Anion Resonance States of Organometallic Molecules - American

11 Anion Resonance States of Organometallic Molecules 1

JUDITH C. GIORDAN , JOHN H. MOORE, and JOHN A. TOSSELL

Downloaded by SWINBURNE UNIV OF TECHNOLOGY on May 25, 2018 | https://pubs.acs.org Publication Date: September 28, 1984 | doi: 10.1021/bk-1984-0263.ch011

Department of Chemistry, University of Maryland, College Park, MD 20742 The technique of electron transmission spectroscopy has only recently been used to measure the energies of anion states of organometallics, following a long and very s i g n i f i c a n t history of application to the study of di- and triatomics and unsaturated hydrocarbons. Two classes of investigation have been carried out: the study of t r a n s i t i o n metal compounds such as Cr(CO) , Fe(CO) , ferrocene, and other metallocenes; and the study of organic species such as benzene on which organometallic and other main group organo ligands have been substituted. Theoretical calculations, the most successful of which employed the multiple scattering Xα formalism, have been used to describe the scattering process and assign anion states in the t r a n s i t i o n metal compounds as well as model compounds such as S i H . Most recently mass spectrometric techniques have been used to identify fragments arising from d i s s o c i a t i v e anion states. 6

5

4

Electron scattering experiments, once considered valuable only to study atoms and small molecules, are now being used to aid in understanding the structure and dynamics of large organic and even organometallic molecules. Over the l a s t two decades the ionization potentials of a great variety of organic, main group organo, and organometallic molecules have been determined by photoelectron spectroscopy (PES). These energies have been shown to correlate with the energies of occupied molecular o r b i t a l s . Since e s s e n t i a l l y a l l descriptions of molecular spectra and reactions between molecu les can be explained using a molecular o r b i t a l picture, these data have proven quite useful. To complete this picture, however, the

1

Current address: Polaroid Corporation, Waltham, MA 02254.

0097-6156/ 84/0263-0193$06.00/0 © 1984 American Chemical Society

Truhlar; Resonances ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


194

RESONANCES

c h e m i s t mast a l s o have a v a i l a b l e a measure of the e n e r g i e s of u n o c cupied o r b i t a l s . M o l e c u l a r e l e c t r o n a f f i n i t i e s can p r o v i d e t h i s data. Over the p a s t decade, the t e c h n i q u e of e l e c t r o n t r a n s m i s s i o n s p e c t r o s c o p y (ETS), which a l l o w s f o r the measurement of these e l e c t r o n a f f i n i t i e s , has been d e v e l o p e d and e f f e c t i v e l y a p p l i e d i n the study of a g r e a t number of o r g a n i c s y s t e m s . As we w i l l i l l u s t r a t e t h i s t e c h n i q u e i s now f i n d i n g a p p l i c a t i o n t o t r a n s i t i o n m e t a l and main-group o r g a n i c s p e c i e s . This is e s p e c i a l l y exciting since we now have a t o o l t o d i r e c t l y probe the e n e r g i e s of those u n o c c u p i e d o r b i t a l s which are i n v o k e d i n l i g a n d f i e l d s p l i t t i n g , t h r o u g h - s p a c e i n t e r a c t i o n , and d - o r b i t a l bonding arguments. Concommitant with these e x p e r i m e n t s , m o l e c u l a r o r b i t a l methods a r e b e i n g d e v e l o p e d t o a i d i n i n t e r p r e t i n g the r e s u l t s . In t h i s summary f o c u s w i l l be g i v e n t o examples of o r g a n o m e t a l l i c and m a i n g r o u p - o r g a n i c s p e c i e s which i l l u s t r a t e the t h r u s t of our work i n this area. O v e r a l l , we wish t o use ETS i n c o n j u n c t i o n with o t h e r s p e c t r a l , c o m p u t a t i o n a l , t h e r m o c h e m i c a l , and p h o t o c h e m i c a l d a t a , some of which are a l r e a d y a v a i l a b l e , t o s t u d y m e t a l - l i g a n d i n t e r a c t i o n s and t h e i r i m p l i c a t i o n s f o r l i g a n d s u b s t i t u t i o n and c a t a l y t i c reactions. In the f o l l o w i n g we p r e s e n t the r e s u l t s of s t u d i e s of the a n i o n s t a t e s of the f i r s t , row m e t a l l o c e n e s , a s e r i e s of t r a n s i t i o n m e t a l h e x a c a r b o n y l s and a v a r i e t y of main-group organo l i g a n d s s u b s t i t u t e d on benzene. Work on the t r a n s i t i o n m e t a l h e x a c a r b o n y l s and m e t a l l o c e n e s p r o v i d e s benchmark d a t a to which t h a t f o r o t h e r complexes may be compared. The work on the s u b s t i t u t e d benzenes a n t i c i p a t e s the s t u d y of the e l e c t r o n t r a n s m i s s i o n s p e c t r a of more complex o r g a n o m e t a l l i c s such as C r ( C O ) ( w h e r e L i s a t w o - e l e c t r o n donor l i g a n d ) • In t h i s case the IT - s y s t e m of benzene s e r v e s as a probe of the empty a c c e p t o r o r b i t a l s of L. For example, by s t u d y i n g p a r a - b i s - ( d i m e t h y l p h o s p h i n o ) b e n z e n e (and t r i m e t h y l p h o s p i n e , as w e l l ) we g a i n knowledge of the e n e r g i e s of the unoccupied o r b i t a l s a s s o c i a t e d w i t h the - P M e group and the n a t u r e of the i n t e r a c t i o n of the - P M e group o r b i t a l s with the benzene T T * o r b i t a l s . 2

