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aa The f-elements > Meet the Actinides > Flashcards

Meet the Actinides Flashcards

1
Q

Which of the actinides are naturally occuring

A
  1. Actinium, thorium, protactinium and uranium naturally occurring
  2. Ac and Pa only in trace amounts.
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2
Q

Which actinides are found in minute amounts

A
  1. Neptunium and plutonium occur in minute amounts in U minerals (were synthesised artificially before discovered naturally!)
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3
Q

Which elements does the study of actinides focus on

A
  1. Thorium and Uranium
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4
Q

Describe how thorium is found

A
  1. Widely dispersed - >3 ppm of the earth’s crust.
  2. Natural thorium is essentially 100% 232Th.
  3. Occurs in monazite [with the rare earths] and in uranothorite [a mixed Th,U silicate]
  4. Is obtained as ThO2, thoria, from mineral extraction processes.
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5
Q

Describe how Uranium is found

A
  1. widely distributed found scattered in the faults of old igneous rocks.
  2. Natural uranium is 99.27% 238U and 0.72% 235U
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6
Q

Describe the difference between 4f and 5f orbitals

A
  1. The 4f and 5f orbitals do not differ in the angular part of their wave functions (i.e. they share the same shape)
  2. however the 5f orbitals possess a radial node.
  3. The 5f orbitals have greater radial extension compared to the 7s and 7p than the 4f have compared to the 6s and 6p.
  4. The 5f orbitals can interact with ligands conferring a degree of covalency in metal-ligand bonding.
  5. This is particularly true for the early actinides.
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7
Q

Summarise 5f orbital properties

A
  1. Not contracted and are not core-like
  2. They interact with their environment
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8
Q

Describe ground state electronic structure of actinides

A
  1. Actinide electronic configurations are complex and difficult to interpret as 7s, 6d, 5f orbitals are all close in energy
  2. Early actinides show easy 5f–>6d promotion to provide more bonding electrons.
  3. (More accessible than 4f–>5d promotion in lanthanides).
  4. Early actinides like Th fill 6d preferentially but uses 5f in bonding
  5. Later actinides are lanthanide-like, 5f is more stabilised than 6d
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9
Q

What is electronic configuration of Th

A
  1. 5f0 6d2 7s2
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10
Q

Describe oxidation states of actinides

A
  1. Far more variation than the lanthanides.
  2. +3 accessible for all but not most stable in early actinides.
  3. +3 most stable in later actinides.
  4. Due to stabilisation of 5f orbitals relative to 7s and 6d.
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11
Q

What is most common oxidation state for Th

A
  1. Th4+
  2. Higher oxidation state due to 7s ,5f, 6d being close in energy
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12
Q

What are most common oxidation states for U

A
  1. U6+
  2. U3+ accessible but not most common
  3. Originates from 7s ,5f, 6d being close in energy
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13
Q

Why do later actinides have lower Oxidation states

A
  1. +3 most stable
  2. Higher oxidation states inaccessible
  3. Due to gaps between 7s ,5f, 6d being more extreme
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14
Q

Describe radius of actinides across the group

A
  1. Actinides show an analagous contraction to lanthanides -actinide contraction
  2. Due to increasing Zeff
  3. This yields a contraction in the 5f orbitals
  4. This makes the 5f orbitals increasingly core-like across the series.
  5. Means late actinides are lanthanide like
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15
Q

Describe the electronic spectra of early lanthanides

A
  1. 5f- ligand interactions in the early actinides leads to vibronic coupling yielding broad, intense bands for the f-f transitions.
  2. 5f-6d is lower energy
  3. Intense visible colours
  4. Ligand-5f interactions due to covalency allows vibronic coupling which distributes the energy and relaxes the selection rules
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16
Q

Describe the electronic spectra of late lanthanides

A
  1. The later actinides show sharp, low intensity lines more closely resembling the lanthanides.
  2. Reduced covalency leads to lanthanide-like spectra
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17
Q

Describe magnetic properties of actinides

A
  1. The magnetic properties of the actinides are complex.
  2. Spin-orbit coupling is strong (2000-4000 cm-1)
  3. but because the 5f electrons do interact with the ligands, ligand field effects are of comparable magnitude
  4. Crystal field splitting is similar magnitude to spin-orbit coupling
18
Q

What are results of magnetic properties of actinides

A
  1. J is no longer a good quantum number as the J states are split by the ligand field.
  2. The spin-only and Landé formulae are both inadequate to predict magnetism
  3. Experimental values of μeff vary with temperature and are generally lower than for the corresponding lanthanides (i.e. L is partially quenched)
19
Q

Why is Ueff for U3+ lower than UJ

A
  1. Get interaction between f orbitals and ligands
  2. Quenches orbital angular momentum
  3. So electrons can no longer freely rotate in system as before
20
Q

