Photoelectron Spectroscopy - American Chemical Society

Chapter 11

Photoelectron Spectroscopy Experimental Characterization of Electronic Structure and Bonding in Organometallic Molecules Dennis L. Lichtenberger, Glen Eugene Kellogg1, and Louis S. K. Pang

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Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: November 24, 1987 | doi: 10.1021/bk-1987-0357.ch011

Laboratory for Electron Spectroscopy and Surface Analysis, Department of Chemistry, University of Arizona, Tucson, AZ 85721

The basic approach, instrumentation, sample require ments, and principles of interpretation for the photoelectron spectroscopy of organometallic molecules are briefly described. The ionization information is directly related to the formal oxidation states and d electron counts of the metals, the actual effective metal charges and charge potentials, the electronic symmetry around the metal centers, and the individual σ, π, etc. bonding capabilities of ligands and metal fragments. This technique contributes to a level of experimental characterization of organometallic mole cules, beyond the usual three-dimensional structure description, that is essential for interpretation of many physical and chemical properties. Application of the technique is illustrated with examples in which the individual bonding capabilities of several different ligands with a common metal fragment are compared, and in which the bonding of different metal fragments with a common ligand are compared. The present level of understanding of organometallic chemistry and catalysis i s rooted i n the detail and breadth of characterization that i s possible for organometallic molecules. An important key to this knowledge i s provided by characterization of the structural arrangement of atoms i n the molecule by crystal structure determina tions or other techniques. However, the understanding of the proper ties and behavior of a molecule, and the a b i l i t y to extend this understanding to new systems, requires knowledge of more than just the particular three-dimensional arrangement of atoms. A further, taore detailed step i n characterization of a molecule i s to obtain information on the electronic structure i n terms of electron dis tributions, bonding, and s t a b i l i t y . The characters of the bonds, lone pairs, regions of charge build-up and depletion, local dipoles, electronic perturbation effects, etc. define the physical and dynamic properties of the molecule and i t s reactions with i t s environment. F u l l characterization of a molecule must include i t s electronic structural features i n addition to the molecular structure. The principles and models of organometallic bonding are some of the most 1

Current address: Department of Chemistry, Northwestern University, Evanston, IL 60201 Current address: University of Tasmania, GPO Box 252C, Hobart, Tasmania, 7001, Australia 0097-6156/87/0357-0265$07.25/0 © 1987 American Chemical Society