2

Experiment and Theory E l e c t r o n t r a n s m i s s i o n s p e c t r o s c o p y (1) i s the experiment c o n j u g a t e t o PES. Whereas PES measures the energy needed t o remove an e l e c t r o n from an o c c u p i e d o r b i t a l , ETS measures the energy of a n e g a t i v e i o n s t a t e a r i s i n g from e l e c t r o n c a p t u r e i n t o an u n o c c u p i e d orbital. The ETS experiment i n v o l v e s measurement of the t r a n s p a r e n c y of a gas t o an e l e c t r o n beam as a f u n c t i o n of e n e r g y . The t r a n s p a r e n c y depends i n an i n v e r s e f a s h i o n upon the e l e c t r o n s c a t tering cross s e c t i o n . Temporary n e g a t i v e i o n f o r m a t i o n o c c u r s w i t h l a r g e c r o s s s e c t i o n over o n l y a narrow energy range. S i n c e the n e g a t i v e i o n p r o m p t l y decays by g i v i n g up the t r a p p e d e l e c t r o n , the f o r m a t i o n and decay p r o c e s s appears as a s h a r p f l u c t u a t i o n i n the e l e c t r o n - s c a t t e r i n g c r o s s s e c t i o n . The p r o c e s s , as w e l l as the c o r r e s p o n d i n g f e a t u r e i n the t r a n s m i s s i o n v s . e l e c t r o n energy s p e c t r u m , i s r e f e r r e d t o as a " r e s o n a n c e " . The e l e c t r o n s p e c t r o m e t e r c o n s i s t s of an e l e c t r o n s o u r c e f o l l o w e d by an e l e c t r o n monochromator, a gas c e l l , and an e l e c t r o n


11.

GIORDAN E T A L .

195

Anion Resonance States


c o l l e c t o r (2)• In p r a c t i c e the f i r s t d e r i v a t i v e o f the t r a n s m i t t e d c u r r e n t as a f u n c t i o n o f energy i s r e c o r d e d s i n c e the d e r i v a t i v e i s s e n s i t i v e t o the abrupt change i n t r a n s m i t t e d c u r r e n t a s s o c i a t e d w i t h a resonance (3)• The energy o f a resonance i s known as an " a t t a c h m e n t energy" (AE) a n d , w i t h r e s p e c t t o the d e r i v a t i v e s p e c t r u m , i s d e f i n e d as the p o i n t v e r t i c a l l y midway between the minimum and maximum which c h a r a c t e r i z e the r e s o n a n c e . F o r the p r e s e n t purposes an attachment energy may be i d e n t i f i e d w i t h t h e n e g a t i v e o f the c o r r e s p o n d i n g e l e c t r o n a f f i n i t y ( E A ) . The c h i e f l i m i t a t i o n o f ETS i s t h a t i t g i v e s o n l y the energy a s s o c i a t e d with u n s t a b l e n e g a t i v e i o n s . That i s , only negative e l e c t r o n a f f i n i t i e s can be o b t a i n e d w i t h ETS. In the d e r i v a t i v e e l e c t r o n t r a n s m i s s i o n spectrum t h e r e i s always a sharp f e a t u r e near 0 eV which c o r r e s p o n d s t o the a b r u p t t u r n i n g on o f the e l e c t r o n c u r r e n t a t t h r e s h o l d (see F i g s . 1 and 4). The e x i s t e n c e o f an a n i o n s t a t e which i s m a r g i n a l l y s t a b l e or m a r g i n a l l y u n s t a b l e w i l l f r e q u e n t l y be m a n i f e s t i n a s i g n i f i c a n t b r o a d e n i n g of the t h r e s h o l d f e a t u r e . To o b t a i n i n f o r m a t i o n on d i s s o c i a t i v e attachment p r o c e s s e s o r i g i n a t i n g i n u n s t a b l e a n i o n s produced i n the e l e c t r o n t r a n s m i s s i o n s p e c t r o m e t e r , a t i m e - o f - f l i g h t s p e c t r o m e t e r has r e c e n t l y been appended t o our a p p a r a t u s . The r e s o l u t i o n o f t h i s d e v i c e i s s u f f i c i e n t t o determine both the mass and k i n e t i c energy o f n e g a t i v e ion fragments. The c o m p l e x i t y o f the o r g a n o m e t a l l i c m o l e c u l e s we have s t u d i e d p r e v e n t s us from using q u a l i t a t i v e , symmetry-based, MO models f o r assignment o f the o b s e r v e d r e s o n a n c e s . R a t h e r , we have employed the M u l t i p l e S c a t t e r i n g Xa (MS-Xa) method ( £ ) which has proven v a l u a b l e f o r assignment o f the PES and UV s p e c t r a o f m o l e c u l e s c o n t a i n i n g t r a n s i t i o n m e t a l atoms (5)• In e a r l y s t u d i e s on t h e d^ t r a n s i t i o n metal h e x a c a r b o n y l s (6) a n d t h e f i r s t t r a n s i t i o n s e r i e s m e t a l l o c e n e s (7), we employed the b o u n d - s t a t e v e r s i o n o f the MS-Xa method, s t a b i l i z i n g the a n i o n by e n c l o s u r e i n a s p h e r i c a l charge s h e l l . In l a t e r work, the continuum MS-Xa method (8) has been a p p l i e d t o C r ( C O ) (9) and t o a s e r i e s o f tetrahedral""c and S i compounds (10). -

6

Metallocenes D e r i v a t i v e e l e c t r o n t r a n s m i s s i o n s p e c t r a f o r the 3d m e t a l l o c e n e s {7) a r e p r e s e n t e d i n F i g . 1. V e r y s i m i l a r r e s u l t s have been o b t a i n e d by M o d e l l i , e t a l . (11). O c c u p a t i o n s o f the p r e d o m i n a n t l y M3d h i g h e s t o c c u p i e d o r b i t a l s and ground e l e c t r o n i c s t a t e s a r e g i v e n i n T a b l e I.