Describe key properties of Actinide coordination complexes

A
  1. 5f orbitals accessible for ligand-orbital overlap and covalent character. 2. Declines across the series and later actinides are “lanthanide-like”.
  2. Ionic bonding stronger across the series due to lanthanide contraction effect on charge density.
  3. Large ionic radii of actinides yields very high coordination numbers (up to 15 coordinate), and a propensity towards oligomerisation when unbulky ligands are present.
21
Q

Describe Lanthanide halides

A
  1. Group valency (i.e. all valence electrons lost) accessible up to U (as U6+).
  2. Thereafter AnX3 becomes most stable and the compounds resemble LnX3.
22
Q

What is the most important actinide halide

A
  1. UF6 is the most important actinide halide
23
Q

Describe how UF6 is generated

A
  1. generated from UO2 and HF then F2:
  2. UO2 +4HF –> UF4 +3H2O
  3. UF4 + F2 –> UF6
24
Q

What happens to the coordination number of an actinide in a halide complex as the halide gets bigger

A
  1. Coordination number drops
25
Q

Describe properties of UF6

A
  1. Used in 235U enrichment
  2. UF6 has a melting point of 64°C and a high vapour pressure.
  3. It is made on a large scale to separate uranium isotopes (235UF6 and 238UF6) by gas diffusion or centrifugation.
  4. Volatile, isolated monomeric molecules
26
Q

Describe coordination chemistry of early actinides

A
  1. High coordination numbers
  2. Range of oxidation states
  3. 5f-ligand covalency
27
Q

Describe coordination chemistry of late actinides

A
  1. Stronger ionic bonding
  2. +3 Oxidation state
  3. 5f orbitals not involved in bonding
  4. Lanthanide like
28
Q

What is a concern about actinide coordination chemistry

A
  1. Environmental outcomes of releasing of product from the nuclear and fuel reprocessing industries
29
Q

Describe aqueous actinide compounds

A
  1. The early actinides (U - Am) are capable of forming pentavalent and hexavalent actinyl cations AnO2+ and AnO2 2+, in contrast to the lanthanides.
    2.Thus, these actinides in the environment are usually found in this form.
  2. Late actinides form dioxo compounds
30
Q

Describe actinyl cations

A
  1. O-An-O always linear, trans
  2. No analogue in lanthanides or late actinides
  3. Reflects 5f involvement in covalency
31
Q

Describe transition metal dioxo compounds

A
  1. Always cis
  2. Orientation of p-orbitals of oxygen with d orbital
32
Q

What does the linear,trans O-An-O means for actinides

A
  1. Can add donors in equatorial plane
  2. Auxiliary ligands are bound in the equatorial plane
  3. yielding octahedral, pentagonal bipyramidal,
  4. or where bidentate ligands (CO32-, NO3-) are present, hexagonal bipyramidal structures.
33
Q

What are the steps of drawing MO diagram

A
  1. Draw AOs
  2. Bring together to make bonds, antibonds and non-bonding. nMO=nAO
  3. Add e-
  4. Label orbitals
34
Q

Which orbitals interact in an Actinyl MO diagram

A
  1. pi g = dzx-px and dyz- py
  2. pi u = 5fxz^2 - px and 5fyz^2 -py
  3. sigma g = dz^2 - pz
  4. sigma u = fz^3 - pz
35
Q

How many bonding orbitals are present in [UO2]2 2+ digram

A
  1. 12 electrons, 6 bonding MOs
  2. 3 per U-O bond
36
Q

What order should you write the orbitals in [UO2]2 2+ MO diagram

A
  1. pi g
  2. pi u
  3. sigma g
  4. sigma u
37
Q

Describe bonding in [UO2]2 2+

A
  1. Not all of the f and d orbitals are involved and some remain non-bonding.
  2. In the case of UO22+, there are zero electrons from U6+ and 6 p electrons from each O 2- making a total of 12.
  3. This completely fills the bonding MOs.
  4. Since all 12 electrons participate in U-O bonding, the bond order is three. 5. This can only arise by the participation of the 5f orbitals.
38
Q

Describe properties of uranyl and then later Actinyl cations

A
  1. Uranyl is extremely stable
  2. but the later actinyls (Np-Am) are less stable due to the stabilisation of the 5f orbitals
  3. rendering them less accessible for bonding, and the occupation of non-bonding MOs.
  4. Beyond Am, the oxidation states required for formation of actinyls are inaccessible.
39
Q

Why are 2s orbitals ignored in uranyl MO diagram

A
  1. We ignore the 2s orbitals because they’re too low in energy to interact with the orbitals on U
  2. but they do accommodate 4 electrons as lone pairs.
40
Q

Describe electron counting in Uranyl Mo diagram

A
  1. The oxygens provide 6 electrons each, the uranium provides 6 electrons to give 18
  2. We remove 2 due to the dicationic nature to leave 16.
  3. We lose 4 to the lone pairs on O (the low energy 2s orbitals)
  4. This leaves 12 electrons
41
Q

Describe bond order of Uranyl MO diagram

A
  1. 12 electrons fill 6 bonding orbitals to give:
  2. BO = 6 overall
  3. BO = 3 per U-O bond

aa The f-elements (11 decks)

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