• No results found

Electron energy loss spectroscopic study of Cr(CO)6, Mo(CO)6 and W(CO)6 in the vapour phase

N/A
N/A
Protected

Academic year: 2022

Share "Electron energy loss spectroscopic study of Cr(CO)6, Mo(CO)6 and W(CO)6 in the vapour phase"

Copied!
5
0
0

Loading.... (view fulltext now)

Full text

(1)

Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 102, No. I, February 1990, pp. I-5.

,!~ Printed in India.

E l e c t r o n e n e r g y l o s s s p e c t r o s c o p i c

study of

C r ( C O ) 6 , M o ( C O ) 6

and W(CO)6 in the vapour phase*

T P R A D E E P

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India

MS received 28 August 1989

Abstract. Electron energy loss spectra (EELS) of Cr, Mo and W hexacarbonyls in the vapour phase are reported. Most of the bands observed are similar to those in optical spectra, but the two high energy transitions in the 9-8-1 l-2eV region are reported here for the first time. ,Based on the orbital energies from the ultraviolet photoelectron spectra and

the

electronic transition energies from EELS and earlier optical studies, the molecular energy level schemes of these molecules are constructed.

Keywords.

Electron energy loss spectra; transition metal hexacarbonyls; molecular energy level schemes.

I. Introduction

Transition metal carbonyls have been a subject of active investigation because of their importance both in theoretical chemistry and in photochemical processes.

Theoretical and electronic spectroscopic studies have hitherto been concerned with the estimation of the a m o u n t of 7, bonding. Studies have clearly established the important role of the n*(CO) orbital in the stability of the carbonyls. The electronic absorption spectra of the carbonyls reported in the literature cover transitions only upto ~ 8eV (Robin 1975). T h e o r y however, predicts the presence of transitions at still higher energies (Trogler etal 1979; Johnson and Klemperer 1977). The present investigation is aimed at obtaining information on higher energy excitations in three metal hexacarbonyls by employing electron energy loss spectroscopy (EELS). This technique is specially suited to probe transitions in the vacuum ultraviolet region.

Besides showing the close similarity of the electronic spectra of the hexacarbonyls, we have been able to construct molecular energy level diagrams by m a k i n g use of ultraviolet photoelectron spectroscopic data (Turner et al 1970; Higginson et al 1973).

2. Experimental

The spectrometer employed in the present study has been fully described earlier (Hedge etal 1985). An electron b e a m of 50eV with a full width at half m a x i m u m

'Contribution No. 644 from the Solid State and Structural Chemistry Unit

(2)

2 7 Pradeep

( F W H M ) of 300meV was used in the present study. Commercial samples of the hexacarbonyis were used without further purification. Samples were kept at room temperature and the vapours were admitted into the collision chamber by a needle valve. Room temperature vapour pressure was sufficient to give an energy loss count rate of 104 c/s. All these spectra were recorded at least five times to establish the peak positions. We did not observe any C O impurity which could be present as a dissociation product.

3. R e s u l t s a n d d i s c u s s i o n

in figure 1 we show the electron energy loss spectra of the hexacarbonyls. The spectra are similar and exhibit clear band maxima. In order to assign the E E L features we

w(co) 6 /~

C r (CO) 6 ~ IL~I~

I i L l I ! I 1

lZ, 12 10 B 6 4 2 0 Energy loss (eV)

Figure I. Electron energy loss spectra of CttCO)~,, Mo(CO) 6 and W(CO)6.

(3)

Electron energy loss spectrometry of transition metal carbonyls 3

need to have information on the occupied orbitais as well as the unoccupied ones.

Ionization energies of the hexacarbonyls have been determined by photoelectron spectroscopy (Turner et al 1970; Higginson et al 1973). The predominantly metal d (t2g) orbital ionization is at .~, 8-5 eV in all these carbonyls. The 13.3-16.2 eV region has ionizations due to orbitals derived from the 50 and In orbitals of CO. In the 18 eV region, ionization occurs from the ligand 46 derived levels. With regard to the unoccupied levels of the carbonyis, we follow the ordering obtained by Johnson and Kiemperer (1977) from their SCF-X,-MSW calculations on Cr(CO)6 and assume the same ordering to hold for the other two carbonyls. The first few unoccupied orbitals of interest are 9a~g, 9t~, 3t2o, 2t2~, 6%, 10t~ and 2ttg in order of increasing energy.

