Lanthanide and Actinide Chemistry and Spectroscopy - American

15 Electronic Structure of Actinyl Ions

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R. G. DENNING, J. O. W. NORRIS, I. G. SHORT, T. R. SNELLGROVE, and D. R. WOODWARK Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, United Kingdom

The covalent bond in a c t i n i d e chemistry is seen in its simplest and most striking form in the actinyl ions, MO . These i o n s , t h e r e f o r e , provide the most s t r a i g h t f o r w a r d t e s t o f our understanding o f the covalent bond in these elements. Although superficially similar to transition metal oxy-cations there are many striking d i f f e r e n c e s . A u s e f u l example can be made o f M o O C l ( P P h O ) and UO Cl ( P P h O ) whose X-ray crystal s t r u c t u r e s have r e c e n t l y been reported ( 1, 2). The approximate geometries are shown in Figure 1. Apart from the l a r g e r r a d i u s o f uranium, as observed i n the m e t a l - c h l o r i n e d i s t a n c e s , the most striking p o i n t is the change from c i s - d i o x o geometry in the molybdenum compound t o t r a n s - d i o x o geometry in the uranium compound. A c t u a l l y these compounds are only prototypes o f general stereochemical d i f f e r e n c e s between dioxo compounds o f the transition metals and of the a c t i n i d e s . From the examples i n Table 1 it seems t h a t the principal f a c t o r determining the geometry is the nature o f the lowest energy metal valence s h e l l and its occupancy. I t i s striking that the a d d i t i o n o f 'd' e l e c t r o n s t o the valence s h e l l causes a change in geometry, whereas the a d d i t i o n o f 'f' e l e c t r o n s causes no change in t h e actinyl i o n s . 22+

2

2

3

2

2

2

3

2

Stereochemistry o f dioxo compounds The stereochemistry o f the t r a n s i t i o n metal compounds can be r a t i o n a l i s e d i n a simple way which i s i l l u s t r a t e d i n Figure 2. I f only the d o r b i t a l s are considered t o be important i n the bond, the l i n e a r dioxo ions have metal o r b i t a l s o f a , TT^ and 6 symmetry w h i l e the oxygen bonding o r b i t a l s have a ,a , TT_, and symmetry. The argument may be i l l u s t r a t e d by c o n i i d e r i n g only the a - o r b i t a l s . The upper p a r t o f Figure 2 shows the r e s u l t o f bending the M0 u n i t . In the d° ions the oxide o r b i t a l s are f o r m a l l y f u l l and the metal o r b i t a l s vacant. The system i s theref o r e s t a b i l i s e d on changing from the l i n e a r c o n f i g u r a t i o n , where there are two bonding and two non-bonding e l e c t r o n s t o the bent geometry f o r which a l l four e l e c t r o n s are i n bonding o r b i t a l s . f

f

U

2

0-8412-0568-X/80/47-131-313$05.00/0 © 1980 American Chemical Society Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


314

Table I.

LANTHANIDE

A N DACTINIDE

CHEMISTRY

A N D SPECTROSCOPY

Geometry and Metal Oxygen Bond Lengths of Some Metal Dioxo Compounds.

d°

d

CJS

f°
6

9

1

6*,^

2

2p

Figure 3.