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In Experimental Organometallic Chemistry; Wayda, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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i m p o r t a n t c o n t r i b u t i o n s t o t h e modern r e n a i s s a n c e o f i n o r g a n i c and o r g a n o m e t a l l i c c h e m i s t r y , and t h e t e c h n i q u e s f o r o b t a i n i n g t h i s i n f o r m a t i o n have e x p e r i e n c e d r a p i d development r e c e n t l y . ( 1 ) T h e o r e t i c a l c a l c u l a t i o n s o f e l e c t r o n i c s t r u c t u r e have h i s t o r i c a l l y had an i m p o r t a n t impact on o r g a n o m e t a l l i c c h e m i s t r y , and t h e o r e t i c a l approaches and c o m p u t a t i o n a l hardware c o n t i n u e t o improve. E x p e r i m e n t a l approaches d i r e c t e d toward e l e c t r o n i c s t r u c t u r e c h a r a c t e r i z a t i o n are s i m i l a r l y e s s e n t i a l . I n r e c e n t y e a r s p h o t o e l e c t r o n s p e c t r o s c o p y has been i n c r e a s i n g l y u t i l i z e d as an e x p e r i m e n t a l t o o l t o r e v e a l e l e c t r o n i c s t r u c t u r e and b o n d i n g f e a t u r e s o f s o l i d s , l i q u i d s , and gases.(2-12) Photoelectron s p e c t r a o f f e r w e l l - d e f i n e d and d e t a i l e d e x p e r i m e n t a l i n f o r m a t i o n about e l e c t r o n r i c h n e s s , e l e c t r o n d i s t r i b u t i o n s , and t h e s t r e n g t h o f b o n d i n g i n t e r a c t i o n s i n o r between m o l e c u l e s . The g e n e r a l e l e c t r o n " r i c h n e s s " o f a m o l e c u l e may be c o r r e l a t e d w i t h i t s f i r s t i o n i z a t i o n p o t e n t i a l ( I P ) . A h i g h i o n i z a t i o n p o t e n t i a l ( b i n d i n g energy) i n d i c a t e s r e l a t i v e l y s t a b l e and i n a c c e s s i b l e e l e c t r o n s , whereas a low i o n i z a t i o n p o t e n t i a l i n d i c a t e s r e l a t i v e l y u n s t a b l e and a v a i l a b l e e l e c t r o n d e n s i t y . These f e a t u r e s a r e d i r e c t l y r e l a t e d t o Lewis a c i d base and o x i d a t i o n - r e d u c t i o n r e a c t i v i t y . The b o n d i n g w i t h i n a molec u l e d e f i n e s t h e m o l e c u l a r geometry and s t a b i l i t y . Strong covalent b o n d i n g i n t e r a c t i o n s r e s u l t i n i o n i z a t i o n bands w i t h g e n e r a l l y h i g h e r IP's, whereas non-bonding and a n t i b o n d i n g o r b i t a l s r e s u l t i n i o n i z a t i o n s w i t h g e n e r a l l y lower I P ' s . The b r e a d t h o f an i o n i z a t i o n band i s a l s o s i g n i f i c a n t because i t i n d i c a t e s t h e b o n d i n g c h a r a c t e r i n t h e i o n i z e d o r b i t a l and t h e e x t e n t o f bond d i s t a n c e changes w i t h t h e ionization. I o n i z a t i o n a s s o c i a t e d w i t h a s t r o n g l y bonding o r b i t a l r e s u l t