RESONANCES

ENERGY

(eV)

F i g u r e 1 . D e r i v a t i v e e l e c t r o n t r a n s m i s s i o n s p e c t r a o f the 3d metallocenes, M(Cp) (M=V, C r , Mn, F e , Co, Ni) (from r e f . 7 ) . Each spectrum was c a l i b r a t e d against, t h a t of N . 2

2


11.

G I O R D A N ET A L .

Table

I.

V(C ) Cr(C ) Mn(C ) P

Oe )2 a ) (3e ) 3 a ) (3e )4 a ) or ( 3 e ) ( 5 a 2

2

P

2

2

2 g

( 5

l g

( 5

l g

( 5

Fe(Cp) Co(Cp)

2

(3e (3e

No(Cp)

2

(3e

2

A 3 2 6

l g

A ] [ g

l g

)(4e

)2

l g

lA 2

)

3

l Q

2

l g

) (5a 4

2 g

4

) (4e

l g

2

l g

l g

A l g

)

2

(

4

2 g

2 g

E

)4 5a ) ) (5a ) ( e

2

of

4

2

2 g


197

O r b i t a l O c c u p a t i o n s and Ground E l e c t r o n i c S t a t e s Metallocenes

2

P


l g

E

2

A 2 g

A c o r r e l a t i o n diagram showing the o b s e r v e d A E ' s i s p r e s e n t e d i n F i g * 2. The r e s u l t s have been i n t e r p r e t e d u s i n g s t a b i l i z e d bound s t a t e MS-Xa c a l c u l a t i o n s . A q u a l i t a t i v e MO scheme f o r the M ( C p ) compounds i s p r e s e n t e d i n F i g . 3 (where C p = C H , the c y c l o p e n t a d i e n y l group). For the F e , Co, and N i compounds the c a l c u l a t e d attachment e n e r g i e s f o r the 4 e M 3 d - o r b i t a l and f o r the 4 e and 3 e Cp i r * o r b i t a l s are g i v e n i n T a b l e II. For the 4 e orbital, calculation 2

5

5

l g

2 g

2 u

l g

Table

II.

C a l c u l a t e d Attachment E n e r g i e s of

4e

l g

,

4e

2 g

, and 3 e

(Spin-Polarized Case*,

Fe(Cp)

4e ig 4e 2g 3e 2u

a

b

Co(Cp)

2

1.15 1.19 3.26

2 u

and

a v e r a g e AE f o r s i n g l e t , s t a t e s A E for A state.

Occupation 2

Co(Cp)

2

2

Ni(Cp)

:

-0.13 1.14 3.68

b

^A^g

for

in Fe(Cp) ,

Ni(Cp)

2

0.93 ,-0.47 1.16 3.49 a

(eV)

Orbitals

+

^ g ) .

3

2 g

and experiment, agree t o w i t h i n 0.5eV and the o b s e r v e d s t a b i l i z a t i o n i n going from Fe t o Ni i s r e p r o d u c e d . The Cp ir* t y p e o r b i t a l s o f e symmetry a l s o have a c a l c u l a t e d AE c l o s e t o the e x p e r i m e n t a l v a l u e , but the 4 e a r e c o n s i d e r a b l y lower i n energy w i t h s u b s t a n t i a l Rydberg c h a r a c t e r . L a t e r MS-Xa c a l c u l a t i o n s (11) u s i n g a s l i g h t l y l a r g e r p a r t i a l wave b a s i s s e t o b t a i n e d an a d d i t i o n a l e s t a t e w i t h AE c l o s e t o t h a t f o r the 3 e , i n agreement, w i t h e x p e r i ment. In the l a t t e r s t u d y , s p i n - p o l a r i z e d MS-Xa c a l c u l a t i o n s were a l s o performed on V ( C p ) and C r ( C p ) y i e l d i n g the c a l c u l a t e d AE o f T a b l e III (8). 2 u

2 g

2 g

2 u

2

2



198

RESONANCES

3 -

2 95

271

2 73

2 62

260

2 45

2 05

2 03

195 1 49

I -

0 88

0 68

063

0 50

^00

7777/,V(Cp)

Cr(Cp)

2

Mn(Cp)

2

Fe(Cp)

2

2

Co(Cp)

2

Ni(Cp)

2

Figure 2. Correlation diagram giving attachment energies of the 3d metallocenes (from r e f . 7).

Cp M 2

M

Cp

2

(D d)

Cp tD l

5

5h

/ v 2u / e (jr*) \ / /e g / eig(d) / / / ' 1/ ain(d) / x

e

x

e (Jt*) 2

2u

2

3d

e

2g(d)

\

Figure 3.Qualitative diagram of the frontier o r b i t a l s of M(Cp>2 and i t s fragments (from r e f . 11).


11.

Table

III.

199


GIORDAN E T A L .

C a l c u l a t e d Attachment E n e r g i e s (eV) Empty O r b i t a l s i n V ( C p ) , C r ( C p ) , and F e ( C p )

for Occupation of

2

2

V(Cp)

5 a

3 e


4 e

4 e

3 e

a

D

lg 2g lg 2g 2u

Cr(Cp)

a 2

0.14 0.33 1.35,2.89 2.88,3.04 2.66,2.71

2

Fe(Cp)

b 2

-0.58 -0.19 1.14,2.20 2.94,3.05 2.80,2.85

q u i n t e t and t r i p l e t s t a t e s , q u a r t e t and d o u b l e t s t a t e s ,

2

1.26 2.98 2.78

respectively. respectively.