Schematic orbital energies in actinyl ions

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

318

LANTHANIDE

AND

ACTINIDE CHEMISTRY

AND

SPECTROSCOPY

v i r t u a l l y u n a f f e c t e d by the s u b s t i t u t i o n while those a s s o c i a t e d with the progressions are s t r o n g l y s h i f t e d (10). F o r t u n a t e l y the c r y s t a l s t r u c t u r e of Cs U0 C l ^ i s p a r t i c u l a r l y simple, there being only one molecule per u n i t c e l l , the uranium atom l y i n g at a C ^ s i t e with i n v e r s i o n symmetry (11). In the monoclinic system i t proves p o s s i b l e to propagate the l i g h t i n three orthogonal d i r e c t i o n s X, Y and Z with respect to the molecul a r a x i s system (Figure 5) and to choose the e l e c t r i c v e c t o r of the r a d i a t i o n (x), (y) and (z) i n such a way as to define s i x d i f f e r e n t experiments. The outcome i s shown i n Figure 6. By comparing the X(y) and Z(y) s p e c t r a and the Z(x) and Y(x) s p e c t r a the bands l a b e l l e d I and II i n the f i g u r e are seen to be magneticd i p o l e allowed, while a c a r e f u l study of band I I I (9) shows i t to be e l e c t r i c - q u a d r u p o l e allowed. S i m i l a r evidence shows that a l l twelve e l e c t r o n i c e x c i t e d s t a t e s observed i n t h i s spectrum (10) are p a r i t y forbidden. Since the lowest energy empty o r b i t a l s are ungerade f o r b i t a l s i t follows that the e x c i t a t i o n must come from e i t h e r a or TT f i l l e d o r b i t a l s . More evidence about the nature of the e x c i t e d s t a t e s comes from Zeeman e f f e c t measurements. In the C ^ s i t e i n C s ^ O ^ C l ^ there i s no degeneracy p o s s i b l e so that a l l Zeeman e f f e c t s are second order, nevertheless the symmetry i s s u f f i c i e n t l y c l o s e to D ^ that the second order e f f e c t s are e a s i l y measured. Figure 7 shows some examples. The most important observation i s that the f i r s t e x c i t e d s t a t e has a magnetic moment of 0.16 Bohr Magnetons. Apparently the magnetic moment of the hole i n the oxygen o r b i t a l s almost cancels that of the f e l e c t r o n . Two s t a t e s , with the rig ( D ^ ) symmetry implied by the magnetic d i p o l e i n t e n s i t y , seem 1 p o s s i b l e , with the wavefunctions | 5 6 > and | T T 'itu1^1There i s no simple choice at t h i s p o i n t between these p o s s i b i l i t ies. Nevertheless the observed symmetries o f the remaining e x c i t e d s t a t e s are b e t t e r described i n terms of the former c o n f i g u r a t i o n . Figures 8 and 9 show the energies o f the various e x c i t e d s t a t e s a r i s i n g from the a 6 and ^ ^ configurations using r e a l i s t i c s p i n - o r b i t coupling parameters and v a r y i n g the i n t e r - e l e c t r o n r e p u l s i o n parameters. Figure 8 p r e d i c t s that the second e x c i t e d s t a t e w i l l be of A ( D ^ ) symmetry while Figure 9 p r e d i c t s r g ( D ^ ) symmetry. The e l e c t r i c quadrupole i n t e n s i t y of band I I I i n Figure 6 i s only c o n s i s t e n t with B g(D | ) and A g ( D ^ ) symmetry suggesting that Figure 8 and the a 6 c o n f i g u r a t i o n give the best d e s c r i p t i o n . There are many a d d i t i o n a l pieces of evidence to support t h i s a s s e r t i o n , the most powerful of which i s a t h e o r e t i c a l argument f i r s t advanced by GtJrller-Walrand and Vanquickenborne (12) and s l i g h t l y r e c a s t by us (13) which shows that i n a strong a x i a l f i e l d i t i s not p o s s i b l e to observe f i r s t - o r d e r e q u a t o r i a l f i e l d s p l i t t i n g s i n a two-open-shell system unless the c o n f i g u r a t i o n i s o f the type ay, where y i s a general representation. Since there i s ample evidence of f i r s t order e q u a t o r i a l f i e l d s p l i t t i n g s the e x c i t a t i o n of a a„ r a t h e r than a IT e l e c t r o n i s s t r o n g l y 2


2

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15.

DENNING E T A L .

Electronic

Structure

of Actinyl

Ions

Figure 5. Crystallographic axes, crystal habit, and molecular axes of Cs^t/O^C^

(9)


319

320

LANTHANIDE

A N D ACTINIDE

CHEMISTRY

A N D SPECTROSCOPY

Xly)


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Molecular Physics

Figure 6.

Absorption spectrum of single crystals of C5 C70 CZ at 4.2K in six different polarizations. Notation is explained in the text (9). 2

2

4


Electronic

Structure

of Actinyl

Ions


DENNING E T AL.