s i n a b r o a d i o n i z a t i o n e n v e l o p e , whereas i o n i z a t i o n a s s o c i a t e d w i t h a nonbonding o r b i t a l g e n e r a l l y r e s u l t s i n a narrow i o n i z a t i o n peak. P r i n c i p l e s such as t h e s e , a l o n g w i t h t h e i n c r e a s i n g l i b r a r y o f p h o t o e l e c t r o n i o n i z a t i o n s , have l e d n a t u r a l l y t o a v a r i e t y o f i o n i z a t i o n - s t r u c t u r e - r e a c t i v i t y r e l a t i o n s h i p s . However, i n o r d e r t o o b t a i n m e a n i n g f u l i n f o r m a t i o n from p h o t o e l e c t r o n measurements i t i s import a n t t h a t t h e s p e c t r o s c o p i c s t u d i e s a r e a d e q u a t e l y d e s i g n e d and p r o p e r l y executed. I n p a r t i c u l a r , t h e b o n d i n g i n t e r a c t i o n s a r e most c l e a r l y r e v e a l e d by t h e a p p r o p r i a t e c o m b i n a t i o n o f s p e c t r o s c o p i c measurements ( H e l UPS, H e l l UPS, MgKa XPS, e t c . ) c o n d u c t e d on s e r i e s o f f r e e and complexed l i g a n d s and s e r i e s o f complexes w i t h s p e c i f i cally useful interrelationships. Perhaps t h e most r e w a r d i n g a s p e c t o f t h i s r e s e a r c h i s t h e e x c i t i n g i n t e r a c t i o n s between c h e m i s t s i n a l l a r e a s o f r e s e a r c h . The development o f a complete b o n d i n g p i c t u r e must combine t h e s k i l l s o f s y n t h e t i c , t h e o r e t i c a l , and p h y s i c a l organometallic chemists. Syntheses o f new c l a s s e s o f compounds p r o v i d e s t h e o p p o r t u n i t y t o i n v e s t i g a t e t h e s p e c t r o s c o p y o f new bonding s i t u a t i o n s . Synthesis o f a s e r i e s o f r e l a t e d molecules provides the opportunity t o i n v e s t i g a t e a p a r t i c u l a r bonding s i t u a t i o n i n a v a r i e t y o f environments. Theory a i d s i n d e f i n i n g t h e p e r t i n e n t q u e s t i o n s and p r i n c i p l e s under i n v e s t i g a t i o n . T h i s c h a p t e r i s d i r e c t e d toward b o t h new s t u d e n t s and e s t a b l i s h e d r e s e a r c h e r s i n o r g a n o m e t a l l i c c h e m i s t r y whose e x p e r t i s e i s n o t i n the area o f photoelectron spectroscopy. I t i s recognized that r e l a t i v e l y few l a b o r a t o r i e s have d i r e c t access t o p h o t o e l e c t r o n i n s t r u m e n t a t i o n , b u t t h a t many a r e i n t e r e s t e d i n t h e i n f o r m a t i o n t h a t can be o b t a i n e d . T h i s c h a p t e r w i l l b r i e f l y d e s c r i b e t h e photoe l e c t r o n e x p e r i m e n t and g e n e r a l sample r e q u i r e m e n t s , and t h e p r i n c i p l e s f o r understanding the information contained i n the data. The p r e s e n t a t i o n o f some "case s t u d i e s " w i l l i l l u s t r a t e t o t h e r e a d e r


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LICHTENBERGER ET AL.

