Inorganic Chemistry: Toward the 21st Century

12 Metal-Metal Quadruple Bonds Direct Experimental Determination of the Bonding Contribution of a δ-Orbital Electron DENNIS L. LICHTENBERGER

Downloaded by TUFTS UNIV on December 9, 2015 | http://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch012

University of Arizona, Department of Chemistry, Tucson, AZ 85721

The newly-developed capability to observe metal -metal vibrational fine structure in the valence ionizations of quadruply bonded dimers is illus trated for the delta-bond ionization of Mo (O CCH ) . Observation of this structure pro vides direct information on the bonding influence of an electron in a delta-bonding orbital by show ing the significant changes in metal-metal force constant and bond distance that occur when that electron is removed. 2

2

3 4

I wish t o describe the f i r s t a p p l i c a t i o n o f a s i g n i f i c a n t new experimental c a p a b i l i t y t o a r a t h e r " c l a s s i c " question about a " c l a s s i c " molecule. The beauty o f the s t o r y i s that d i r e c t and unique information i s provided by the technique, and the expla n a t i o n i s short and simple. The " c l a s s i c " molecule i s Mo ( 0 2 ^ 3 ) 4 , which i s an impor tant r e p r e s e n t a t i v e member o f di-metal molecules c o n t a i n i n g a quadruple bond. The occupation o f the delta-bonding o r b i t a l , which completes formation o f the quadruple bond, i s a s p e c i a l feature o f these molecules. The c l a s s i c question i s the f o l l o w i n g : To what extent does an e l e c t r o n i n the delta-bonding o r b i t a l c o n t r i b u t e t o the t o t a l bond strength and f o r c e constant between the two metal centers? The obvious approach t o answering t h i s question i s t o remove an e l e c t r o n from t h i s o r b i t a l and observe the e f f e c t on, f o r example, the metal-metal s t r e t c h i n g frequency or metal-metal bond d i s t a n c e . Of course, removal o f an e l e c t r o n from the d e l t a bond ing o r b i t a l creates a p o s i t i v e molecular i o n f o r which determina t i o n o f these p r o p e r t i e s may not be p o s s i b l e using normal tech niques. In those cases where the i o n i s s u f f i c i e n t l y s t a b l e that these p r o p e r t i e s can be measured, the meaning o f the information may be clouded by changes i n i n t e r m o l e c u l a r i n t e r a c t i o n s o r other internal factors. One simple method o f t a k i n g the e l e c t r o n from the molecule 2

0097-615 6 / 8 3 / 0 2 1 1 -0221 $ 0 6 . 0 0 / 0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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TOWARD T H E 2 1 S T C E N T U R Y

i s photoelectron spectroscopy. Advantages o f t h i s technique are that the molecule i s i n the gas phase, so that s o l u t i o n o r s o l i d s t a t e e f f e c t s are non-existent, and the technique i s f a s t , so that subsequent molecular occurrences are g e n e r a l l y not observed. A l s o , t h i s i s the best technique f o r measuring accurate i o n i z a t i o n energies. Professor Cotton discussed the importance o f i o n i z a t i o n energies t o understanding bonding i n t e r a c t i o n s i n h i s talk. I wish t o s t r e s s that these i o n i z a t i o n s can a l s o provide a d i r e c t measure o f both the metal-metal s t r e t c h i n g frequency and the e q u i l i b r i u m bond d i s t a n c e i n the p o s i t i v e i o n . This i n f o r mation i s obtained i f the v i b r a t i o n a l f i n e s t r u c t u r e comprising the i o n i z a t i o n band envelope i s observed. The p o s s i b i l i t y o f o b t a i n i n g t h i s d i r e c t information has not been discussed p r e v i o u s l y i n t h i s context because v i b r a t i o n a l f i n e s t r u c t u r e i s not g e n e r a l l y observed i n the i o n i z a t i o n s o f molecules o f t h i s s i z e , and has never before been observed f o r any t r a n s i t i o n metal-metal v i b r a t i o n a l mode. Our breakthrough i n demonstrating t h a t t h i s f i n e s t r u c t u r e can be observed has f o l lowed from s e v e r a l developments o f our instrumentation. The d e t a i l s o f these developments have r e c e n t l y been published along with our r e p o r t o f the f i r s t observations o f metal-ligand v i b r a t i o n a l f i n e s t r u c t u r e i n the i o n i z a t i o n s o f metal carbonyls Q J . Figure 1 d i s p l a y s the i o n i z a t i o n band o f M02(C^CCHs)** c o r responding t o l o s s o f one e l e c t r o n from the delta-bonding o r b i t a l ( B2g positive ion state). The f i n e s t r u c t u r e due t o the t o t a l l y symmetric metal-metal v i b r a t i o n a l mode l e v e l s i n the p o s i t i v e i o n i s c l e a r l y observed. As Figure 1 shows, the band i s well r e p r e sented by an evenly spaced progression o f v i b r a t i o n a l components. No anharmonicity i s detected. The p r o g r e s s i o n has a skewed Gauss i a n i n t e n s i t y p r o f i l e as expected f o r e x c i t a t i o n t o a p o t e n t i a l well with a d i s p l a c e d e q u i l i b r i u m bond d i s t a n c e . This i s i l l u s t r a t e d i n Figure 2. The spacing between the v i b r a t i o n a l components gives a metal-metal s t r e t c h i n g frequency i n the p o s i t i v e i o n o f 360(10) cm" , which i s c o n s i d e r a b l y l e s s than the 406 cm" metal-metal s t r e t c h i n g frequency i n the n e u t r a l molecule. Thus, the f o r c e constant between the metals has decreased with removal o f an e l e c t r o n from the delta-bonding o r b i t a l . Franck-Condon a n a l y s i s of the p r o g r e s s i o n shows that the metal-metal bond d i s t a n c e i n t h i s positive- i o n i s about 0.17 A longer than i n the n e u t r a l molecule. This a l s o shows that an e l e c t r o n i n the delta-bonding o r b i t a l has a s u b s t a n t i a l i n f l u e n c e on the strength o f the metalmetal i n t e r a c t i o n . A d d i t i o n a l i n s i g h t i s obtained i f these r e s u l t s are compared with the r e l a t e d absorption experiments i n which an e l e c t r o n from the delta-bonding o r b i t a l i s e x c i t e d t o the d e l t a - a n t i b o n d i n g o r b i t a l (2). The p e r t i n e n t data i s summarized i n the Table. The s t a t e obtained by 6 i o n i z a t i o n has a greater formal bond order than the s t a t e obtained by 6->6* e x c i t a t i o n , but has a weaker metal-metal f o r c e constant and a longer metal-metal bond. I t i s 2