321

322


LANTHANIDE

A N D ACTINIDE CHEMISTRY

A N D SPECTROSCOPY

2nd 1st order order s o c. 0

25

50

75

100

125

150

>

(cm ) -1

Molecular Physics

Figure 8.

Correlation diagram for the states arising from the o-S configuration (IS)


ET AL.

Electronic

Structure

of Actinyl

323

Ions


DENNING

Molecular Physics

Figure 9.

3

Correlation digram for the states arising from the Tr configuration (13)


324

LANTHANIDE

A N D SPECTROSCOPY

supported. For example, i n the approximately s i t e symmetry found i n C s U 0 ( N 0 ) , a x i a l f i e l d s t a t e s o f $ symmetry ( a r i s i n g from a o $ c o n f i g u r a t i o n ) should be s p l i t by the e q u a t o r i a l f i e l d into A and A components (14). The absence o f f i r s t order Zeeman e f f e c t s makes these s t a t e s d i f f i c u l t to i d e n t i f y . Nevert h e l e s s we have found that the n i t r a t e i n t e r n a l modes couple appreciably to the e l e c t r o n i c t r a n s i t i o n s i n t h i s compound (15). Figure 10 shows the nitrogen-15 isotope s h i f t o f one such f e a t u r e . The magnitude o f the s h i f t i d e n t i f i e s the mode, whose frequency i s known from the pure v i b r a t i o n a l spectrum, and the symmetry o f the r e p r e s e n t a t i o n s which i t spans i n D ^ . Taken with the p o l a r i s a t i o n data the symmetry o f the e l e c t r o n i c e x c i t e d s t a t e t o which t h i s mode couples can then be constrained to e i t h e r A or E ( D h ) . The absence o f a magnetic moment narrows the choice to A j " . The A " component o f the $g ( D ^ ) s t a t e can a l s o be i d e n t i f i e d v i a the s i m i l a r isotope c h a r a c t e r i s a t i o n o f a second n i t r a t e i n t e r n a l mode. Using a v a r i e t y o f experimental techniques o f t h i s kind we have been able to f i x the energies and, with a few exceptions, the symmetries o f twelve e l e c t r o n i c e x c i t e d s t a t e s i n C s U 0 C l (10), and seven e x c i t e d s t a t e s i n both C s U 0 ( N 0 ) and NaUO (acetate) (14). S u p e r f i c i a l l y the s t a t e s appear to a r i s e from the e x c i t a t i o n o f a a e l e c t r o n and so we have t e s t e d a simple t h e o r e t i c a l model based on the a c o n f i g u r a t i o n l i e s 2900cm" above the a 6 c o n f i g u r a t i o n i s important. This i s not the same as the d i f f e r e n c e between the and 6 v i r t u a l o r b i t a l s on account o f the a t t r a c t i o n between the e l e c t r o n i n these o r b i t a l s and the hole i n the a s h e l l . Making a reasonable estimate o f t h i s a t t r a c t i o n sets the v i r t u a l o r b i t a l between 1500cm" and 2700cm" above the 6 v i r t u a l o r b i t a l (13). J^rgensen (16) takes the view, opposed to ours, that the f i r s t e x c i t e d s t a t e s o f the uranyl i o n stem from the ff conf i g u r a t i o n . The i m p l i c a t i o n s f o r the r e l a t i v e c|> and 6 v i r t u a l o r b i t a l energies have not been i n v e s t i g a t e d but i t seems u n l i k e l y that t h i s assignment i s c o n s i s t e n t with a ^ o r b i t a l 2000cm" above the 6 o r b i t a l . The simplest way to independently i n v e s t i gate the energies o f these two o r b i t a l s i s through the p r o p e r t i e s of the s i n g l e f e l e c t r o n i n the neptunyl i o n . To t h i s end we have confirmed, by Zeeman e f f e c t measurements, the p e c u l i a r ESR r e s u l t s , due to Leung and Wong (17), that i n C s U ( N p ) 0 C l the 2

u

3

3

u

11


A N D ACTINIDE CHEMISTRY

11

11

M

3

2

2

2

3

2

l f

3

u

u

u

u

u

2

3

3

1

u

u

u

u

u

U

u

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u

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U

3

u

u

u

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U

f

f



DENNING E T A L .

