Lecture 2: General Overview • Presentation from typical actinide lecture from inorganic chemistry • Chapter 24, Advanced inorganic chemistry • http://www.chem.ox.ac.uk/icl/heyes/LanthAct/lanthact.html • Occurrence • Ac, Th, Pa, U natural • Ac and Pa daughters of Th and U • Traces of 244Pu in Ce ores • Properties based on filling 5f orbitals
Electronic structure • Electronic Configurations of Actinides are not always easy to confirm • atomic spectra of heavy elements are very difficult to interpret in terms of configuration • Competition between 5fn7s2 and 5fn-16d7s2 configurations • for early actinides promotion 5f 6d occurs to provide more bonding electrons much easier than corresponding 4f 5d promotion in lanthanides • second half of actinide series resemble lanthanides more closely • Similarities for trivalent lanthanides and actinides • 5f orbitals have greater extension with respect to 7s and 7p than do 4f relative to 6s and 6p orbitals • The 5 f electrons can become involved in bonding • ESR evidence for bonding contribution in UF3, but not in NdF3 • Actinide f covalent bond contribution to ionic bond • Lanthanide 4f occupy inner orbits that are not accessable • Basis for chemical differences between lanthanides and actinides
Electronic Structure • 5f / 6d / 7s / 7p orbitals are of comparable energies over a range of atomic numbers • especially U - Am • Bonding can include any orbitals since energetically similar • Explains tendency towards variable valency • greater tendency towards (covalent) complex formation than for lanthanides • Lanthanide complexes tend to be primarily ionic • Actinide complexes complexation with p-bonding ligands • Hybrid bonds involving f electrons • Since 5f / 6d / 7s / 7p orbital energies are similar orbital shifts may be on the order of chemical binding energies • Electronic structure of element in given oxidation state may vary with ligand • Difficult to state which orbitals are involved in bonding
Ionic Radii and trends Trends based on ionic radii Actinide contraction
Absorption Spectra and Magnetic Properties • Electronic Spectra • 5fn transitions • narrow bands (compared to transition metal spectra) • relatively uninfluenced by ligand field effects • intensities are ca. 10x those of lanthanide bands • complex to interpret • Magnetic Properties • hard to interpret • spin-orbit coupling is large • Russell-Saunders (L.S) Coupling scheme doesn't work, lower values than those calculated • LS (http://hyperphysics.phy-astr.gsu.edu/hbase/atomic/lcoup.html) • Weak spin orbit coupling • Sum spin and orbital angular momentum • J=S+L • ligand field effects are expected where 5f orbitals are involved in bonding http://www.sciencedirect.com/science/article/pii/S0020169300924873#
Example: Puabsorbance spectrum • Ability to distinguish between Pu oxidation states • Variation in molar absorptivity • Determine speciation of Pu by spectroscopy f electrons and hybrid orbitals • Various orbital combinations similar to sp or d orbital mixing • Linear: sf • Tetrahedral: sf3 • Square: sf2d • Octahedral: d2sf3 • A number of orbital sets could be energetically accessible • General geometries • Trivalent: octahedral • Tetravalent: 8 coordination • Pentavalent and hexavalent actinides have double bonded oxygens • O=U=O2+
Redox chemistry • actinides are electropositive • From 2+ to 7+ • Pa - Pu show significant redox chemistry • all 4 oxidation states of Pu can co-exist in appropriate conditions • stability of high oxidation states peaks at U (Np) • redox potentials show strong dependence on pH (data for Ac - Cm) • high oxidation states are more stable in basic conditions • even at low pH hydrolysis occurs • tendency to disproportionation is particularly dependent on pH • at high pH 3Pu4+ + 2H2O PuO22+ + 2Pu3+ + 4H+ • early actinides have a tendency to form complexes • complex formation influences reduction potentials • Am4+(aq) exists when complexed by fluoride (15 M NH4F(aq)) • radiation-induced solvent decomposition produces H• and OH• radicals • lead to reduction of higher oxidation states e.g. PuV/VI, AmIV/VI
Actinide metals • Preparation of actinide metals • Reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca or Ba at 1100 – 1400 °C • Other redox methods are possible • Thermal decomposition of iodine species • Am from Am2O3 with La • Am volatility provides method of separation • Metals tend to be very dense • U 19.07 g/mL • Np 20.45 g/mL • Am lighter at 13.7 g/mL • Some metals glow due to activity • Ac, Cm, Cf
Pu metal • Some controversy surrounding behavior of metal http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818030.pdf
Organometallic • Organometallic chemistry of actinides is relatively recent • Interest is expanding but still focused on U • Similar to lanthanides in range of cyclopentadienides / cyclooctatetraenides / alkyls • Cyclopentadienides are p-bonded to actinides
Uranocene • Paramagnetic • Pyrophoric • Stable to hydrolysis • Planar 'sandwich' • Eclipsed D8h conformation • UV-PES studies show that bonding in uranocene has 5f & 6d contributions • e2u symmetry interaction shown can only occur via f-orbitals
Overview • Radius trends for ions and metals of the actinides • General trends in actinide electronic structure • Electronic and magnetic spectroscopy • Variations in the actinides • Actinide stereochemistry • Range of oxidation states for the actinides • Role of organometallic chemistry for understanding f-electrons
Questions • What is the trend in for the ionic radii of actinides? • Which electrons are more likely to be involved in bonding, 4f or 5f? Why? • What is the spectroscopic nature of 5f electrons and how is this observed? • What are examples of f electron hybridization? • What is the relationship between molecular geometry and coordination number? • Describe a method for the preparation of actinide metals? • How many phases of Pu metal exist under normal pressure? What drives the change in phases?
Pop Quiz • List 3 pentavalent actinides.