It i s i n t e r e s t i n g that i n V ( C p ) there are t r i p l e t s t a t e anions a r i s i n g from 5 a and 3 e o r b i t a l s l y i n g j u s t above t h r e s h o l d and both q u i n t e t and t r i p l e t E a n i o n s t a t e s o c c u r r i n g i n the 1-3 eV range. U n f o r t u n a t e l y , a complete s p e c t r a l i n t e r p r e t a t i o n has not y e t been performed f o r M n ( C p ) , f o r which the ground e l e c t r o n i c s t a t e i s not d e f i n i t i v e l y known ( 1 2 ) . N o n e t h e l e s s , the f o l l o w i n g g e n e r a l c h a r a c t e r i s t i c s o f the M3d o r b i t a l s emerge from the M ( C p ) d a t a : (1) s t a t e s o f d i f f e r e n t s p i n m u l t i p l i c i t y may d i f f e r i n energy by more than 1 eV, (2) s p i n averaged A E ' s f o r the 4 e orbital becomes l e s s p o s i t i v e a c r o s s the s e r i e s , (3) t h i s d r o p i s s m a l l e r from V t o Fe s i n c e the d e c r e a s i n g M - t o - r i n g d i s t a n c e (13) tends t o d e s t a b i l i z e the 4 e and l a r g e r from Fe t o Co and N i s i n c e the i n c r e a s i n g M - t o - r i n g d i s t a n c e augments the g e n e r a l M3d s t a b i l i z a t i o n e f f e c t , i t i s a l s o apparent t h a t the t r a n s i t i o n s t a t e approach (14) used i n r e f s . 7 and 11 f a c i l i t a t e s the comparison o f ETS r e s u l t s w i t h o t h e r s p e c t r a l q u a n t i t i e s . For example, i n N i ( C p ) the MS-Xa ground s t a t e o r b i t a l e i g e n v a l u e f o r the 4 e o r b i t a l i s - 2 . 8 eV, d e c r e a s i n g t o - 5 . 4 eV i n the p h o t o i o n i z a t i o n t r a n s i t i o n s t a t e (15) (0.5 e l e c t r o n s removed from 4 e ) • In the e l e c t r o n attachment t r a n s i t i o n s t a t e (0.5 e l e c t r o n s added to 4 e ) the e i g e n v a l u e becomes - 0 . 1 e V . The c a l c u l a t e d i o n i z a t i o n p o t e n t i a l (IP) and e l e c t r o n a f f i n i t y (EA) a r e thus 5.4 and 0.1 eV, r e s p e c t i v e l y . The c a l c u l a t e d IP compares w e l l w i t h the e x p e r i m e n t a l v a l u e o f 6.4 eV w h i l e the b r o a d e n i n g of the ETS t h r e s h o l d f e a t u r e i s c o n s i s t e n t w i t h an a n i o n s t a t e j u s t below t h r e s h o l d . I t t h e r e f o r e appears t h a t the bounds t a t e MS-Xa method y i e l d s a c c u r a t e e n e r g e t i c s f o r both o c c u p i e d and u n o c c u p i e d M3d o r b i t a l s o f the m e t a l l o c e n e s . 2

l g

2 g

l g

2

2

l g

l g

2

l g

l g

l g

T r a n s i t i o n Metal

Hexacarbonyls

T h i s s e r i e s a f f o r d s the c h e m i s t an o p p o r t u n i t y t o see the p o t e n t i a l o f e l e c t r o n s c a t t e r i n g e x p e r i m e n t s such as ETS f o r s t u d y i n g m e t a l l i g a n d bonding. U s i n g the e n e r g i e s o f the l i g a n d - b a s e d c a r b o n monoxide o r b i t a l s and p r i m a r i l y m e t a l - b a s e d o r b i t a l s o f the d h e x a c a r b o n y l s as r e f e r e n c e p o i n t s , l i g a n d s (L) on the m e t a l can be v a r i e d l e a v i n g 1 t o 5 c a r b o n monoxides i n p l a c e . The e f f e c t o f t h i s 6



200

RESONANCES

p e r t u r b a t i o n on the energy of the CO-based 2TT o r b i t a l as measured by ETS can then be a s s e s s e d i n terms of i n c r e a s e d or d e c r e a s e d bonding between the metal and the c a r b o n y l as a f u n c t i o n of the new ligand(s). U l t i m a t e l y , what i s being probed i s the a b l i t y of the l i g a n d t o h i n d e r CO s u b s t i t u t i o n on the m o l e c u l e , i . e . , l i g a n d exchange or c a t a l y t i c b e h a v i o r . To a s s e s s the e f f i c a c y of t h i s c o n c e p t , resonances i n the e l e c t r o n t r a n s m i s s i o n s p e c t r a of these mole c u l e s must, be a s s i g n e d to n e g a t i v e i o n s t a t e s or u n f i l l e d m o l e c u l a r o r b i t a l s i n each compound. D e r i v a t i v e e l e c t r o n t r a n s m i s s i o n s p e c t r a are p r e s e n t e d i n F i g . 4 f o r the t r a n s i t i o n metal h e x a c a r b o n y l s M(CO)g, (M=Cr, Mo, W). In a d d i t i o n , F i g . 5 shows as a f u n c t i o n of e l e c t r o n impact, energy the t o t a l n e g a t i v e i o n current, from d i s s o c i a t i v e attachment, to C r ( C O ) g . MS-Xa o r b i t a l e n e r g i e s c a l c u l a t e d f o r the ground s t a t e s of Mo(CO)g and CO are shown i n F i g . 6. The broadness of the t h r e s h o l d peak i n the spectrum i s i n d i c a t i v e of an attachment p r o c e s s o c c u r r i n g near threshold. T h i s i s q u i t e e v i d e n t i n the i o n c u r r e n t spectrum of F i g . 5. S t a b i l i z a t i o n e l e c t r o n attachment, t r a n s i t i o n s t a t e c a l c u l a t i o n s i n d i c a t e d that a number of M ( C O ) " a n i o n s t a t e s [e.g., those produced by adding an e l e c t r o n t o the l o w e s t empty t ^ (9t^ ) o r b i t a l of C r ( C O ) ] were a c t u a l l y bound. The ETS peaks t h e r e f o r e c o r r e s p o n d e d to o c c u p a t i o n of h i g h e r - e n e r g y empty o r b i t a l s . For example, the f e a t u r e below 1 eV i n the ETS and i o n c u r r e n t spectrum of Cr (CO) g was a t t r i b u t e d t o e l e c t r o n c a p t u r e i n t o the 4 t . (second lowest, empty t ) and the 6 e (lowest, empty e ) w i t h c a l c u l a t e d AE's of 0.9 and 0.2 eV, r e s p e c t i v e l y , w h i l e peaks A and B were a t t r i b u t e d t o o c c u p a t i o n of the 3 t and 7 e o r b i t a l s and C t o o c c u p a t i o n of the 5 t orbital. Subsequent, r e s t r i c t e d H a r t r e e - F o c k (RHF) c a l c u l a t i o n s (JL6) i n d i c a t e d that, the lowest, energy C r ( C O ) " a n i o n o b t a i n e d by o c c u p a t i o n of the 9 t o r b i t a l l i e s 1.5 eV above t h r e s h o l d . However, the RHF c a l c u l a t i o n s gave no e x p l a n a t i o n f o r the t h r e s h o l d ETS or f o r the maximum around 0.4eV i n the i o n c u r r e n t spectrum. 6