Photo electron Spectroscopy

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how p h o t o e l e c t r o n s p e c t r o s c o p y c o n t r i b u t e s t o t h e e x p e r i m e n t a l characterization of electronic structure.


The

Experiment

Instrumentation. C o n c e p t u a l l y , the p h o t o e l e c t r o n s p e c t r o s c o p y ex p e r i m e n t i s q u i t e s i m p l e . I t s r o o t s a r e b a s e d i n E i n s t e i n ' s photo e l e c t r i c e f f e c t , i n w h i c h l i g h t i m p i n g i n g on a m e t a l l i c s u r f a c e w i l l i o n i z e t h e s u r f a c e i f the energy o f the p h o t o n o f l i g h t exceeds t h e energy r e q u i r e d t o remove an e l e c t r o n from the m e t a l . I n gas phase p h o t o e l e c t r o n s p e c t r o s c o p y t h e excess energy o f the i o n i z i n g p h o t o n i s c o n v e r t e d t o the k i n e t i c energy o f the e j e c t e d e l e c t r o n . Thus t h e k i n e t i c energy (E^) o f the e j e c t e d e l e c t r o n i s t h e d i f f e r e n c e between the p h o t o n energy (hi/) and the i o n i z a t i o n energy, E^ ( e q u i v a l e n t l y c a l l e d the i o n i z a t i o n p o t e n t i a l , I P , o r b i n d i n g energy, B E ) . E, - hi/ - E k I T

The p h o t o n energy o f t h e l i g h t source i s known, and measurement o f the k i n e t i c energy o f t h e e j e c t e d e l e c t r o n s y i e l d s t h e i o n i z a t i o n e n e r g i e s c o r r e s p o n d i n g t o t h e e l e c t r o n - o c c u p i e d bound s t a t e s . K i n e t i c e n e r g i e s a r e e a s i l y measured w i t h a n e l e c t r o s t a t i c a n a l y z e r as shown i n F i g u r e 1. The i n n e r sphere i s a t a p o s i t i v e p o t e n t i a l w h i l e the o u t e r sphere i s m a i n t a i n e d a t a n e g a t i v e p o t e n t i a l . Electrons f o l l o w a c u r v e d p a t h as they are r e p e l l e d from the o u t e r sphere p o t e n t i a l and a t t r a c t e d b y the i n n e r sphere p o t e n t i a l , and o n l y e l e c t r o n s w i t h i n a narrow range o f k i n e t i c energy w i l l have t h e c o r r e c t r a d i u s o f c u r v a t u r e t o pass through the e x i t s l i t t o t h e d e t e c t o r a t t h e end o f the a n a l y z e r . The p o t e n t i a l s on the spheres are scanned under computer c o n t r o l t o observe d i f f e r e n t k i n e t i c energy e l e c t r o n s . An a n a l y z e r vacuum o f b e t t e r t h a n 1 0 ~ T o r r i s n e c e s s a r y f o r t h e e l e c t r o n mean f r e e p a t h t o be s u f f i c i e n t l y l o n g f o r e l e c t r o n s t o reach the detector. S e v e r a l d i f f e r e n t monochromatic i o n i z a t i o n s o u r c e s a r e commonly used. U l t r a v i o l e t sources (photon energy g e n e r a l l y l e s s t h a n 50 eV) such as H e l , H e l l , o r N e l p r o v i d e i n f o r m a t i o n about v a l e n c e e l e c t r o n i c s t r u c t u r e , and the experiment i s l a b e l l e d UPS f o r u l t r a v i o l e t photoelectron spectroscopy. X - r a y s o u r c e s (photon energy g e n e r a l l y g r e a t e r t h a n 1000 eV) such as MgKa, A l K a , e t c . , have s u f f i c i e n t energy t o probe c o r e i o n i z a t i o n s , and the experiment i s l a b e l l e d XPS for x-ray photoelectron spectroscopy. Comparison o f p h o t o e l e c t r o n s p e c t r a o b t a i n e d w i t h d i f f e r e n t energy p h o t o n s o u r c e s p r o v i d e s i n f o r m a t i o n on the o r b i t a l c h a r a c t e r o f t h e i o n i z a t i o n s because each o r b i t a l type h a s a d i f f e r e n t i o n i z a t i o n p r o b a b i l i t y ( c r o s s - s e c t i o n ) dependence on photon energy. M e t a l d o r b i t a l based i o n i z a t i o n s show s t r o n g i n t e n s i t y w i t h H e l l e x c i t a t i o n , b u t r e l a t i v e l y weak i n t e n s i t y w i t h Hel e x c i t a t i o n . I o n i z a t i o n s o f s and ρ o r b i t a l s l o c a l i z e d on l i g a n d s c o m p r i s e d o f c a r b o n and o t h e r main group elements show t h e o p p o s i t e t r e n d . These changes a r e i d e a l l y s u i t e d f o r t h e s t u d y o f organometallic molecules. H e l and H e l l i n t e n s i t y comparisons a r e commonly used t o r e v e a l the m e t a l and l i g a n d c h a r a c t e r a s s o c i a t e d w i t h the observed i o n i z a t i o n s . A l l components o f the p h o t o e l e c t r o n s p e c t r o s c o p y i n s t r u m e n t a t i o n have c o n t i n u e d t o e v o l v e over t h e l a s t decade. New commercial s o u r c e s f o r XPS w i t h the anode a t h i g h p o s i t i v e p o t e n t i a l have an o r d e r o f magnitude improvement i n photon f l u x over t h e o l d e r grounded anode d e s i g n s . A n a l y z e r s w i t h e l e c t r o n l e n s p r e f o c u s i n g a r e f a r superior t o unmodified hemispherical, p a r a l l e l p l a t e , o r c y l i n d r i c a l 4