Electronic

Structure

of Actinyl

Ions

325

Figure 10. Nitrogen-15 isotopic shift in the ir-polarized, single-crystal absorption spectrum of CsU0 (N0 ) at 4.2K 2

3

3


LANTHANIDE

A N D ACTINIDE

CHEMISTRY A N D SPECTROSCOPY


326

Molecular Physics

Figure 11. Calculated and observed energy levels for Cs U0 Cl . the diagram indicate magnetic moments (13). 2

2

If

Numbers on


15.

DENNING ET AL.

Electronic

Structure

of Actinyl

Csuo (N0 )


2

3

327

Ions

;

u 131

Molecular Physics

Figure 12. Calculated and observed energy levels for CsU0 (N0 ) . on the diagram indicate magnetic moments (13). 2

3

3

Numbers


328

LANTHANIDE

f

AND

ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

f

ground s t a t e g values are g„-gj.-1.32. The apparent i s o t r o p y of the *g value seems to c o n t r a d i c t the extreme anisotropy of the ligand f i e l d . The reason can be uncovered by a c a l c u l a t i o n of the g values as a f u n c t i o n o f the energy d i f f e r e n c e of the 0 and 6 o r b i t a l s (18). Figure 13 shows that when t h i s d i f f e r e n c e i s large i n e i t h e r sense g tends to zero and g to the appropriate value for the ground s t a t e . Intermediate values can be seen to a r i s e because of the mutual i n t e r a c t i o n of the a consequence of both the t e t r a g o n a l f i e l d and the second-order spin o r b i t coupling. The e x c e l l e n t agreement between the t h e o r e t i c a l p r e d i c t i o n that gn-gi=1.4 and the experimental values sets t i g h t l i m i t s on the o r b i t a l energy d i f f e r e n c e at 2100cm""-. This i s e x c e l l e n t support f o r the parameter choice used i n our model of the uranyl e x c i t e d s t a t e s . f

T

!

x

u


a n c

s

t

a

t

e

s

a s

1

A l l i n a l l our work implies that the highest f i l l e d o r b i t a l s are o f a symmetry. To anyone r e f l e c t i n g on the e l e c t r o n i c s t r u c t u r e of carbon dioxide i t i s e x t r a o r d i n a r y to f i n d the a o r b i t a l above the TT o r b i t a l , implying that the l a t t e r forms fee stronger bond. Nevertheless t h i s s t a t e of a f f a i r s was a n t i c i p a t e d many years ago i n the overlap c a l c u l a t i o n s of B e l f o r d and B e l f o r d (4). They pointed out that the angular nodal p r o p e r t i e s of the f and f o r b i t a l s are such that at short distances the f -p overlap may a c t u a l l y be l e s s than the f -p overlap; a r e s u l t con?irmed i n a c a l c u l a t i o n by Newman (5). The s i t u a t i o n i s , however, more complicated than t h i s argument implies because the ff-rr ) antibonding o r b i t a l energy i s observed, i n the spectra of the neptunyl i o n , to be about 15,000cm" above the 0 and 6. o r b i t a l s (18,19), while the f ( a ) o r b i t a l , presumably at much higher energy i s not observed. ¥t seems l i k e l y , from recent comprehensive c a l c u l a t i o n s (8), that the r e l a t i v e l y high energy o f the f i l l e d (and empty) a o r b i t a l s a r i s e s from the r o l e o f the f i l l e d 6p(a ) o r b i t a l of the c l o s e d s h e l l w i t h i n the valence s h e l l ; i t s i n t e r a c t i o n with oxygen o r b i t a l s being greater than that of 6p(7r ). Whatever the explanation i t i s c l e a r from the drop of the u r a n y l symmetric s t r e t c h i n g frequency i n the e x c i t e d states (from 835cm" to 710cm" ) that the a e l e c t r o n i s q u i t e s t r o n g l y bonding. Since the TT , a and IT o r H i t a l s must a l l be placed below the a o r b i t a l ¥hey^too mult be seen as s t r o n g l y bonding. The best evidence t h e r e f o r e suggests an energy l e v e l scheme of the type shown i n Figure 14. The i m p l i c a t i o n i s that a l l twelve valence e l e c t r o n s are i n bonding o r b i t a l s , o f f e r i n g an explanation f o r the extraordinary s t a b i l i t y and shortness o f the a c t i n y l bond. Formally each metal oxygen bond i s a t r i p l e bond. Moreover because the o and TT o r b i t a l s are already bonding i n the l i n e a r geometry, by v i r t u e of t h e i r i n t e r a c t i o n with f o r b i t a l s , there i s no tendency f o r the l i n e a r dioxo u n i t to bend as i s the case i n the t r a n s i t i o n metal oxy c a t i o n s . A d d i t i o n of f u r t h e r f e l e c t r o n s leads to the f i l l i n g of o r b i t a l s which are non-bonding towards oxygen so that the remaining a c t i n y l ions are a l s o l i n e a r . 1