1

1

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

LiCHTENBERGER

Metal-Metal

Quadruple

223

Bonds


12.

2

Figure 1. The Β ionization band ( B positive ion state) of Mo (0 CCH ) equally spaced symmetric vibrational components. 2g

2

2

s h


fit with

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INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

Figure 2.

The relationship between the δ ionization band and the potential wells of the ground state and the displaced B positive ion state. 2

Xg


12.

LiCHTENBERGER

TABLE.

Quadruple

Bonds

225

Related data f o r three e l e c t r o n i c perturbations o f Mo ( 0 ^ Η ) ι * . 2

2

3

formal bond order

state

Mo-Mo distance

Mo-Mo stretch

4.0

406 cm"

2.093 A

δ-δ*

3.0

370(5)

2.20

δ ionization

3.5

360(10)

2.26(1)

neutral


Metal-Metal

1

important t o note t h a t , i n a d d i t i o n t o the change i n formal bond order, the i o n i z a t i o n process a l s o changes the formal o x i d a t i o n s t a t e o f the metal centers. The i n c r e a s e i n p o s i t i v e charge a t the metal centers w i l l c o n t r a c t the metal d o r b i t a l s and reduce t h e i r overlap. This w i l l decrease the bonding a b i l i t y o f the remaining delta-bonding e l e c t r o n , as w e l l as t h a t o f the p i bonding e l e c t r o n s and q u i t e p o s s i b l y a l s o the sigma-bonding e l e c t r o n s . This e f f e c t o f p o s i t i v e charge i s seldom discussed f o r v i b r a t i o n a l f i n e s t r u c t u r e i n photoelectron spectroscopy, but has been discussed f o r quadruply bonded metal complexes (3). The important p o i n t t o remember i s that an e l e c t r o n i n the delta-bonding o r b i t a l o f Mo (0 CCH3)i,. has a s u b s t a n t i a l i n f l u e n c e on the strength o f the metal-metal i n t e r a c t i o n . This i n f l u e n c e i s d i r e c t l y evidenced by the metal-metal v i b r a t i o n a l f i n e s t r u c ture observed with i o n i z a t i o n from the d e l t a o r b i t a l , which shows a lowering o f the metal-metal s t r e t c h i n g frequency and a l e n g t h ening o f the e q u i l i b r i u m metal-metal bond d i s t a n c e . 2

2

Acknowl edgment These experiments were conducted by Mr. Charles H. B l e v i n s I I . We wish t o thank the Department o f Energy, c o n t r a c t DE-AC0280ER10746 and the u n i v e r s i t y o f Arizona f o r p a r t i a l support o f t h i s work. D.L.L. i s an A l f r e d P. Sloan Fellow. literature Cited

1. Hubbard, J.L.; Lichtenberger, D.L., J. Am. Chem.Soc.,1982, 104, 2132. 2. Martin, D.S.; Newman, R.A.; Fanwick, P.E., Inorg. Chem., 1979, 18, 2511. 3. Bursten, B.E.; Cotton, F.A., Farad. Disc. Roy. Soc. Chem., 1980, 14, 180. RECEIVED

September 1 6 , 1982


Inorganic Chemistry: Toward the 21st Century

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