Ionization Source

F i g u r e 1. Schematic o f h e m i s p h e r i c a l a n a l y z e r showing e l e c t r o n s w i t h low k i n e t i c energy h i t t i n g t h e i n n e r s p h e r e , e l e c t r o n s w i t h h i g h k i n e t i c energy h i t t i n g t h e o u t e r s p h e r e , and e l e c t r o n s w i t h k i n e t i c energy i n t h e c o r r e c t range t o r e a c h t h e e x i t s l i t and detector.

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Photoelectron Spectroscopy

m i r r o r a n a l y z e r s . Development o f m i n i and microcomputers e n a b l e s much more p r e c i s e c o n t r o l o f the a n a l y z e r f o c u s i n g v o l t a g e s and e l e c t r o n c o u n t i n g e l e c t r o n i c s . ( 1 3 ) The most i m p o r t a n t developments, however, have been i n the a r e a s o f u l t r a - h i g h vacuum (UHV) and c l e a n sample h a n d l i n g / p r e p a r a t i o n i n the UHV environment. These developments have made p o s s i b l e many experiments f o r i n v e s t i g a t i n g s u r f a c e s t h a t were i m p o s s i b l e a decade ago. A l t h o u g h the p h o t o e l e c t r o n experiment i s c o n c e p t u a l l y s i m p l e , u t i l i z a t i o n o f photoelectron spectroscopy requires a s u b s t a n t i a l commitment o f t a l e n t e d manpower and f i n a n c i a l r e s o u r c e s . The gas phase p h o t o e l e c t r o n experiment i s p a r t i c u l a r l y c a p r i c i o u s because i n the c o u r s e o f r u n n i n g the experiment t h e e n t i r e vacuum chamber i s exposed t o a p a r t i a l p r e s s u r e o f the s t u d i e d compound, thus degenerat i v e l y c o a t i n g the e l e c t r o n o p t i c s , e l e c t r o n d e t e c t o r , i o n i z a t i o n s o u r c e , and pumping hardware. D i l i g e n t maintenance o f s u r f a c e c o n d i t i o n s i n the a n a l y z e r i s n e c e s s a r y t o r e t a i n s e n s i t i v i t y and r e s o l u t i o n , and i n many c a s e s , depending on the i n s t r u m e n t , a s i n g l e exp e r i m e n t cannot be completed b e f o r e l o s s o f i n s t r u m e n t performance. The b e s t gas-phase p h o t o e l e c t r o n s p e c t r o m e t e r s f o r o r g a n o m e t a l l i c compounds a r e those l e a s t a f f e c t e d b y these changes i n s u r f a c e c o n d i t i o n s . Our i n s t r u m e n t h a s a number o f advantages i n t h i s r e g a r d . We u t i l i z e a r e l a t i v e l y l a r g e 36 cm r a d i u s h e m i s p h e r i c a l a n a l y z e r w i t h a 10 cm gap between the spheres ( F i g u r e 1 ) . The d e s i g n p r o v i d e s a l a r g e e l e c t r o n s a m p l i n g a n g l e and h i g h e l e c t r o n t h r o u g h p u t w i t h good r e s o l u t i o n (15-20 mV on the argon 2 p line). The h i g h s e n s i t i v i t y o f the a n a l y z e r means t h a t l e s s sample i s r e q u i r e d , and the l a r g e s u r f a c e a r e a o f the s p e c t r o m e t e r means t h a t more c o n t a m i n a t i o n i s tolerated. I n a d d i t i o n , t h e sample i o n i z a t i o n chamber i s s e p a r a t e from the a n a l y z e r chamber, and f a s t d i f f e r e n t i a l pumping systems a r e used t o reduce t h e r a t e a t w h i c h i n s t r u m e n t s u r f a c e s become contaminated. Even w i t h t h e s e advantages, a slow d r i f t o f the i o n i z a t i o n energy s c a l e i s observed as the v a p o r p r e s s u r e o f the sample i n the i o n i z a t i o n chamber i n c r e a s e s and d e p o s i t i o n o f sample on t h e s l i t s and a n a l y z e r s u r f a c e s changes t h e e l e c t r o s t a t i c s . Our h i g h s e n s i t i v i t y and r e s o l u t i o n a l l o w us t o u s e t h e sharp A r 2 p , i o n i z a t i o n r e f e r e n c e l i n e as a r a p i d i n t e r n a l l o c k (14) o f the energy s c a l e t h a t m a i n t a i n s an energy s c a l e c o n s t a n t t o b e t t e r t h a n ± 0.003 eV. T h i s i n t e r n a l l o c k i s e s s e n t i a l f o r o b s e r v a t i o n o f the d e t a i l e d i o n i z a t i o n band shapes and v i b r a t i o n a l f i n e s t r u c t u r e we a r e a b l e t o obtain. 3 / 2

3

2

Samples. P h o t o e l e c t r o n s p e c t r o s c o p y h a s been used t o s t u d y samples i n the gas, l i q u i d , and s o l i d s t a t e . As w i t h most forms o f s p e c t r o s copy the sample type a f f e c t s ( s e v e r e l y ) s p e c t r a l r e s o l u t i o n and s e n s i t i v i t y , and d e t e r m i n e s t h e amount and k i n d o f i n f o r m a t i o n t h a t can be o b t a i n e d from a g i v e n study. The t r a d e - o f f between r e s o l u t i o n and s e n s i t i v i t y i s most e v i d e n t i n the comparison o f the p h o t o e l e c t r o n s p e c t r a o f s o l i d s and gases. The h i g h s e n s i t i v i t y from s o l i d s u r f a c e s i s compensated b y s o l i d s t a t e e f f e c t s g i v i n g b r o a d e r bands and a more c o m p l i c a t e d background. The r e l a t i v e l y poor s e n s i t i v i t y o f gas phase p h o t o e l e c t r o n s p e c t r o s c o p y i s compensated b y good r e s o l u t i o n . A second major advantage o f gas phase s p e c t r o s c o p y over s o l i d s t a t e ( s u r f a c e , powder) s p e c t r o s c o p y i s t h a t t h e energy s c a l e i s e a s i l y and d i r e c t l y r e f e r e n c e d t o the vacuum l e v e l . We have r e c e n t l y made s i g n i f i c a n t p r o g r e s s on r e d u c i n g the problems o f r e s o l u t i o n , b a s e l i n e , and energy r e f e r e n c i n g i n the p h o t o e l e c t r o n s p e c t r a o f s o l i d o r g a n o m e t a l l i c samples on s u r f a c e s t h a t p r o m i s e s t o expand the f u t u r e a p p l i c a t i o n s o f i o n i z a t i o n s p e c t r o s c o p y . However, a t t h e



