Oxidation State Ambiguity in f Element Organometallics - a Spectroscopic and Quantum Chemical Journey Nik Kaltsoyannis Department of Chemistry, University.

Oxidation State Ambiguity in f Element Organometallics -

a Spectroscopic and Quantum Chemical Journey

Nik KaltsoyannisDepartment of Chemistry, University College London

Outline of presentation

Story 1: X-ray absorption spectroscopy of Ce compounds

Story 2: Gas-phase photoelectron spectroscopy of CeCp3

Story 3: Multiconfigurational quantum chemical calculations of M(COT)2

(COT = 8-C8H8; M = Th, Pa, U, Pu, Cm and Ce)

Story 4: Multiconfigurational quantum chemical calculations of CeCp3 and CeCp3+ (Cp = 5-C5H5)

Ln(COT)2 the “lanthanocenes”An(COT)2 the “actinocenes”

Ce

CeCp3

Story 1: X-ray absorption spectroscopy of Ce compounds

Qualitative molecular orbital diagram for

M(COT)2 M = f element

COT(COT)2

0 (a2u)

1 (e1g)

2 (e2u)

3 (e3g)

4 (b1u)

a2u

a1g

e1u

e1g

e2g

e2u

e3u

e3g

b2g

b1u

f

d

f (e3u)

f (e1u)

f (e2u)

f (a2u)

d (a1g)

d (e1g)

d (e2g)

M(COT)2M

The traditional view of Ce(COT)2 and Th(COT)2

2 (e2u)e2g

e2u

ff (e3u)

f (e1u)

f (e2u)

f (a2u)

Ground state is 1A1g with an electronic configuration e2u(p2)4f0

Þ M(IV) and 2 x COT2-

Correct description of Th(COT)2 BUT NOT Ce(COT)2

M. Dolg, P. Fulde, H. Stoll, H. Preuss, A. Chang and R. M. Pitzer J. Chem. Phys. 195 (1995) 71

Dolg et al.’s view of Ce(COT)2

2 (e2u)e2g

e2u

ff (e3u)

f (e1u)

f (e2u)

f (a2u)

2 (e2u)e2g

e2u

ff (e3u)

f (e1u)

f (e2u)

f (a2u)

Ground state is 1A1g with two contributing electronic configurations e2u(p2)4f0 (20%) + e2u(p2)3f1 (80%)

Þ Ce(III) and 2 x COT1.5-

20%

80%

Can we test this experimentally (how do we measure oxidation state)?

N. M. Edelstein, P. G. Allen, J. J. Bucher, D. K. Shuh, C. D. Sofield, A. Sella, M. Russo, N. Kaltsoyannis and G. Maunder J. Am. Chem. Soc. 118 (1996) 13115

X-ray Absorption Near Edge Spectroscopy (XANES)

Ce K edge (1s electrons)

Need a variable energy light source capable of delivering c. 40 keV photons (Stanford Synchrotron)

Representative K-edge spectra of Ce compounds

CeO2 (Ce(IV))

Ce K-edge XANES results

1 CeO2 (solid) 8 Ce2(SO4)3 (solid) 15 Ce(NO3)3 (1.2 M HCl solution)

2 Ce(NH4)4(SO4)4.2H2O (solid) 9 CeSi2 (solid)

3 Ce(NH4)4(SO4)4.2H2O (1.6 M HNO3 soln.) 10 CeI3.(THF)x (THF soln.) 16 Ce[1,4(TMS)2C8H6]2 (toluene soln.)

4 Ce(CH3C(O)CHC(O)CH3)4 (toluene soln.) 11 Ce[(Me3C)2C5H3]3 (toluene soln.) 17 Ce[1,3,6(TMS)3C8H5]2 (toluene soln.)

5 CeCl3.6H2O (solid) 12 Ce2(SO4)3 (1.6 M HNO3 soln.) 18 Li{Ce[1,4(TMS)2C8H6]2} (toluene soln.)

6 CeF3 (solid) 13 Ce2(SO4)3 (1.2 M HCl soln.) 19 K{Ce(C8H8)2} (toluene soln.)

7 Ce2O2S (solid) 14 Ce(NO3)3 (1.6 M HNO3 soln.)

▼ Ce(IV) compounds

■ Ce(III) compounds

0

5

10

Compound

Shi

ft fr

om C

eB6

Sta

ndar

d (e

V)

Ce3+compoundsQ

Ce4+compoundsWSubstituted cerocenesL

Substitued cerocenes

Ce(III) !!

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 

Increasing tendency toward lanthanide-like chemistry (An(III) dominant)

Þ Are the ground states of the later actinocenes multiconfigurational? Need: high-level ab initio calculations (see story 3….)

