4: Coordination Chemistry

Question 1

What are 4f orbitals like?

what effect does this have on bonding?

Answer 1

4f orbitals are core like and uninvolved in covalent bonding
Resulting in ionin, non directional bonding in lanthanide complexes
Hard lewis acids- high affinity for hard bases such as F- and H2O
more similar to s-block elements than d block

Question 2

4 additional bonding facts

Answer 2

1) f elements form large cations and support high coordination numbers (8/9 for h20 solid 12ish)
2)bonding is largely non-directional and electrostatic in origin –> coord geom determined by size and shape of coordinating ligands
3)Size of the 3+ cation decreases across the series –> heaver elements have higher charge densities and hence stronger ionic bonds
4) Rates of ligand exchange are very fast and on the order of diffusion nano s


Question 3

How do you work out a stability constant?

Answer 3

Picture

Question 4

Study of logK values tell us?

Answer 4

larger values, mean more stable the ligand complexes
negative values mean aqua complex is more stable than the ligand complex
F only halide that forms stable complexes relative to aquo complexes
Chelating compounds form most stable complexes
late Ln higher than earky Ln- charge density

Question 5

Aquo complexes

Answer 5

-hydrated Lanthanides studied by X-ray crystallography
-have a tricapped trigonal prismatic structure
-Dissolution of Ln(Cl)3 Ln3+ forms 8 or 9 coordinate
[Ln(H2O)x]3+
-Early tend to be 9 and later tend to be 8

Question 6

aqeous solutions of lanthanides

Answer 6

-they are acidic as a result of hydrolysis
PICTURE
-result of the highly polarizing nature of the LN3+ ions and the ionization of the lanthanide coordinated H2O which becomes increasingly facile as the Ln3+ions reduce in size
exchange of H2O very rapid, lifetimes nanoseconds long
Ln2+ reduce and 4+ oxidise H2O respectively

Question 7

What is the Gadolinium break

Answer 7

The hypothesised point where H2O coordination number drops from 9 to 8
Decrease in the change in entropy

occurs w other ligands edta
number of H2o kicked out/remains
changes

Question 8

Chelate and macrocyclic complexes

Answer 8

polydentate ligands form espesially stable complexes due to the chelate effect. The favourable change in entropy results in a more favourable deltaG of the reaction
formation constant much higher than for monodentate ligands


Question 9

High coordinate Lanthanides

Answer 9

-Negliible CSFE between 4f and ligand orbitals no coordination geometry preffered
Achieved by small hard polydentate ligands with small bite angles (LML angle)
eg N-crown-n w/ halides
eg. NO3, bidentate ligand
UP TO 12

Question 10

Low coordinate ligands

Answer 10

-Negliible CSFE between 4f and ligand orbitals no coordination geometry preffered
DOWN to 3
requires use of very bulky ligands eg, N (SiMe3) and tbu3 etc etc
Siz of the R group has a profound effect on nuclearity and reactivity
MUST BE PREPARED in absense of water aka in schlenk

Question 11

Application: NMR chemistry; intro

Answer 11

Ln3+ complexes strongly paramagnetic- not give useful NMR spectroscopic data (unusual peaks and broad peaks

Lanthanide shift reagents are added to diamagnetic samples to induced chemical shift changes through space interactions upon coordination of the organic molecule to the ion.

Common reagents are Eu(III) and Pr(III) as they have short relaxation times that reduce broadning usually w acac or acac based ligands

used to be used before better magnets developed now only used for chiral separation

Question 12

Application NMR chemistry how it works

Answer 12

Ligands impart solubility in organic solvents
Transient coordination of a heteroatom to the lanthanide centre- simplifies the spectrum

Pr(facam)3, a chiral reagent
spectra of complexes formedallows the determination of ee by integration

Each enantiomer of an organic molecule can interact differently with a single enantiomer of the NMR shift reagent. Forming two diastereomeric intermediates that can be identified by 1H NMR spectroscopy.