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p r e s e n t t i m e the p r i n c i p l e s o f the p h o t o e l e c t r o n s p e c t r o s c o p y t e c h n i q u e a r e b e s t i l l u s t r a t e d w i t h s t u d i e s o f gas phase samples. The remainder o f t h i s c h a p t e r w i l l f o c u s on gas phase r e s u l t s . S y n t h e t i c c h e m i s t s who a r e i n t e r e s t e d i n o b t a i n i n g p h o t o e l e c t r o n i n f o r m a t i o n on t h e i r systems commonly w i s h t o know whether t h e i r m o l e c u l e s are s u i t a b l e f o r the experiment and how much sample i s r e q u i r e d . N e a r l y any sample t h a t can be s u b l i m e d can be s t u d i e d i n the gas phase w i t h p h o t o e l e c t r o n s p e c t r o s c o p y . A i r and/or m o i s t u r e s e n s i t i v i t y i s n o t an i s s u e because sample t r a n s f e r s a r e p e r f o r m e d w i t h i n e r t atmosphere t e c h n i q u e s and the a c t u a l e x p e r i m e n t i s performed i n h i g h vacuum. Another good i n d i c a t i o n t h a t the gas phase p h o t o e l e c t r o n e x p e r i m e n t w i l l be s u c c e s s f u l i s p r o v i d e d by a normal mass spectrum o f the m o l e c u l e showing the p a r e n t i o n i n h i g h y i e l d . However, n e i t h e r b u l k s u b l i m a t i o n s nor mass s p e c t r o s c o p y e x a c t l y match the c o n d i t i o n s o f the p h o t o e l e c t r o n e x p e r i m e n t , and t h e s e e x p e r i m e n t s can n o t be t a k e n as a b s o l u t e i n d i c a t o r s f o r o b t a i n i n g s u i t a b l e v a p o r p r e s s u r e o f the e x p e c t e d s p e c i e s i n the gas phase f o r photoelectron spectroscopy. In fact, q u a l i t y photoelectron spectra have been o b t a i n e d f o r many s p e c i e s t h a t do n o t h o l d t o g e t h e r i n b u l k s u b l i m a t i o n s o r mass s p e c t r o s c o p y . The b e s t experiment t o answer the q u e s t i o n i s s i m p l y t o g i v e a sample a t r y i n the s p e c t r o m e t e r . I f the sample happens t o decompose i n the s p e c t r o m e t e r , the decomposit i o n p r o d u c t s a r e u s u a l l y s i m p l e r m o l e c u l e s w h i c h are e a s i l y i d e n t i f i e d i n the spectrum. For samples w h i c h a r e s o l i d s a t room temp e r a t u r e , the e x p e r i m e n t i s c a r r i e d out by s l o w l y r a i s i n g the temp e r a t u r e and thus the sample v a p o r p r e s s u r e t o o b t a i n a s e r i e s o f s p e c t r a u n t i l the sample i s d e p l e t e d . T h i s i s e s s e n t i a l l y a temperat u r e programmed d e c o m p o s i t i o n t e c h n i q u e , and i f no changes o c c u r i n the s p e c t r a w i t h time and temperature and no r e s i d u e r e m a i n s , t h e n a c l e a n v a p o r i z a t i o n has t a k e n p l a c e . F u r t h e r c o n f i r m a t i o n o f sample i n t e g r i t y i s p r o v i d e d by comparing the s p e c t r a o f r e l a t e d s e r i e s o f molecules. The amount o f sample r e q u i r e d depends on the b e h a v i o r o f the m o l e c u l e and the i n f o r m a t i o n t h a t i s d e s i r e d . We have o b t a i n e d h i g h q u a l i t y ( f u l l v a l e n c e r e g i o n H e l ) s p e c t r a on l e s s t h a n 10 mg o f sample. T h i s amount i s g e n e r a l l y s u i t a b l e f o r an i n i t i a l e x p e r i m e n t . The H e l l e x p e r i m e n t i s l e s s s e n s i t i v e and r e q u i r e s more sample. A f u l l H e l / H e l l s t u d y might be a c c o m p l i s h e d w i t h about 100 mg o f a w e l l - b e h a v e d sample. The gas phase XPS experiment a l s o i n v o l v e s low s e n s i t i v i t y s i g n a l p r o c e s s i n g , and the c o r e i o n i z a t i o n energy measurement o f each element i n the m o l e c u l e i s r e p e a t e d s e v e r a l t i m e s t o d e t e r m i n e the c o n f i d e n c e l i m i t s o f the measurement. Complete gas phase XPS s t u d i e s may r e q u i r e s e v e r a l hundred m i l l i g r a m s o f sample with current instrumentation. I o n i z a t i o n Energy R e l a t i o n s h i p s I n a r e c e n t a c c o u n t we d i s c u s s e d i n d e t a i l the i n f o r m a t i o n a v a i l a b l e from i o n i z a t i o n band shapes, i n t e n s i t i e s , s p l i t t i n g s , and s h i f t s i n the p h o t o e l e c t r o n s p e c t r a o f t r a n s i t i o n m e t a l compounds.(15) Here we would l i k e t o g i v e the r e a d e r a f e e l i n g f o r the r e l a t i o n s h i p s between i o n i z a t i o n e n e r g i e s and the t r a d i t i o n a l c o n c e p t s o f e l e c t r o n i c s t r u c t u r e and s t a b i l i t y o f o r g a n o m e t a l l i c m o l e c u l e s . F i r s t i t must be emp h a s i z e d t h a t the i o n i z a t i o n energy i s s i m p l y a w e l l - d e f i n e d thermodynamic q u a n t i t y t h a t , u n l i k e many o t h e r e n t h a l p y and e n t r o p y q u a n t i t i e s f o r o r g a n o m e t a l l i c m o l e c u l e s , can be p r e c i s e l y and r e l a t i v e l y e a s i l y measured. The fundamental s i g n i f i c a n c e o f t h i s q u a n t i t y i s most e a s i l y a p p r e c i a t e d by remembering the q u a n t i t a t i v e d e s c r i p t i o n o f the p r o p e r t i e s o f atoms i n freshman c h e m i s t r y t e x t b o o k s . An



