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Inorganic > Actinides > Flashcards

Actinides Flashcards

1
Q

Naturally-occurring actinides

A

Actinium (Ac)
Thorium (Th)
Protactinium (Pa)
Uranium (U)

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2
Q

5f orbitals

A

4f and 5f orbitals do not different in their angular wavefunction (they have the same shape)
But 5f orbitals have a radial node (PQN has increased by 1)

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3
Q

Why can 5f orbitals interact with ligands?

A

5f orbitals have greater radial extension compared to 7s/7p than 4f orbitals have compared to 6s/6p
This confers some covalency to the metal-ligand bonding

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4
Q

Actinide Contraction

A

Zeff increases across the series which leads to a contraction of the 5f orbitals
The 5f orbitals become increasingly core-like across the series - the later actinides (from Am/Cm onwards) are “lanthanide-like”

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5
Q

Electronic configurations of actinides

A

Early actinides show easy promotion of electrons from 5f to 6d to provide more bonding electrons (because the orbitals are close in energy)
e.g. Th ground state electron configuration is 5f0 6d2 7d2 (because 6d lower energy than 5f)
Later actinides resemble the lanthanides in that that do not show this easy promotion

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6
Q

Electronic spectra of the actinides

A

Early actinides experience ‘vibrionic coupling’ due to interactions between 5f and ligand orbitals
This means f–>f transitions yield broad, intense bands in the absorption spectra
Later actinides show sharp, low intensity lines in their absorption spectra - resembling the lanthanides

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7
Q

Magnetic properties of the actinides

A

Actinides show strong spin-orbit coupling (2000-4000 cm-1), but the ligand field effects are also of comparable magnitude because of the interaction of the 5f electrons with the ligands
This means J is no longer a useful quantum number, because the J states are split by the ligand field
The ‘spin-only’ and Lande formulae are also inadequate

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8
Q

Why is J not a useful quantum for calculating the magnetic moment of actinide complexes?

A

Because the J states are split by the ligand field as a result of the interactions between 5f electrons and the ligand field

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9
Q

Why are the experimental magnetic moment values for the actinides lower than for the corresponding lanthanides?

A

Due to partial quenching of L.

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10
Q

+3 oxidation state

A

Accessible for all actinides
Only the most stable oxidation state for the later actinides due to the stabilisation of the 5f orbitals relative to 7s/6d

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11
Q

Which oxidation state of thorium dominates its chemistry?

A

+4

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12
Q

What is the most stable oxidation state of Pa and Np?

A

+5

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13
Q

What is the most common oxidation state of U?

A

+6

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14
Q

What is the preferred oxidation state of Am?

A

+3

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15
Q

Actinide halides

A

Group valency (i.e. loss of all valence electrons) is accessible up to U (6+)
After U, AnX3 is the most stable
AnX3 compounds resemble LnX3

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16
Q

What is the most important actinide halide?

A

UF6

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17
Q

Synthesis of UF6

A

UO2 + 4HF —> UF4 + 2H2O

UF4 + F2 —> UF6

18
Q

Actinide coordination chemistry

A

5f orbitals can overlap with ligand orbitals which introduces covalency into the bonding
Level of covalency decreases across the series as the later actinides become more “lanthanide-like”

19
Q

Why can actinides support high coordination numbers (up to 15)?

A

Actinides have large ionic radii

actinides have a propensity towards oligomerisation with non-bulky ligands

20
Q

What are the “late-early” actinides?

A

U–>Am

21
Q

“Late-early” actinides can form…

A

…pentavalent/hexavalent actinyl cations

i.e. AnO2(+), AnO2(2+)

22
Q

Shape of O-An-O units

A

Linear (trans)

This provides evidence for the use of 5f orbitals in O-An bonding

23
Q

Shape of TM ‘dioxo’ compounds

A

Bent (cis)

24
Q

Why is the uranyl cation so stable?

A

Because all the bonding orbitals are completely filled and there are no electrons in antibonding orbitals

25
Q

Why are later actinyl cations (Np–>Am) less stable than the uranyl cation?

A

5f orbitals are stabilised so are less available for bonding

Non-bonding MOs are occupied

26
Q

PUREX

A

= Plutonium Uranium Redox Extraction Process
Allows for the isolation for plutonium, uranium, americium and other fission products from spent nuclear fuel
An application of the variable oxidation states of the actinides and the high stability of UO2(2+) complexes

27
Q

PUREX process

A
  1. Spent fuel rods are dissolved in 7 M HNO3 and filtered to remove the insoluble Zr casing. This yields an aqueous solution of UO2(NO3)2(H2O)2, Pu(NO3)4 and Am(NO3)3
  2. Kerosene and TBP are added to yield UO2(NO3)2(TBP)2 and PuNO34(TBP)2 which can be extracted into the organic
  3. The aqueous layer is removed using “pulsed” columns which isolates Pu and U from Am and the other fission products
  4. Pu(IV) is more easily reduced than U(VI) (higher reduction potential for Pu(IV)) so a reducing agent is added alongside some distilled water to reduced Pu(IV) —> Pu(III) (Pu(NO3)4 —> Pu(NO3)3)
  5. Pu(NO3)3 can be extracted into the aqueous layer
28
Q

Actinide organometallics

A

Dominated by the +4 oxidation state

but some highly reactive +3 compounds are now known

29
Q

Cp ligands are installed by…

A

…salt metathesis with UX4

UCl4 + 3KCp —> Cp3UCl + 3KCl

30
Q

Cp3UCl + KOPh —>

A

Cp3UOPh

31
Q

Cp3UCl + LiSiPh3 —>

A

Cp3USiPh3

32
Q

Cp3UCl + KCp —>

A

Cp4U

33
Q

Cp4U

A

= the only n5-tetracyclopentadienyl compound, reflecting the size of the early actinides and their ability to engage in covalency
Does NOT transfer Cp to FeCl2 to form ferrocene

34
Q

Cp3UCl + LiR —>

A

Cp3UR

35
Q

Cp3UCl + NEt2 —>

A

Cp3UNEt2

36
Q

Cp* ligand

A

Gives access to Cp*2AnCl2 which are useful starting materials

37
Q

Salt metathesis reactions of Cp*2ThCl2

A

Cp2ThCl2 + 2RLi —> Cp2ThR2 + 2LiCl

38
Q

Reactions of Cp*2ThR2

A
  1. Sigma-bond metathesis e.g. with H2

2. Insertion of unsaturation into the An-R bond e.g. with CO2/SO2/CO/ethene

39
Q

Synthesis of uranium carbonyls

A

3KCp* + UCl4 —> Cp3UCl
Cp3UCl + Na —> Cp3U + NaCl
Cp3U + CO —> Cp*3UCO

40
Q

Methods for stabilising homoleptic sigma-alkyls and aryls

A
  1. Sterically bulky ligands without beta-substituents

2. Coordinate saturation of the metal centre with smaller ligands to form “ate” complexes

41
Q

Uranocene

A 
UCl4 + 2K2COT ---> COT2U + 4KCl
Green
Pyrophoric
Paramagnetic
Stable to hydrolysis
22 electron complex - means 5f orbitals must be involved in forming additional bonding MOs

Inorganic (4 decks)

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