u

u

6

2g

2 g

g

2 u

g

g

2 g

6

l u

Of c o u r s e the ETS experiment can o n l y be f o r m u l a t e d c o r r e c t l y as a s c a t t e r i n g e x p e r i m e n t . Any b o u n d - s t a t e a p p r o a c h , whether MS-Xa or RHF, must, be t r e a t e d with c a u t i o n . We have t h e r e f o r e performed continuum MS-Xa c a l c u l a t i o n s of e l a s t i c e l e c t r o n s c a t t e r i n g c r o s s s e c t i o n s f o r C r ( C O ) u s i n g the methods of r e f . 8. Elastic scat t e r i n g c r o s s s e c t i o n s f o r s e v e r a l d i f f e r e n t c h o i c e s of s c a t t e r i n g p o t e n t i a l were c a l c u l a t e d and a d e c o m p o s i t i o n of the c r o s s s e c t i o n by the symmetry of the continuum e l e c t r o n f o r a r e p r e s e n t a t i v e p o t e n t i a l i s g i v e n i n F i g . 7. The c h o i c e of s c a t t e r i n g p o t e n t i a l f o r such a c a l c u l a t i o n i s d i f f i c u l t . (17). We have i n i t i a l l y used p o t e n t i a l s o b t a i n e d from s e l f - c o n s i s t e n t e l e c t r o n attachment t r a n s i t i o n s t a t e s s u b t r a c t i n g out the s t a b i l i z i n g s h e l l p o t e n t i a l . The c a l c u l a t e d c r o s s s e c t i o n s appear t o be i n r e a s o n a b l e agreement, w i t h the i o n c u r r e n t r e s u l t s and t h e i r i n t e r p r e t a t i o n i s s i m i l a r t o that, from our bound s t a t e c a l c u l a t i o n s : e.g., the maximum i n i o n current, at about. 0.4eV i s a s s o c i a t e d with the 4 t o r b i t a l and t h a t around 1.6eV with the 3 t orbital. N o n e t h e l e s s , it. i s apparent t h a t s e v e r a l c h a n n e l s , both resonant, and nonresonant, c o n t r i b u t e t o the t o t a l c r o s s s e c t i o n and the c a l c u l a t e d v a r i a t i o n of t o t a l c r o s s s e c t i o n i n the 1-3 eV r e g i o n i s i n s u f f i c i e n t t o q u a n t i t a t i v e l y e x p l a i n the ETS. T h i s d i s c r e p a n c y may be a s s o c i a t e d with the s m a l l p a r t i a l wave b a s i s (sp o n l y ) used f o r C and 0. I t i s w e l l known t h a t 6

2g

2 u


11.

GIORDAN E T A L .

201



no sample

E N E R G Y (eV)

Figure 4.Derivative Mo~(CO) and W(CO) 6

6

e l e c t r o n t r a n s m i s s i o n s p e c t r a of (from r e f . 6).

ELECTRON

Cr(CO)g*

ENERGY (eV)

F i g u r e 5.Ion c u r r e n t v s . e l e c t r o n impact energy on C r ( C O ) g . i o n s are e s s e n t i a l l y a l l C r ( C O ) ~ 5

(from r e f . 9).


The


RESONANCES

Mo(CO)

•Il2t| .4t J2a u

2 u

| g > 9

4

3t

| g N

6

CO

^

e

g 2g »«lg -

6 t

8e„ 3t 2ug

c

6o-

5t "»lu 7e 2 g

g

>ot,S"

3

9

V

'lu

F i g u r e 6 . C a l c u l a t e d o r b i t a l e n e r g i e s i n the ground s t a t e s o f n e u t r a l Mo(CO) and CO. The h i g h e s t o c c u p i e d o r b i t a l s a r e the 3 t g and 5it, r e s p e c t i v e l y (from r e f . 6 ) . 6

2


11.

GIORDAN E T A L .

