A lack of synergy? An unusual actinide-ligand bonding mode

Nik Kaltsoyannis

Department of ChemistryUniversity College London

What should I talk about?

“Anything you like, as long as you are enthusiastic”

Outline of presentation

Part 1 A 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, 3rd Edition, L. R.

Morss, N. Edelstein, and J. Fuger (eds), Springer (2006)

Part 2 Unusual metal-ligand bonding modes in molecular uranium complexes

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ac Rf Db Sg Bh Hs Mt

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 

Element 90

Just checking…..

Element 89

Element 103

Element Electronic configurationThorium [Rn]6d27s2

Protactinium [Rn]5f26d17s2

Uranium [Rn]5f36d17s2

Neptunium [Rn]5f46d17s2

Plutonium [Rn]5f67s2

Americium [Rn]5f77s2

Curium [Rn]5f76d17s2

Berkelium [Rn]5f97s2

Californium [Rn]5f107s2

Einsteinium [Rn]5f117s2

Fermium [Rn]5f127s2

Mendelevium [Rn]5f137s2

Nobelium [Rn]5f147s2

Lawrencium [Rn]5f146d17s2

The ground electronic configurations of the actinides

The shapes of the seven 5f orbitals (cubic set).

5fy3, 5fx3, 5fz3

5fx(z2-y2), 5fy(z2-x2), 5fz(x2-y2)

5fxyz

Th Pa U Np Pu Am Cm Bk Cf Es

+3

+4

+5

+6

For

mal

Oxi

datio

n S

tate

Fm Md No Lr

+2

+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.

Radial distribution functions of selected atomic orbitals of U6+

(Enrique Batista, B3LYP, all-electron, 2nd order DK)

The particular challenges posed to quantum chemistry by the actinides

1 Lots of electrons.

2 Heavy elements relativistic effects are important (scalar - modification of atomic orbital energies – and spin-orbit).

3 Large 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.

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

Qualitative MO scheme for octahedral ML6 with acceptor ligands (e.g. CO)

Are there CO complexes of the actinides?

Two views of (C5Me5)3U(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”

f orbital to carbonyl 2 backbonding: the electronic structures of (C5H5)3U(CO) and (C5H5)3U(OC)

“Two major interactions of (C5H5)3U(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.

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

N

UN

N

R

R

N

R

N

U N

N

R

R

N

R

N

NR=SiMe2But

N

UN

N

R

R

N

R

N

U N

N

R

R

N

R

N

N

N.Kaltsoyannis and P. Scott, Chem. Commun. (1998) 1665.

NH2

UH3N

H2N

H2N

H2N

U NH3

NH2

NH2N

N

Back bonding without bonding

What is the oxidation state of the uranium atoms in [(C5Me5)2U]2(-µ6:µ6-C6H6)?

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-

Interatomic distance/Å Exp. Calc.

U-U 4.396 4.406

U1-Cp* (av) 2.840 2.860

U2-Cp* (av) 2.830 2.840

C-C (benzene, complex) 1.440 1.440

C-C (benzene, free) 1.390 1.394

U1-C (benzene, av) 2.621 2.634

U1-C (benzene, max) 2.733 2.719

U1-C (benzene, min) 2.547 2.591

U2-C (benzene, av) 2.628 2.627

U2-C (benzene, max) 2.730 2.674

U2-C (benzene, min) 2.538 2.532

How well does calculation reproduce the experimental geometry?

So why is the benzene ring so non-planar?

Hückel energies of the carbocyclic ring orbitals

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

•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 e2u 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.

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.

(-C7H8)[U(N[Ad]Ar)2]2

The two near degenerate backbonding orbitals of (-C6H6)[U(NH2)2]2