Out of these, 9 t t , , 3t20 , 2t2~ and 2t~g are CO 2rr-derived orbitals while 9atg, 10t~

and 6% are predominantly metal- (s, p and d) derived orbitals.

In table 1 we have summarized the electronic transitions of the three hexacarbonyls.

Since the metal t2u MO is at 8.5 eV and the other occupied ligand MOs are below 13'3 eV, all the lower energy transitions arise from t2g MO. The lowest energy transition (2t2~9a~o), is observed around 3-8 eV (Beach and Gray 1968); this transition is not observed in EELS. The next transition is due to a charge-transfer excitation ( 2 t ~ 9 t t~) and is observed around 4-4eV in optical spectroscopy (Beach and Gray 1968). The first EEL feature is due to this transition. The band in the 4-5-4.8 eV region (Beach and Gray 1968) found in the optical spectra is very weak and is not observed in EELS. The optical transition around 5-5eV (Beach and Gray 1968) has the highest intensity and is due to the second charge-transfer transition (2t2g~2t2~). This is observed in EELS in the 5.2-5-4eV region. Asymmetry found in the optical spectra in the blue side of this band is considered to be due to the presence of a transition-centred around 5"9eV (Beach and Gray 1968). EEL spectra do not show any apparent asymmetry probably due to the low resolution. The asymmetry in the vacuum ultraviolet spectra (lverson and Russell 1970) is also not significant. The transition in the 6-2-7.0eV region has been observed in the optical spectra (Beach and Gray 1968) and has been assigned to the 2t2o--,2t~ transition. The splitting of this band in W(CO)6, observed in the optical spectra (Iverson and Russell 1970) is not seen in EELS.

Table !. E l e c t r o n i c t r a n s i t i o n s of C t I C O ) 6 , M o ( C O ) o a n d W ( C O ) 6 a n d t h e a s s i g n m e n t s .

T r a n s i t i o n e n e r g y

C r ( C O ) o Mo{CO)6 W~CO)6

E E L S O p t i c a l E E L S O p t i c a l E E L S O p t i c a l A s s i g n m e n t

- 3.60r.~ _ 3.63 r.a _ 3.53r.a 2t2r

3.91 s.a 4.05s.~ 3.96s.a

4"31 4"44 a 4"31 4"32 a 4"30 4"29" 2t2g--*9tl~

- 4"83a - 4"66 a - 4-54" 2tzg --* 3t2a

5" 19 5.48 a 5-28 5'44 a 5"40 5.52 a 2t2g ~ 2t2~

6"16 6"32 b 6-34 6"42 ~ 7"00 6.35 ~ 2tzg--,2tl~

6.56 b

10"39 - 9"86 - 9"80 - 8tl -..2t2~

I 1"10 - 11"19 - 11"00 8t1~-.+ 2ttg

T - t r i p l e t : S - s i n g l e t ; ' - f r o m Beach a n d G r a y ( 1 9 6 8 ) ; b - F r o m lverson and Russell ([970).

(4)

4 T Pradeep

An earlier optical study of lverson and Russell (1970) reported two transitions in the 7.4-8.9 eV region, but the EEL spectra do not show evidence for them clearly.

There are, however, two transitions in the EEL spectra above the first ionization energy in the 9.8-11.2eV region; these two transitions, reported for the first time, probably arise from the second occupied level, 8tt. as shown intable 1. An SCF-X~-DV calculation (Trogler et al 1979) predicts the 8tl,--*9alg transition at 12.39eV. From the molecular energy level scheme (figure 2) constructed on the basis of previous studies (Beach and Gray 1968; Iverson and Russell 1970; Higginson et al 1973), this transition works out to have an energy of 8"5 eV, but we do not see any EELS feature

_

- 5

- 1 0

t

w-15

E

- 2 (

2ttg 2t2u 3t2g

9tlu -- 9Ol 9

2t2g

8tl u l t l g

- - 5eg

7ti u lt2g

~ulg

It2u

Ztl+j 2t2u.

~~ ,L

2t 2g

2tlg .