203


e l e c t r o n s c a t t e r i n g r e s o n a n c e s i n m o l e c u l e s such as CO o f t e n i n v o l v e h i g h e r p a r t i a l waves (d or f ) • The p r e s e n t continuum MS-Xa r e s u l t s f o r Cr(CO)g must t h e r e f o r e be c o n s i d e r e d p r e l i m i n a r y . The case o f C r ( C O ) i l l u s t r a t e s the extreme d i f f i c u l t y o f o b t a i n i n g a c o n v i n c i n g ETS assignment u s i n g c a l c u l a t i o n a l o n e . The MS-Xa method i s of c o u r s e l i m i t e d by i t s use of a m u f f i n - t i n p o t e n tial. For unoccupied o r b i t a l s a s i g n i f i c a n t f r a c t i o n o f the e l e c t r o n d e n s i t y o c c u r s i n the i n t e r a t o m i c r e g i o n , the l e a s t a c c u r a t e l y t r e a t e d p a r t of the m o l e c u l e . In g e n e r a l , d i f f u s e o r b i t a l s t e n d t o be o v e r s t a b i l i z e d by the c o n s t a n t i n t e r a t o m i c p o t e n t i a l . T h i s problem can be reduced t o some e x t e n t by the use of o v e r l a p p i n g a t o m i c spheres (as we have done) but the i n f l u e n c e o f degree of o v e r l a p on e n e r g e t i c s i s not n e g l i g i b l e (18). Schemes which employ more a c c u r a t e MS-Xa type p o t e n t i a l s (19) g i v e o r b i t a l e n e r g i e s somewhat d i f f e r e n t from c o n v e n t i o n a l v a l u e s . The RHF scheme a l s o e n c o u n t e r s s e v e r e d i f f i c u l t y i n the d e s c r i p t i o n o f a n i o n r e s o n a n c e s due t o b a s i s s e t l i m i t a t i o n s and c o r r e l a t i o n e f f e c t s , which seem t o be l a r g e r i n H a r t r e e - F o c k c a l c u l a t i o n s than i n l o c a l exchange c a l c u lations. In g e n e r a l , one would e x p e c t MS-Xa t o g i v e m o l e c u l a r a n i o n s which a r e too s t a b l e and RHF t o g i v e them as not s t a b l e enough, so t h a t the c o r r e c t v a l u e would l i e somewhere i n between. We may use e x p e r i m e n t a l PES (20) and UV e n e r g i e s (21) and c a l c u l a t e d changes i n o r b i t a l e i g e n v a l u e t o e s t i m a t e the EA o f the C r ( C O ) 9t o r b i t a l as shown i n F i g . 8. The IP of the 2 t orbital c o r r e s p o n d s t o i t s e i g e n v a l u e i n a t r a n s i t i o n s t a t e i n which i t s o c c u p a t i o n number i s 5.5 (and a l l o t h e r o r b i t a l s have t h e i r g r o u n d state occupations). A d d i t i o n o f 0.5 e l e c t r o n s t o the 9 t orbital r a i s e d the c a l c u l a t e d 2 t e i g e n v a l u e by 2.2 eV. The 2 t *• 9 t e x c i t a t i o n energy c o r r e s p o n d s t o the 9 t - 2t o r b i t a l energy d i f ference in a t r a n s i t i o n s t a t e with occupations 2 t » 9t^ . A d d i t i o n of 0.5 e l e c t r o n s t o the 2 t o r b i t a l r a i s e s the c a l c u l a t e d 9t o r b i t a l e i g e n v a l u e by 1.9 eV. T h i s a n a l y s i s i n d i c a t e s t h a t the 9t o r b i t a l l i e s v e r y c l o s e t o t h r e s h o l d and t h a t the MS-Xa v a l u e i n d e e d l i e s below experiment and the RHF v a l u e above. It i s c l e a r t h a t a d e f i n i t i v e i n t e r p r e t a t i o n of the ETS of C r ( C O ) must a w a i t more r i g o r o u s continuum MS-Xa and H a r t r e e - F o c k CI c a l c u l a t i o n s but t h a t such c a l c u l a t i o n s a l o n e , w i t h o u t a comprehensive c o n s i d e r a t i o n o f o t h e r s p e c t r a l d a t a , cannot be d e f i n i t i v e .


6

6

l u

2 g

l u

2 g

2 g

l u

l u

2 g

5

0 # 5

5

u

2 g

2 g

l u

l u

6

Main-Group

Compounds

In o r d e r t o b e t t e r u n d e r s t a n d bonding i n main-group organo compounds and t o o b t a i n d a t a on main-group s p e c i e s which can a c t as l i g a n d s i n t r a n s i t i o n m e t a l complexes, s t u d y o f the n e g a t i v e i o n s t a t e s o f v a r i o u s s a t u r a t e d and u n s a t u r a t e d Group IV, V , V I , and V I I hydroc a r b o n s was u n d e r t a k e n . One example o f t h i s work i s the s e r i e s o f p a r a - d i s u b s t i t u t e d benzenes:

M(CH ) 3

n


RESONANCES

2.0r


Cr(C0)

6

F i g u r e 7.Decomposition o f continuum MS-Xa c r o s s s e c t i o n by sym metry o f continuum e l e c t r o n f o r the s c a t t e r i n g p o t e n t i a l g i v e n by o c c u p a t i o n o f the 3 t orbital (e and t channels give a

F i g u r e 8 . E s t i m a t i o n o f EA o f C r ( C 0 ) 9 t i o r b i t a l using e x p e r i mental 2t. I P and 2 t + 9t.^ e x c i t a t i o n energy and c a l c u l a t e d o r b i t a l energy changes w i t h o c c u p a t i o n number from MS-Xa c a l c u lations. 6

2g

2 g

u

u


11.

GIORDAN E T A L .