Question 13

Application: MRI scanner

Answer 13

MRI scanners detect the 1H NMR signals of water in the tissues of the human body, contrast in images is due to different relaxation times of water protons in different environments within the body (shorter relaxation time give brighter images

The contrastin a MRI can be enhanced with a contrast agent

Question 14

MRI: what is used as a contrast agent?

Answer 14

Gd3+ widely used
paramagnetic cause greatly shortened 1H NMR relaxation time for coordinated H2O molecules with rapid exchange with bulk H2O
used w DPTA and one H2O

complexes can be distributed in extracellular fluids to see where they travel
can be attacthed to a label to be concentrated in tumours
-signal enhancement shown where complex travels.

Question 15

Actinide bonding summary

Answer 15

5f orbitals greater relative radial extension as relativistic effects play a greater role
Early- TM like
Late- Ln like
Bonding has a degree of covalency

Question 16

Low coordination Actinides

Answer 16

-Anhydrous conditions (schlenk)- to avoid formation of the actinyl ions, water must be excluded
-similar to Ln complexes by we are not limited to a single oxidation state, Uranium coordination complexes are known for 3,4,5 and 6 oxidation states
Th 4 is favoured but can vary
U and Th bulk of the chemistry

Question 17

General coordination chemistry of the actinides

Answer 17

Coordination numbers are generally high due to the large size of the actinide ions and geometry is largely determined by the sterics of the ligands

Coordination numbers mainly between 3 and 14

Question 18

Aqeous actinide complexes

Answer 18

Up to U:
Behaves like a group 3/4 TM::ThO2 eg in gas phase has a bent structure-like TM

U-Am
forms pentavalent and hexavalent actinyl ions AnO2(+) AnO2(2+) - essentially linear

Cm -Lr
Behave like the Lanthanides

Question 19

Actinyl ions

Answer 19

essentially linear O=An=O units (cf. TMO2 usually bent)
most actinides complexes with OS states higher than +4 contain these linear actinyl groups

The auxillary ligands are thus in the equatorial plane and the O atoms are trans to each other (in TM they are mostly cis to each other) This provides evidence for 5f orbital participation and covalency

Can fit in 4 to 6 donor ligands in the equatorial plane
(octahedral, pentagonal/hexagonal bipyramidal)
hex only possible w/ 2 or more bidentate ligands eg CO32- or NO3-

Question 20

what does the structure of the actinyl ion show?

Answer 20

Structure provides evidence of the involvement of 5f orbitals U-O distance v short 1.8A ish cg. 2.2A., they are IR active
Short distance and force constant not as expected for IR theory
not just U=O double bond but involves donation of a LP into the metal (dative bond)

Th due to 5f orbitals being lower in energy than 6d and hence accesible for bonding

Question 21

MO of UO2(2+)

Answer 21

d and f orbitals of the uranium overlap with oxygen p-orbitals to give both sigma and pi bonding MO’s

f orbitals have ungerade symmetry, d orbitals are gerade
MO: LHS: 5d>5f> 6p>6s RHS O2 MO

sigma orbitals are higher in energy due to unfavourable out of phase interactions 3su>3sg>1pg>2pu 2/2/4/4
bond order=16-4=12 % 2=6 - two triple bonds UO2(2+)

PICTURE

Question 22

Actinyl ions: how are they activated

Answer 22

uranium actinyl: urinyl
-Chemists have devised a trap in which to catch and modify UO2(2+) which is usually chemically inert
-Pacman ligand created which can bind with the UO2 and complex it to a TM
-The additional metal atoms interact with one of the U=O bonds
-After which a one electron reduction and silylation of the UO2 group
-UO2 to O=U-OR(+)
therefore the U=O strong bond is weakened and hence activated

Question 23

Dealing with nuclear waste

Answer 23

Uk nucelar power- 18% of electricity and only increasing
reprossing of nuclear fuel done in the UK
Disposal of spent nuclear fuel rods is a massive environmental problem