2t2u l

3t2g

~tlu 9~ig |

2t2g

8tlu 8tlu

ltlg 1rig

lt2u 5eg 5eg

7tlu 7tlti

lt2~ It2g

8~ 8ol, 3

4eg,6tlu 4eg~6tlu 4eg, 6tfu

7a 1 g 7Ol g

7o1( 3

Cr (CO) 6 Mo (CO) 6 W (CO) 6

It2u

Figure 2. Molecular orbital energy level schemes of the hexacarbonyls.

(5)

Electron e n e r g y loss s p e c t r o m e t r y o f transition m e t a l c a r b o n y l s 5

in this energy region. T h e transition at 10.39eV in the case of Cr(CO)6 has been assigned to the 8tlu--.2t2u excitation o n the basis of term-value arguments. Similarly, the second transition is assigned to 8tlu--,2t2g excitation. This can also be assigned to the 8 t i ~ 10tl~ transition with a q u a n t u m defect of 0"6. We have m a d e similar assignments for the b a n d s in the other two c a r b o n y l s as well as those shown in the energy level scheme (figure 2).

It is interesting to c o m p a r e the carbonyl spectra with that of C O adsorbed on the respective metal surfaces. U n f o r t u n a t e l y d a t a are available only for the W(100) surface (Chesters et a! 1976). There is only one transition in the a b o v e discussed energy region.

This transition at 5.3 eV has been assigned to the C O valence excitation (5a--.27r).

However, it could be regarded as a charge-transfer transition from the metal to the 27r level of C O also (Shin-ichi lshi a n d O h n o 1984). In the absence of m o r e experimental data, n o t h i n g further could be stated.

Acknowledgements

The a u t h o r wishes to thank Prof. C N R R a o for his kind e n c o u r a g e m e n t a n d D r M S Hegde for useful discussions. Financial assistance from the D e p a r t m e n t of Science and T e c h n o l o g y , G o v e r n m e n t of India, is gratefully acknowledged.

References

Beach N A and Gray H B 1968 d. Am. Chem. Soc. 90 5713 Chesters M A, Hopkins B J and Winton R 1 1976 Surf. Sci. 59 46

Hegde M S, Jayaram V, Kamath P V and Rao C N R 1985 Pramana- J. Phys. 24 293

Higginson B R, Lloyd D R, Burroughs P, Gibson D M and Orchard A F 1973 d. Chem. Sot., Faraday Trans. II 69 1659

Iverson A and Rus~ll B R 1970 Chem. Phys. Lett. 6 307

Johnson J B and Klemperer W G 1977 J. Am. Chem, Soc. 99 7132

Robin M B 1975 Higher excited states c~fpolyatomic molecules (New York: Academic Press) vol. 2 Shin-ichi Ishi and Ohno Y 1984 Surf Sci. 139 L219

Trogler W C, Desjardins S R and Solomon E 1 1979 Inorg. Chem. 18 2131

Turner D W, Baker C, Baker A D and Brundle C R 1970 Molecular photoelectron spectroscopy (London:

Wiley Interscience)

References

Related documents

SINGH solemnly declare that this dissertation titled “ A STUDY OF THE TOPOGRAPHICAL ANATOMY OF THE EXTERNAL LARYNGEAL BRANCH OF SUPERIOR LARYNGEAL NERVE IN PATIENTS

Providing cer- tainty that avoided deforestation credits will be recognized in future climate change mitigation policy will encourage the development of a pre-2012 market in

Energy Monitor. The combined scoring looks at the level of ambition of renewable energy targets against a pathway towards full decarbonisation in 2050 and at whether there is

The necessary set of data includes a panel of country-level exports from Sub-Saharan African countries to the United States; a set of macroeconomic variables that would

In summary, compared with what is happening in the rest of the world, where the lockdown measures and the economic crisis are driving the decrease in energy demand, the general

Percentage of countries with DRR integrated in climate change adaptation frameworks, mechanisms and processes Disaster risk reduction is an integral objective of

The Congo has ratified CITES and other international conventions relevant to shark conservation and management, notably the Convention on the Conservation of Migratory

These gains in crop production are unprecedented which is why 5 million small farmers in India in 2008 elected to plant 7.6 million hectares of Bt cotton which