205


and t h e i r p a r e n t model compounds M ( C H 3 ) and M H , where n=3 f o r M=C,Si,Ge,Sn; n=2 f o r M=N,P,As; n=l f o r M=0,S; and n=0 f o r M = F , C l , B r . Here we t e s t the e f f i c a c y of employing a n i o n s t a t e e n e r g i e s d e t e r m i n e d from ETS measurements t o a i d i n e x p l a i n i n g (1) the e f f e c t of lone p a i r p a r t i c i p a t i o n i n b o n d i n g , (2) the e x t e n t of d - o r b i t a l p a r t i c i p a t i o n of row 2, 3, and 4 elements i n b o n d i n g , (3) bonding o f 1st row v e r s u s 2nd and 3rd row e l e m e n t s , and (4) the a b i l i t y of these l i g a n d s t o p a r t i c i p a t e i n m e t a i - t o - l i g a n d b a c k - b o n d i n g . In the p a r e n t compounds, resonances a s s o c i a t e d w i t h l i g a n d o* o r b i t a l s are e v i d e n t , i n the s u b s t i t u t e d benzenes the e f f e c t of such o* o r b i t a l s a r e m a n i f e s t as p e r t u r b a t i o n s of the b e n zene ir* o r b i t a l e n e r g i e s w h i l e i n metal c a r bony Is the e f f e c t can be seen i n the s p l i t t i n g of the t orbital. n + 1


n+1

2 g

As an example of our o b s e r v a t i o n s on the l i g a n d model compounds, we show i n F i g . 9 a c o r r e l a t i o n diagram g i v i n g the e n e r g i e s o f the o* resonances i n the s e r i e s S i H - HC1 and S i M e - M e C l . An immediate o b s e r v a t i o n i s t h a t the empty a * - o r b i t a l s are d e s t a b i l i z e d by s u b s t i t u t i o n of Me f o r H. T h u s , P(Me)3 s h o u l d be a weaker a c c e p t o r than PH3 i n compounds such as Cr(CO) PH3, as i s i n d e e d i n f e r r e d on the b a s i s of PES (22a) and IR e v i d e n c e (22b). Analysis o f the p r o p e r t i e s of s u b s t i t u t e d c a r b o n y l s such as C r ( C O ) L , where L i s a two e l e c t r o n donor l i g a n d such as PH3, shows a c o r r e l a t i o n b e t ween l i g a n d AE and Cr3d-L ir-bonding e f f e c t s . Representative data a r e g i v e n i n T a b l e IV. When L i s a s t r o n g ir a c c e p t o r ( e . g . , PMe ) 4

4

5

5

3

Table

IV.

L

CO NMe

a

b

c

d

5

AE(eV)

of I J

"t

2 g

"

2.0 4.8 4.8° 3.1 3.3 b

3

2

splitting

0 0.31 0.13 0.14 0.20

a

3

PH3 PMe SMe

C o r r e l a t i o n of P r o p e r t i e s of C r ( C O ) L w i t h Attachment E n e r g i e s of L

(eV)

c

"IT acceptance" of L (mdyn/A)

d

0.74 0.0 0.48 0.15

ref 1 present result r e f . 23 r e f . 24

e f f e c t i v e symmetry at the Cr atom i s n e a r l y o c t a h e d r a l and the o c c u p i e d m e t a l 3d o r b i t a l (of t symmetry i n o c t a h e d r a l symmetry) i s almost u n s p l i t . When L i s a poor i r - a c c e p t o r ( e . g . , NMe3) the s p l i t t i n g of t h i s o r b i t a l i s s u b s t a n t i a l (23) • IR and Raman s p e c t r a may a l s o be used t o g e n e r a t e s c a l e s of a-donor and i r - a c c e p t o r s t r e n g t h for l i g a n d , L (24). CO i s found t o have the s t r o n g e s t ira c c e p t o r c h a r a c t e r and N bases the weakest. Again, a general c o r r e l a t i o n i s o b s e r v e d between the AE of the l i g a n d and i t s i r - a c c e p t o r 2 g


206

RESONANCES

character.

Thus, measurements of AE's

understand

other

for free

l i g a n d s may

help

to

bonding p r o p e r t i e s .

In o r d e r t o a d d r e s s the q u e s t i o n of d - o r b i t a l p a r t i c i p a t i o n i n the bonding schemes of m o l e c u l e s c o n t a i n i n g S i , P, and the l i k e , MS-Xa c a l c u l a t i o n s have been p e r f o r m e d . As an example of the i n t e r p r e t a t i o n of t h i s d a t a we w i l l f o c u s on the s e r i e s ( C , S i ) X (10) where X=H,F,C1. T h i s work was i n i t i a t e d t o p r o v i d e an i n t e r p r e t a t i o n of the ETS of S i H (25), which e x h i b i t s a resonance a t 2.1 eV whose shape s u g g e s t s a c r o s s s e c t i o n which r i s e s q u i c k l y around 2 eV and then l e v e l s o u t . The t symmetry a * - o r b i t a l r e s p o n s i b l e f o r the change i n c r o s s s e c t i o n shown i n F i g . 10 has a s i g n i f i c a n t amount of S i p and d c h a r a c t e r , b e i n g much more l o c a l i z e d than the c o r r e s p o n d i n g o r b i t a l in CH . T h i s o r b i t a l a l s o y i e l d s the dominant peak i n the S i L x-ray a b s o r p t i o n spectrum (XAS) o f S i H (26) and a c o r r e s p o n d e n c e of S i L XAS, S i 2p IP, and ETS resonance energy may be drawn u s i n g the t r a n s i t i o n s t a t e approach as shown i n F i g . 11, which i s s i m i l a r i n c h a r a c t e r t o F i g . 8. Both the bound s t a t e MS-Xa approach and the c a l c u l a t e d s c a t t e r i n g c r o s s - s e c t i o n y i e l d v a l u e s (2.1 and 2.4 eV r e s p e c t i v e l y ) v i r t u a l l y i d e n t i c a l w i t h e x p e r i m e n t . For S i ( M e ) the AE of t h i s t o r b i t a l i n c r e a s e s to 3.9 eV and i n p a r a - b i s ( t r i m e t h y l s i l y l ) benzene the T T * ( e ) r e s o n a n c e i n benzene a t 1.09 eV i s s p l i t , w i t h one component of the T T * - o r b i t a l s t a b i l i z e d to 0.54 eV by hyperconj u g a t i v e i n t e r a c t i o n of the empty - S i (Me) 3 T T * w i t h the r i n g T T * - o r b i t a l w i t h nonzero e l e c t r o n d e n s i t y a t the 1 c a r b o n (27). Thus, u n d e r s t a n d i n g of the a* resonance i n S i H h e l p s t o c l a r i f y our i n t e r p r e t a t i o n of f e a t u r e s i n the more c o m p l i c a t e d s p e c t r a o f - S i ( M e ) s u b s t i t u t e d benzenes. A l t h o u g h c a l c u l a t i o n s have not been performed f o r S i ( M e ) i t s e l f , comparison of the S i 2 p IP's (28) and S i L X-ray a b s o r p t i o n s p e c t r a (29) o f S i H and Si(Me) i n d i c a t e s t h a t the t o r b i t a l l i e s about 2 eV h i g h e r ( c l o s e r to t h r e s h o l d ) i n S i ( M e ) , c o n s i s t e n t , with the ETS r e s u l t s . 4

