Meet the Actinides Flashcards
Which of the actinides are naturally occuring
- Actinium, thorium, protactinium and uranium naturally occurring
- Ac and Pa only in trace amounts.
Which actinides are found in minute amounts
- Neptunium and plutonium occur in minute amounts in U minerals (were synthesised artificially before discovered naturally!)
Which elements does the study of actinides focus on
- Thorium and Uranium
Describe how thorium is found
- Widely dispersed - >3 ppm of the earth’s crust.
- Natural thorium is essentially 100% 232Th.
- Occurs in monazite [with the rare earths] and in uranothorite [a mixed Th,U silicate]
- Is obtained as ThO2, thoria, from mineral extraction processes.
Describe how Uranium is found
- widely distributed found scattered in the faults of old igneous rocks.
- Natural uranium is 99.27% 238U and 0.72% 235U
Describe the difference between 4f and 5f orbitals
- The 4f and 5f orbitals do not differ in the angular part of their wave functions (i.e. they share the same shape)
- however the 5f orbitals possess a radial node.
- The 5f orbitals have greater radial extension compared to the 7s and 7p than the 4f have compared to the 6s and 6p.
- The 5f orbitals can interact with ligands conferring a degree of covalency in metal-ligand bonding.
- This is particularly true for the early actinides.
Summarise 5f orbital properties
- Not contracted and are not core-like
- They interact with their environment
Describe ground state electronic structure of actinides
- Actinide electronic configurations are complex and difficult to interpret as 7s, 6d, 5f orbitals are all close in energy
- Early actinides show easy 5f–>6d promotion to provide more bonding electrons.
- (More accessible than 4f–>5d promotion in lanthanides).
- Early actinides like Th fill 6d preferentially but uses 5f in bonding
- Later actinides are lanthanide-like, 5f is more stabilised than 6d
What is electronic configuration of Th
- 5f0 6d2 7s2
Describe oxidation states of actinides
- Far more variation than the lanthanides.
- +3 accessible for all but not most stable in early actinides.
- +3 most stable in later actinides.
- Due to stabilisation of 5f orbitals relative to 7s and 6d.
What is most common oxidation state for Th
- Th4+
- Higher oxidation state due to 7s ,5f, 6d being close in energy
What are most common oxidation states for U
- U6+
- U3+ accessible but not most common
- Originates from 7s ,5f, 6d being close in energy
Why do later actinides have lower Oxidation states
- +3 most stable
- Higher oxidation states inaccessible
- Due to gaps between 7s ,5f, 6d being more extreme
Describe radius of actinides across the group
- Actinides show an analagous contraction to lanthanides -actinide contraction
- Due to increasing Zeff
- This yields a contraction in the 5f orbitals
- This makes the 5f orbitals increasingly core-like across the series.
- Means late actinides are lanthanide like
Describe the electronic spectra of early lanthanides
- 5f- ligand interactions in the early actinides leads to vibronic coupling yielding broad, intense bands for the f-f transitions.
- 5f-6d is lower energy
- Intense visible colours
- Ligand-5f interactions due to covalency allows vibronic coupling which distributes the energy and relaxes the selection rules
Describe the electronic spectra of late lanthanides
- The later actinides show sharp, low intensity lines more closely resembling the lanthanides.
- Reduced covalency leads to lanthanide-like spectra
Describe magnetic properties of actinides
- The magnetic properties of the actinides are complex.
- Spin-orbit coupling is strong (2000-4000 cm-1)
- but because the 5f electrons do interact with the ligands, ligand field effects are of comparable magnitude
- Crystal field splitting is similar magnitude to spin-orbit coupling
What are results of magnetic properties of actinides
- J is no longer a good quantum number as the J states are split by the ligand field.
- The spin-only and Landé formulae are both inadequate to predict magnetism
- Experimental values of μeff vary with temperature and are generally lower than for the corresponding lanthanides (i.e. L is partially quenched)
Why is Ueff for U3+ lower than UJ
- Get interaction between f orbitals and ligands
- Quenches orbital angular momentum
- So electrons can no longer freely rotate in system as before
Describe key properties of Actinide coordination complexes
- 5f orbitals accessible for ligand-orbital overlap and covalent character. 2. Declines across the series and later actinides are “lanthanide-like”.