Story 2: Gas-phase photoelectron spectroscopy of CeCp3

Gaseousmolecules

The experiment

UV or X-ray light e-

e-

e-

Measure kinetic energy of electrons and determine ionization energy as the difference between the energy of the incident light photons and the electrons’ kinetic energy

Direct probe of electronic energy levels

Compared with d-block complexes, very few lanthanide complexes have been studied in the gas phase, because it is very hard to see f-based bands in spectra

Two main reasons

1. With Ln(III) compounds ionizations from 4f orbitals come at similar ionization energies to those from ligand orbitals

2. With photon energies given by discharge lamps 4f cross sections (ionization probabilities) are low

Ionization cross sections (ionization probabilities)

0

2

4

6

8

10

0 50 100 150 200

Calculated photoionization cross sections for C 2p and Ce 4f electrons

(Lindau and Leh)

C 2pCe 4f

Cro

ss s

ect

ion

Photon energy / eV

Delayed maximum

At photon energies accessible with a

discharge lamp, 4f electrons have very low ionization cross sections

The “Elettra” synchrotron, Trieste, Italy

Photoelectron spectrum of CeCp3

At low incident photon energies only ionizations from the Cp rings are visible

1500

1000

500

0

counts

20181614121086

IE / eV

Cp š

hv = 25 eV

Cp p

Cp p

0

2

4

6

8

10

0 50 100 150 200

Calculated photoionization cross sections for C 2p and Ce 4f electrons

(Lindau and Leh)

C 2pCe 4f

Cro

ss s

ect

ion

Photon energy / eV

Low ionization energy band (A) clearly visible BUT also a band just above 10 eV (D) showing f characteristics

250x106

200

150

100

50

0

Counts

181614121086

IE / eV

hv = 40 eV

A

B

C3

D

E F

C1

C2

Photoelectron spectrum of CeCp3 (again)

Cp p

Resonance structure is observed for bands A and Di.e. ionization of the single 4f electron gives rise to two cation states with f character

Are there really two f bands?

If the incident photon energy is sufficient to excite a Ce 4d core electron to a 4f orbital, a resonance will occur. Ionization of a 4f electron can borrow intensity from this transition and the ionization cross section can show a dramatic increase tune hn to the 4d ionization energy…..

8x106

6

4

2

0

Norm

alize

d c

ounts

181614121086

IE / eV

hv = 122 eV

A (f)

B C

D (f)

Cp p

What the…..?

Assume neutral CeCp3 has a ground state with the configuration Lf1, where L represents the ligand electrons and f1 is the single 4f electron

The matrix element governing the band intensity for f ionization is given by

1 ˆLf | | LεO where is the free electron (g) wave

Note that (a) Le represents a configuration with ligand electrons and no f electrons, i.e. Lf0 and(b) the ion states corresponding to bands A and D in the photoelectron spectrum must have Le (Lf0) as a contributing configuration

Assume that ionization of the f electron leads to ligand to metal charge transfer, generating a cation configuration with a hole in the ligand orbitals and a single Ce 4f electron, i.e. L-1f1 (sound familiar….?)

If Lf0 and L-1f1 have the same symmetry, mixing of the two configurations can generate two states of CeCp3

+

gc1Lf0 + c2L-1f1) band A – ground state of CeCp3+

ec3Lf0 – c4L-1f1) band D – excited state of CeCp3+

Our suggestion was that the ground state of CeCp3+ (formally Ce(IV)) is

multiconfigurational, in a manner comparable with that of neutral Ce(COT)2

What the…..? (continued)

M. Coreno, M. DeSimone, J. C. Green, N. Kaltsoyannis, N. Narband and A. Sella, Chemical Physics Letters 432 (2006) 17

Story 3: Ab initio quantum chemical calculations of M(COT)2 (M = Th, Pa, U, Pu, Cm and Ce)

• CASSCF/CASPT2 method• MOLCAS code• D2h point group

• Basis sets: correlation consistent, all-electron, ANO (27s24p18d14f)/[10s9p7d5f] for An, (25s22p15d11f)/[9s8p5d4f] for Ce, VDZP for C and H

• Scalar relativistic effects incorporated via 2nd order Douglas-Kroll• Spin-orbit free and spin-orbit coupled calculations

Computational details

COT(COT)2

1 (e1g)

2 (e2u)

e1g

e2g

e2u

5f

6d

f (e3u)

f (e1u)

f (e2u)

f (a2u)

d (a1g)

d (e1g)

d (e2g)

M(COT)2M

Active spaces

Partial ground state geometry optimisations performed with ((12+n),16) active spaces (n = 0 (Ce, Th), 1 (Pa) and 2 (U)….)

