groups 1,2 and 13 than that of d-block

Question 2

What are the 3 principles that determine bonding

Answer 2

1. Lanthanide ions behave as hard Lewis acids. They thus have high affinity for hard bases such as F- and H2O.
2. The bonding is largely non-directional and electrostatic in origin. Coordination geometries are largely determined by the size and shape (steric demands) of the coordinating ligands.
3. The f-elements form large cations and thus support high coordination numbers. The size of the Ln3+ cations decreases across the series leading to higher charge densities and stronger ionic bonds for the heavier members.

Question 3

Are Lns contracted

Answer 3

1. No
2. The 4f orbitals are
3. The Ln are big ions

Question 4

What ligands are the Ln happiest to bind to

Answer 4

1. Cl, CCPh, Br, Ou

Question 5

What ligands are the Ln happiest to bind to

Answer 5

1. Cl, CCPh, Br, OBu

Question 6

What happens when LnCl3 is dissoluted in water

Answer 6

1. Dissolution of LnCl3 in H2O results in the formation of [Ln(H2O)x]3+
2. where x = 9 for the first half of the series
3. and x = 8 from Gd onwards.

Question 7

What is structure of hydrated lanthanide ions

Answer 7

1. Tricapped trigonal prismatic - tysonite

Question 8

Describe pH of aqueous Ln

Answer 8

1. Acidic as a result of hydrolysis
2. Due to highly polarising nature of Ln3+ cations
3. [Ln(H2O)9]3+ <–> [Ln(H2O)8(OH)]2+ + H+ <–> [Ln(H2O)7(OH)2]+ + 2H+ etc
4. As water coordinates to lanthanide oh bonds weaken as electron density from Oxygen stabilising OH-

Question 9

What happens to pH of aqueous solutions across the group

Answer 9

1. Increasing acidity
2. Ln3+ ions reduce in size- increasing charge density
3. example of lanthanide contraction

Question 10

Describe of aqueous compounds are labile or stable

Answer 10

1. Aqueous compounds are labile
2. Have high rate of exchange
   . exchange of water - diffusion controlled

Question 11

Describe the stability constant of lanthanide complexes

Answer 11

1. A stability (formation) constant, K1, can be written for any complexing reaction between a Ln3+ ion and a ligand Ln–.
2. M3+(aq) + Ln–(aq) <–>ML(3–n)+ (aq)
3. K1 = [ML(3-n)+(aq)] / ([M3+(aq)][Ln–(aq)] )

Question 12

What is needed for a stable Ln complex

Answer 12

1. The noted high rates of exchange of ligands in Ln3+ complexes makes the isolation of their coordination complexes difficult – a kinetic effect.
2. Polydentate ligands form especially stable complexes because of the chelate effect i.e. the favourable entropy change involved in the process – ie thermodymanic stabilisation.
3. May have strong affinity for F- but not stable so would just be hydrated

Question 13

What halogens do lanthanides form most stable complexes with

Answer 13

1. The lanthanides form more stable complexes with hard fluoride than the softer halogens.

Question 14

Does Lu or La have higher stability constants

Answer 14

1. The values for Lu are consistently higher than for La as expected for the smaller ion with a higher charge/radius ratio.

Question 15

Where are irregularities seen in stability constants

Answer 15

1. Discontinuity often seen between Eu/Gd/Tb called the “gadolinium break”
2. originates from variation in x for [Ln(H2O)x]3+ from 9 to 8 at Gadolinium.
3. Decrease in H2O coordination number - decrease in positive value of deltaS

Question 16

What are ion exchange methods

Answer 16

1. Spin off from manhattan project
2. Makes use of lanthanide contraction - entropic drive- chelate effect
3. THe mixed lanthanides are loaded on to a cation-exchange resin and then eluted with a suitable complexing agent like EDTA or citric acid
4. Heavier, smaller lanthanides are better Lewis acids
5. Removed from resin first in order of their stability constants i.e. decreasing atomic number

Question 17

What can access 4+ oxidation states

Answer 17

1 Only Ce4+ stable in aqueous solution for weeks, for example cerium ammonium nitrate (CAN)- important oxidant:
2. Ce2O3 + HNO3 + NH3 –> Ce(NO3)6(NH4)2 + H2O
3. Ce4+ has higher charge density than Ln3+
4. Much better at ionising water due to higher charge density
5. Stable for weeks at a time (12 coordinate)
6. Also can cerium ammonium sulphate

Question 18

Why is +4 OS accessible for Ce

Answer 18

1. Chemically accessible due to higher energy of 4f orbitals at start of series - similar in energy to 5d orbital
2. Not sufficiently stable to prevent the loss of the 4th e-
3. Ce3+ has 1 f electron - only one away from completely empty shell - closed shell is stable

Question 19

Which elements can access Ln(II) chemistry

Answer 19

1. Sm, Eu, Yb

Question 20

Show how Ln2+ can be accessed

Answer 20

1. LnCl3 + 1/2 Zn –> Ln2+

Question 21

Describe stability of Ln(II) Aqueous complexes

Answer 21

1. not stable long term,
2. with Eu being stable for hours
3. Sm, Yb for minutes - faster oxidation
4. Eu 3+ –> Eu2+ E0 is less negative than for other two