- Ionic bonding stronger across the series due to lanthanide contraction effect on charge density.
- Large ionic radii of actinides yields very high coordination numbers (up to 15 coordinate), and a propensity towards oligomerisation when unbulky ligands are present.
Describe Lanthanide halides
- Group valency (i.e. all valence electrons lost) accessible up to U (as U6+).
- Thereafter AnX3 becomes most stable and the compounds resemble LnX3.
What is the most important actinide halide
- UF6 is the most important actinide halide
Describe how UF6 is generated
- generated from UO2 and HF then F2:
- UO2 +4HF –> UF4 +3H2O
- UF4 + F2 –> UF6
What happens to the coordination number of an actinide in a halide complex as the halide gets bigger
- Coordination number drops
Describe properties of UF6
- Used in 235U enrichment
- UF6 has a melting point of 64°C and a high vapour pressure.
- It is made on a large scale to separate uranium isotopes (235UF6 and 238UF6) by gas diffusion or centrifugation.
- Volatile, isolated monomeric molecules
Describe coordination chemistry of early actinides
- High coordination numbers
- Range of oxidation states
- 5f-ligand covalency
Describe coordination chemistry of late actinides
- Stronger ionic bonding
- +3 Oxidation state
- 5f orbitals not involved in bonding
- Lanthanide like
What is a concern about actinide coordination chemistry
- Environmental outcomes of releasing of product from the nuclear and fuel reprocessing industries
Describe aqueous actinide compounds
- 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. - Late actinides form dioxo compounds
Describe actinyl cations
- O-An-O always linear, trans
- No analogue in lanthanides or late actinides
- Reflects 5f involvement in covalency
Describe transition metal dioxo compounds
- Always cis
- Orientation of p-orbitals of oxygen with d orbital
What does the linear,trans O-An-O means for actinides
- Can add donors in equatorial plane
- Auxiliary ligands are bound in the equatorial plane
- yielding octahedral, pentagonal bipyramidal,
- or where bidentate ligands (CO32-, NO3-) are present, hexagonal bipyramidal structures.
What are the steps of drawing MO diagram
- Draw AOs
- Bring together to make bonds, antibonds and non-bonding. nMO=nAO
- Add e-
- Label orbitals
Which orbitals interact in an Actinyl MO diagram
- pi g = dzx-px and dyz- py
- pi u = 5fxz^2 - px and 5fyz^2 -py
- sigma g = dz^2 - pz
- sigma u = fz^3 - pz
How many bonding orbitals are present in [UO2]2 2+ digram
- 12 electrons, 6 bonding MOs
- 3 per U-O bond
What order should you write the orbitals in [UO2]2 2+ MO diagram
- pi g
- pi u
- sigma g
- sigma u
Describe bonding in [UO2]2 2+
- Not all of the f and d orbitals are involved and some remain non-bonding.
- In the case of UO22+, there are zero electrons from U6+ and 6 p electrons from each O 2- making a total of 12.
- This completely fills the bonding MOs.
- 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.
Describe properties of uranyl and then later Actinyl cations
- Uranyl is extremely stable
- but the later actinyls (Np-Am) are less stable due to the stabilisation of the 5f orbitals
- rendering them less accessible for bonding, and the occupation of non-bonding MOs.
- Beyond Am, the oxidation states required for formation of actinyls are inaccessible.
Why are 2s orbitals ignored in uranyl MO diagram
- We ignore the 2s orbitals because they’re too low in energy to interact with the orbitals on U
- but they do accommodate 4 electrons as lone pairs.
Describe electron counting in Uranyl Mo diagram
- The oxygens provide 6 electrons each, the uranium provides 6 electrons to give 18
- We remove 2 due to the dicationic nature to leave 16.
- We lose 4 to the lone pairs on O (the low energy 2s orbitals)
- This leaves 12 electrons
Describe bond order of Uranyl MO diagram
- 12 electrons fill 6 bonding orbitals to give:
- BO = 6 overall
- BO = 3 per U-O bond