Ground and excited states calculated with ((8+n),14) active spaces (n = 0 (Ce, Th), 1 (Pa), 2 (U)….)

For the partial geometry optimisations of the ground state of Pa(COT)2 (13,16), 11,451,440 configurations were included

Results – Th(COT)2

Ground state is the expected 1Ag (d0f0)

Metal-ring distance; 2.015 Å (calc), 2.004 Å (expt)

Two lowest energy singlet and triplet states of each D2h irrep calculated (32 states)

Lowest energy dipole-allowed transition is to 1B1u (dσ1f0); 2.47 eV (calc), 2.76 eV (expt –

UV/Vis)

Spin-orbit coupling makes essentially no difference to energy spectrum (<0.05 eV).

First excited states (Th(Cp'')3)Ground

state

Th(COT)2 energy level diagram

Results – Pa(COT)2

Ground state is a degenerate pair of spin-orbit free states 2B2u/2B3u (d0ff1)

Metal-ring distance; 1.969 Å (calc), 1.964 Å (“expt”, average of Th(COT)2 and U(COT)2)

Two lowest energy doublet and quartet states of each D2h irrep calculated

Spin-orbit coupling makes a significant difference

Pa(COT)2 energy level diagram(no spin-orbit coupling)

The effect of spin-orbit coupling on the ground and lowest excited states of Pa(COT)2

A comparison of the spin-orbit coupled Pa(COT)2 energy levels (eV) with those from previous calculations

State Symmetry This work Chang et al. a Li & Bursten b

1 E5/2u 0 0 0

2 E1/2u 0.003 0.166 0.049

3 E3/2u 0.459 0.477 0.369

4 E7/2u 0.584 0.362 0.379

5 E1/2u 0.642 0.569 0.541

6 E1/2g 0.880 0.925 0.685

7 E3/2u 1.467 1.222 1.122

a SOCI calculations using the experimental uranocene geometry (1.924 Å)b DFT calculation using the PW91 exchange-correlation functional, using an optimised geometry with ring-metal separation of 1.975 Å

Results – U(COT)2

Metal-ring distance; 1.944 Å (calc), 1.924 Å (expt)

Spin-orbit coupled ground state is E3g

Dominant configuration of

spin-orbit free state

Total spin of spin-orbit free

state

This work Chang et al.

fp1ff1 1 70.7 68.0

fs1ff1 1 22.1 22.7

fs1ff1 0 7.0 5.3

Results – U(COT)2

Comparison of experimental (UV/Vis) excitation energies (eV) with calculation

Expt This work

1.880

1.934 1.65

2.018 1.79

Both calculated transitions are principally f dσ in character

Results – Ce(COT)2

Ground state is the expected 1Ag

Metal-ring distance; 1.964 Å (calc), 1.969 Å (expt)

Lowest energy dipole-allowed transitions are to 1B1u (d0fσ1) and 1B2u/1B3u (d0fp1); 2.47

eV (calc), 2.18 eV (expt – UV/Vis). Second dipole-allowed transition to 1B1u (d0fd1); 2.93 eV (calc), 2.63 eV (expt)

As with Th(COT)2, spin-orbit coupling makes essentially no difference to energy spectrum (<0.05 eV).

Ce(COT)2 energy level diagram

A look at the ground and first excited 1Ag states of Ce(COT)2

58.1% f0, 23.4% fd1, 8.7% fd2

84.6% fd1, 6.2% fd2

How can we square this result with previous

theory and experiment for Ce(COT)2?

(number of states in state-average)

Cha

nge

in g

roun

d st

ate

ener

gy

Single state total energy = -257724.60 eV

Experiment (XANES): 0.89 ± 0.03

C.H. Booth, M.D. Walter, M. Daniel, W.W. Lukens and R.A. Andersen Phys. Rev. Lett. 95 (2005) 267202

Ce(COT)2 f electron occupancy nf

Calculation: 0.90 ± 0.04

Configurational admixture of Ce(COT)2 ground state as a function of ns

Occupation (NOO) of the Ce(COT)2 ground state natural orbitals as a function of ns

A. Kerridge, R. Coates and N. Kaltsoyannis J. Phys. Chem. A 113 (2009) 2896

Th Pa U Pu Cm0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

What about the actinides?Occupation of the ground state e2u “f” natural orbitals in An(COT)2

Ce(COT)2 = 0.216

A. Kerridge and N. Kaltsoyannis J. Phys. Chem. A 113 (2009) 8737

Story 4: Ab initio quantum chemical calculations of CeCp3 and CeCp3+

8x10

6

6

4

2

0

Norm

alize

d c

ounts

181614121086

IE / eV

hv = 122 eV

A (f)

B C

D (f)

Cp p

Recall the PE spectrum of CeCp3…..