Question 22

What is Sm(II) Iodide used for

Answer 22

1. Important reagent in organic synthesis
2. Very bright colour- good colour changes + most reactive

Question 23

Why is Eu2+ aqueous complex more stable than Sm and Yb

Answer 23

1. Eu>Yb>Sm
2. Eu more stable with respect to own +3 forms
3. Eu> Sm due to higher exchange energy
4. Eu> Yb : 7 rounds of pairing energy gives stability away
5. Pairing energy not as important as exchange energy why Yb>Sm
6. Sm is stronger reducing agent

Question 24

Describe paramagnetism of Ln3+ compounds and consequence for spectra

Answer 24

1. Ln3+ compounds are strongly paramagnetic which precludes useful NMR data (peaks are broadened and shifted).

Question 25

How can Ln be used in NMR

Answer 25

1. Lanthanide shift reagents can be added to diamagnetic samples to induce chemical shift changes via through-space interactions.
2. Occurs via transient coordination of substrate to Ln centre.
3. Interaction between proton spin and spin of magnetic moment- sharpens peaks and moves peaks apart- spreads out

Question 26

What are the most common chemical shift reagents

Answer 26

1. Eu3+ and Pr3+ ions with biketonate (acac) ligands
2. Have short relaxation times reducing broadening in 1H NMR spectra
3. Eu3+ shifts downfield
4. Pr3+ upfield

Question 27

Give example of when chemical shift reagents are used

Answer 27

1. Chiral compounds
2. Allows determination of ee by integration
3. Normally wouldn’t see both enantiomers
4. One enantiomer reacts more readily with Sm than the other
5. Relies on dynamic coordination

Question 28

What is needed for ligands in aqueous Ln chemistry

Answer 28

1. Must overcome hydrophilicity
2. Tend to rely on hard donors (N,O)
3. Tend to be polydentate
4. Allows applications in medicine - stops Ln interacting with body as stable

Question 29

Describe how MRI works

Answer 29

1. Contrast in a magnetic resonance image (MRI) is due to the different relaxation times of water protons in different environments within the body
2. Protons with short relaxation times give rise to brighter images
3. Enhance the contrast in a magnetic resonance image with ‘contrast agent’
4. Complexes of Gd3+ are widely used due to paramagnetic (4f7 configuration)
5. Cause greatly shortened 1H NMR relaxation times for coordinated H2O molecules - rapid exchange with bulk H2O

Question 30

What is needed from a ligand in in vivo applications

Answer 30

1. Necessary to have a complex which is soluble in physiological media and in which Gd3+ is strongly bound - stop release in body
2. Must have at least one free site for coordination of H2O
3. Ligands must be water stable, and give solubility in i.e. blood such as DTPA (above).

Question 31

Which of the Ln 2+ ions is the most useful reducing agent

Answer 31

1. three lanthanides form stable but reducing Ln2+ ions.
2. Sm2+ is the least stable (Sm3+/Sm2+ – 1.48 V) and has found use as a selective organic reductant, usually in the form of SmI2.

Question 32

How can SmI2 be made

Answer 32

1. SmI2 may be conveniently prepared as a deep blue ether solution by reaction of Sm metal and ICH2CH2I.
2. Sm + ICH2CH2I –> CH2=CH2 + SmI2(THF)x
3. Need to be free of air otherwise Sm2+ oxidises rapidly to become Sm3+

Question 33

Complete equations for reduction by SmI2
Ph2S=O + SmI2
PhCH2I + SmI2

Answer 33

1. Ph2S=O + 2SmI2 –> Ph2S + SmOI
2. 2PhCH2I + 2SmI2 –> PhCH2CHPh + 2 SmI3

Question 34

What oxidants are widely used

Answer 34

1. Ceric ammonium nitrate [Ce(NH4)2(NO3)6] (CAN) and the analogous sulphate (CAS) are widely used as selective one electron oxidants.
2. Recall Ce4+ (4f0) is the only member of the lanthanide series with a well developed 4+ chemistry.
3. Can turn OH –> =O

Question 35

What are 3 organic oxidations that CAN or CAS can do

Answer 35

1. Benzoin condensations (CHO -> 2x C=O
2. Ortho oxidations (OH –>=O
3. alpha-C oxidations ( C=O added to carbon chain)

Question 36

Describe how self cleaning ovens work

Answer 36

1. Oxidises fat to water
2. Rely on oxidising ability of Ce4+ oxide - Walls are coated in CeO2
3. 2CeO2 + R2CH2 (Fat) –> Ce2O3 + H2O + R2C
4. 4CeO2 + R2C –> 2Ce2O3 + CO2 + R2
5. 3Ce2O3 + 1 1/2 O2 –> 6CeO2
6. Overall R2CH2 + 1 1/2 O2 –> R2 + H2O + CO2
7. Completely catalytic process

Question 37

Describe how catalytic converters work

Answer 37

1.