1. A lack of synergy? An unusual actinide-ligand bonding mode Nik Kaltsoyannis Department of Chemistry University College London

2. What should I talk about? “Anything you like, as long as you are enthusiastic”

3. Outline of presentation Part 1A very brief introduction to actinide chemistry The f elements by N Kaltsoyannis and P Scott, Oxford University Press (1999) The Chemistry of the Actinide and Transactinide Elements, 3 rd Edition, L. R. Morss, N. Edelstein, and J. Fuger (eds), Springer (2006) Part 2Unusual metal-ligand bonding modes in molecular uranium complexes

4. HHe LiBeBCNOFNe NaMgAlSiPSClAr KCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr RbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXe CsBaLaHfTaWReOsIrPtAuHgTlPbBiPoAtRn FrRaAcRfDbSgBhHsMt CePrNdPmSmEuGdTbDyHoErTmYbLu ThPaUNpPuAmCmBkCfEsFmMdNoLr Element 90 Just checking….. Element 89 Element 103

5. Element Electronic configuration Thorium [Rn]6d 2 7s 2 Protactinium [Rn]5f 2 6d 1 7s 2 Uranium [Rn]5f 3 6d 1 7s 2 Neptunium [Rn]5f 4 6d 1 7s 2 Plutonium [Rn]5f 6 7s 2 Americium [Rn]5f 7 7s 2 Curium [Rn]5f 7 6d 1 7s 2 Berkelium [Rn]5f 9 7s 2 Californium [Rn]5f 10 7s 2 Einsteinium [Rn]5f 11 7s 2 Fermium [Rn]5f 12 7s 2 Mendelevium [Rn]5f 13 7s 2 Nobelium [Rn]5f 14 7s 2 Lawrencium [Rn]5f 14 6d 1 7s 2 The ground electronic configurations of the actinides

6. The shapes of the seven 5f orbitals (cubic set). 5f y3, 5f x3, 5f z3 5f x(z2-y2), 5f y(z2-x2), 5f z(x2-y2) 5f xyz

7. The oxidation states adopted by the actinide elements in their compounds The most stable oxidation state in aqueous solution is represented by the black circles. Open circles indicate other oxidation states adopted and squares indicate that the oxidation state is found only in solids.

8. Radial distribution functions of selected atomic orbitals of U 6+ (Enrique Batista, B3LYP, all-electron, 2 nd order DK)

9. The particular challenges posed to quantum chemistry by the actinides 1Lots of electrons. 2Heavy elements  relativistic effects are important (scalar - modification of atomic orbital energies – and spin-orbit). 3Large number of valence atomic orbitals of similar radial distribution and energy (5f, 6p, 6d, 7s, 7p)  actinide complexes are frequently open ‑ shell, with many closely-spaced electronic states. The correct description of electron correlation effects is extremely important (and difficult) in these cases.

10. Part 2 Unusual metal-ligand bonding modes in molecular uranium complexes The classic Dewar-Chatt-Duncanson view of synergic bonding Qualitative MO scheme for CO  donation from filled CO 3  orbital  acceptance “backbonding” into vacant CO 2  orbital Schematic view of synergic bonding between CO and a transition metal

11. Qualitative MO scheme for octahedral ML 6 with  acceptor ligands (e.g. CO)

12. Are there CO complexes of the actinides? Two views of (C 5 Me 5 ) 3 U(CO) Evans et al. JACS 125 (2003) 13831 [{(L)U} 2 (µ:  1,  1 -CO)] Meyer et al. JACS 127 (2005) 11242 “The hard, oxophilic f elements typically have a low binding affinity for the soft  bonding CO ligand, and carbonyl complexes do not readily form”

13. f orbital to carbonyl 2  backbonding: the electronic structures of (C 5 H 5 ) 3 U(CO) and (C 5 H 5 ) 3 U(OC) “Two major interactions of (C 5 H 5 ) 3 U(CO) are discussed. The CO 3  lone pair interacts primarily with the empty U 6d orbitals to form the U-CO  bond, and extensive U 5f → CO 2  backbonding is observed” B.E. Bursten and R.J. Strittmatter, JACS 109 (1987) 6606.

14. P. Roussel and P. Scott, JACS 120 (1998) 1070.

15. N.Kaltsoyannis and P. Scott, Chem. Commun. (1998) 1665.  Back bonding without  bonding

16. What is the oxidation state of the uranium atoms in [(C 5 Me 5 ) 2 U] 2 (  -µ 6 :µ 6 -C 6 H 6 )? Realistic possibilities include (a) U(II) and neutral benzene (b) U(III) and (benzene) 2- (most likely from experiment) and (c) U(IV) and (benzene) 4-

17. Interatomic distance/ÅExp.Calc. U-U4.3964.406 U1-Cp* (av)2.8402.860 U2-Cp* (av)2.8302.840 C-C (benzene, complex)1.440 C-C (benzene, free)1.3901.394 U1-C (benzene, av)2.6212.634 U1-C (benzene, max)2.7332.719 U1-C (benzene, min)2.5472.591 U2-C (benzene, av)2.6282.627 U2-C (benzene, max)2.7302.674 U2-C (benzene, min)2.5382.532 How well does calculation reproduce the experimental geometry? So why is the benzene ring so non-planar?

18. Hückel energies of the carbocyclic ring  orbitals

19. Calculation suggests (a) each uranium gives up two electrons to the cp* ligands (b) each uranium has two 5f-based electrons (c) four electrons (two per uranium) are used to form a uranium/arene  bond

20. The localisation properties of the four uranium/arene δ bonding electrons determine the formal oxidation state of the metal centres. Population analysis indicates that these electrons have an approximately equal contribution from both metal and arene, and hence the oxidation state of the uranium atoms is best described as +3. The benzene ring is not neutral. Rather, it carries a charge close to -2, as there is transfer of uranium 5f electron density into the benzene e 2u C-C π* molecular orbitals. The benzene ring is thus no longer Hückel aromatic, and is significantly non-planar as a result. W.J. Evans, S.A. Kozimor, J. W. Ziller and N. Kaltsoyannis, JACS 126 (2004) 14533.

21. Arene-bridged diuranium complexes: inverted sandwiches supported by  backbonding P.L. Diaconescu, P.L. Arnold, T.A. Baker, D.J. Mindiola and C.C. Cummins, JACS 122 (2000) 6108. (  -C 7 H 8 )[U(N[Ad]Ar) 2 ] 2 The two near degenerate  backbonding orbitals of (  -C 6 H 6 )[U(NH 2 ) 2 ] 2