Electronic Structure of Actinyl Ions - ACS Symposium Series (ACS

15 Electronic Structure of Actinyl Ions

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 13, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch015

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

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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

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3

3

1

u

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u

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U

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U

3

u

u

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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.


Electronic Structure of Actinyl Ions - ACS Symposium Series (ACS

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