Active spaces for CeCp3 and CeCp3+

2,3 (e1'')e

CpCp3CeCp3

e

a1

a2

1e

1a1

1a2

2e

2a1 + 3a1 + 3e + 4e

4a1 (dz2)

2a2

4f

5d

Ce

Inclusion of all 14 MOs too costly (5,8) for CeCp3 and (4,8) for CeCp3+ (4 a, 4 a)

Configurational admixture of CeCp3+ 1A ground state as a function of ns

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10

Number of states in average

Con

trib

utio

n of

con

figur

atio

n (%

)

2000 2000

2000 u0d0

2000 0020

Use natural orbitals and their occupations

NOOs of CeCp3 2A ground state

Active space orbital 48a' 49a' 50a' 51a' 33a'' 34a'' 35a'' 36a''

Occupation 1.967 0.001 0.027 0.005 1.966 0.029 0.005 1.000

Single configurational stateOne 4f-localised NO

NOOs of CeCp3

+ 1A ground state

Active space orbital 48a' 49a' 50a' 51a' 33a'' 34a'' 35a'' 36a''

Occupation 1.961 0.000 0.000 0.038 1.445 0.000 0.555 0.000

Strongly multi-configurational stateNo 4f-localised NO (as expected following 4f ionisation)Energy relative to CeCp3: 7.07 eV (band A in PE spectrum 6.77 eV)

NOOs of CeCp3

+ fifth excited 1A and 1A states

Active space orbital 48a' 49a' 50a' 51a' 33a'' 34a'' 35a'' 36a''

Occupation 1A 1.509 1.000 0.001 0.490 0.963 0.000 0.000 0.037

Occupation 1A 0.985 1.013 0.000 0.002 1.473 0.000 0.526 0.001

Strongly multi-configurational statesNo 4f-localised NO (as expected following 4f ionisation)Energy relative to CeCp3: 10.00 and 10.17 eV (band D in PE spectrum 9.97 eV)

R. Coates, M. Coreno, M. DiSimone, J.C. Green, N. Kaltsoyannis, A. Kerridge, N. Narband and A. Sella Dalton Trans. (2009) 5943

Conclusions - 1

Calculations (Dolg et al.) suggest that Ce(COT)2 has a multiconfigurational ground state, with a dominant f1 (Ce(III)) configuration. XANES results (us and Booth et al.) appear to support this.

Variable energy photoelectron spectroscopy of CeCp3 reveals not one but two f bands during resonance; is the ground state of CeCp3

+ multiconfigurational?

CAS calculations on An(COT)2 (An = Th, Pa, U) yield results consistent with experiment and previous computational studies.

CAS calculations on Ce(COT)2 produce excellent agreement with experiment for metal-ring separation, electronic excitation energies and f electron occupancy (nf).

Total energy of Ce(COT)2 ground state, nf, the natural orbitals and their occupations are essentially invariant to the number of states included in the state-average.

Description of ground state in terms of configurational admixture varies wildly as a function of state average configurational admixture not a reliable tool to describe the electronic structure of Ce(COT)2.

Conclusions - 2

Ce(COT)2 is best described as Ce(IV) system in which transfer of electron density from ligand to metal through occupation of bonding orbitals allows measures of the effective oxidation state to be lower than the formal +4 value, and indeed closer to +3 in certain cases.

Occupation of the ground state e2u “f” natural orbitals increases markedly across the actinide series, indicating that the ground states of the later actinocenes are strongly multiconfigurational.

The ion states which give rise to bands A and D in the photoelectron spectrum of CeCp3

+ are strongly multiconfigurational, and do not possess a Ce 4f-localised natural orbital (i.e. they have the characteristics of f ionization).

And finally……

“The effective oxidation state of Ce in cerocene is intermediate between the formal Ce(IV) and Ce(III) situations. When interpreted as a Ce(IV) system the effective oxidation number is lowered toward III by strong orbital mixing, whereas when interpreted as a Ce(III) system a strong configurational mixing increases the effective oxidation number toward IV. The latter choice however is more compact since only two configurations…..are needed for building a sufficiently accurate zeroth-order wavefunction: the cerocene 1A1g ground state can be described as a…..mixture of about 70% 4f1p3 and 30% 4f0p4.”

The without whom department

National Service for Computational Chemistry Software

BerkeleyNorm EdelsteinPat AllenJerry BucherDave ShuhChad Sofield

UCLAndy KerridgeRosie CoatesAndrea SellaMaria-Rosa RussoGraham MaunderNaima Narband

OxfordJenny Green

Trieste (Elettra)Monica DiSimoneMarcello Coreno