u

1

11

!

f

1


f

DENNING E T AL.


15.

Electronic

Structure

of Actinyl

Ions

329

— O

a

— TI

U

5f

ALL BONDING

Figure

14. A possible energy-level scheme for actinyl ions


330

LANTHANIDE AND ACTINIDE CHEMISTRY A N D SPECTROSCOPY

f

f

f

?

Summarising, there i s c l e a r evidence that both £ and d o r b i t a l s p a r t i c i p a t e i n the a c t i n y l bond and i t i s t h i s j o i n t p a r t i c i p a t i o n which i s r e s p o n s i b l e f o r both the s t a b i l i t y and the l i n e a r i t y o f the dioxo i o n s . Literature Cited 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Butcher, R. J . ; Penfold, B. R.; Sinn, E. J. Chem. Soc.; Dalton Trans., 1979, 668. Bombieri, G.; F o r s e l l i n i , E.; Day, J . P.; Azzeez, W. I. J. Chem. Soc., Dalton Trans., 1978, 677. McGlynn, S. P.; Smith, J. K. J. Mol. Spec., 1961, 6, 164. B e l f o r d , R. L.; B e l f o r d , G. J. Chem. Phys., 1961, 34, 1330. Newman, J. B. J. Chem. Phys., 1965, 43, 1691. Boring, M.; Wood, J. H.; Moscowitz, J. W. J . Chem. Phys., 1975, 63, 638. Yang, C. Y.; Johnson, K. H.; Horsley, J . A. J. Chem. Phys., 1978, 68, 1000. Walch, P. F.; Ellis, D. E. J. Chem. Phys., 1976, 65, 2387. Denning, R. G.; S n e l l g r o v e , T. R.; Woodwark, D. R. Mol. Phys., 1975, 30, 1819. Denning, R. G.; S n e l l g r o v e , T. R.; Woodwark, D. R. Mol. Phys., 1976, 32, 419. H a l l , D.; Rae, A. D.; Water, T. N. Acta C r y s t . , 1966, 20, 160. Gőrller - Walrand, C.; Vanquickenborne, L. G. J. Chem. Phys., 1972, 57, 1436. Denning, R. G.; S n e l l g r o v e , T. R.; Woodwark, D. R. Mol. Phys., 1979, 37, 1109. Denning, R. G.; F o s t e r , D. N. P.; S n e l l g r o v e , T. R.; Woodwark, D. R. Mol. Phys., 1979, 37, 1089. Denning, R. G.; Short, I. G.; Woodwark, D. R. ( i n preparation) Jørgensen, C. K. J. Luminescence, 1979, 18, 63. Leung, A. F.; Wong, E. Y. Phys. Rev., 1969, 180, 380. Brown, D.; Denning, R. G.; N o r r i s , J. O. W. ( i n preparation) Stafsudd, O. M.; Leung, A. F.; Wong, E. Y. Phys. Rev., 1969, 180, 339.

RECEIVED December 26, 1979.


Lanthanide and Actinide Chemistry and Spectroscopy - American

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