4


2

4

4

4

2

2 u

4

3

4

4

4

2

4

Assessment, of the l o n e - p a i r i n t e r a c t i o n i n t h e s e systems can be made by n o t i n g the p e r t u r b a t i o n t o the benzene I T * - o r b i t a l upon s u b s t i t u t i o n w i t h a Group V moiety ( F i g . 12). In g o i n g from the N to the P compound t h e r e i s a s t a b i l i z a t i o n of the a * - o r b i t a l which y i e l d s a s t a b i l i z i n g h y p e r c o n j u g a t i v e i n t e r a c t i o n w i t h the a p p r o p r i a t e r i n g T T * - o r b i t a l , l o w e r i n g i t s AE. In the N compound t h i s o r b i t a l i s d e s t a b i l i z e d by i n t e r a c t i o n with the N l o n e - p a i r and i t s energy i s r a i s e d above t h a t of the n o n i n t e r a c t i n g ir*-com ponent, A s i m i l a r effect, i s seen i n the As compounds. The change i n energy seen i s p r o b a b l y a r e s u l t of both reduced i r * - l o n e p a i r i n t e r a c t i o n s and i n c r e a s e d o*-ir* i n t e r a c t i o n s in the P and As com pounds. In summary, we f i n d ETS to be ct v a l u a b l e t o o l f o r the e l u c i d a t i o n of the e n e r g e t i c s and o t h e r p r o p e r t i e s of low-energy u n o c c u p i e d o r b i t a l s i n o r g a n o m e t a l l i c compounds and t h e i r fragments. Such s t u d i e s complement P E S s t u d i e s of o c c u p i e d o r b i t a l s and UV, X-ray a b s o r p t i o n , and e l e c t r o n energy l o s s s t u d i e s of t r a n s i t i o n s t o lowenergy unoccupied o r b i t a l s y i e l d i n g a complete p i c t u r e of v a l e n c e o r b i t a l i n t e r a c t i o n s i n the compound.


11.

GIORDAN E T A L .


UJ

< 0

Si(Me)

P(Me)


4

SiH

(Me) S

3

F i g u r e 9. Attachment e n e r g i e s

MeCI

2

PH

4

HS

3

HCI

2

from ETS f o r

S i (H,Me) 4"" (H,Me) C I .

5.2 4.< E(eV) F i g u r e 10.Continuum MS-Xa e l a s t i c e l e c t r o n s c a t t e r i n g c r o s s s e c t i o n s f o r SiR^, t o t a l and by symmetry of continuum e l e c t r o n (from r e f . 10)•

. J2.0

2.9 '

Threshold 5

9

0

5

€ (Si2p - t - ) t2

Si2pIP 108.0

5

0

Si2p^t XAS 104.1 2

T" 1

2

1

J

9

9

0

5

€Si2p(Si2p - t - ) 2

9

9

«Si2p(Si2p ' )

F i g u r e 11. R e l a t i o n s h i p between c o r e o r b i t a l i o n i z a t i o n s p o t e n t i a l , X - r a y a b s o r p t i o n e n e r g i e s and ETS resonance e n e r g i e s for S i H u s i n g t r a n s i t o n s t a t e model (from r e f . 10). 4



208

RESONANCES

7r

66

N(Me)

2

P(Me)

2

As(Me)

2

Figure 12. Attachment e n e r g i e s (AE) and i o n i z a t i o n p o t e n t i a l s (IP) o f group V s u b s t i t u t e d benzenes and s u b s t i t u e n t p a r e n t compounds.

Acknowledgments We thank NSF (Grant. No. CHE-81-21125) and the Computer S c i e n c e C e n t e r , U n i v e r s i t y of Maryland f o r support, o f t h i s work, the ACS f o r p e r m i s s i o n t o reproduce F i g . 1, 2, and 4-7, the AIP f o r p e r m i s s i o n t o reproduce F i g . 10 and 11, and E l s e v i e r S c i e n c e P u b l i s h e r s f o r p e r m i s s i o n t o reproduce F i g . 3.


11. GIORDAN et AL.


209

Literature cited 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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RECEIVED July 10, 1984 Truhlar; Resonances ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Anion Resonance States of Organometallic Molecules - American

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