11.

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i m p o r t a n t f e a t u r e o f the c h a r a c t e r i z a t i o n o f atoms i s t h e i r v a l e n c e i o n i z a t i o n e n e r g i e s , which are then r e l a t e d t o e l e c t r o n e g a t i v i t y s c a l e s , b o n d i n g schemes, and thermodynamic c y c l e s . The c h a r a c t e r i z a t i o n s o f m o l e c u l a r i o n i z a t i o n e n e r g i e s a r e s i g n i f i c a n t f o r t h e same reasons. The e l e c t r o n i c s t r u c t u r e f a c t o r s t h a t account f o r s h i f t s i n m o l e c u l a r i o n i z a t i o n e n e r g i e s between two r e l a t e d m o l e c u l e s a r e c l o s e l y s i m i l a r t o t h e f a c t o r s t h a t account f o r s h i f t s i n m o l e c u l a r o r b i t a l e n e r g i e s o r e i g e n v a l u e s . T h i s i s perhaps the p r i m a r y r e a s o n t h a t m o l e c u l a r p h o t o e l e c t r o n s p e c t r o s c o p y h a s a t t r a c t e d so much a t t e n t i o n , a l t h o u g h i t s h o u l d be s t r e s s e d t h a t t h e r e i s c o n s i d e r a b l y more v a l u e c o n t a i n e d i n the m o l e c u l a r i o n i z a t i o n s t h a n j u s t t h i s r e l a t i o n s h i p . The v a l e n c e i o n i z a t i o n e n e r g i e s a r e o f t e n d i s c u s s e d as though t h e y a r e e x p e r i m e n t a l l y measured m o l e c u l a r o r b i t a l e n e r g i e s (vide i n f r a ) . The f i r s t c o n s i d e r a t i o n f o r the energy o f a m o l e c u l a r o r b i t a l i s t h e i n h e r e n t s t a b i l i t i e s o f the c o n s t i t u e n t atomic o r b i t a l s and t h e i r c o n t r i b u t i o n t o the m o l e c u l a r o r b i t a l . The i n h e r e n t s t a b i l i t i e s o f the o u t e r v a l e n c e e l e c t r o n s a r e r e l a t e d t o t h e e l e c t r o n e g a t i v i t i e s o f the atoms. The second f a c t o r c o n t r i b u t i n g t o t h e s t a b i l i t y i s t h e amount and k i n d o f b o n d i n g c h a r a c t e r i n the molecu lar orbital. T h i r d l y , t h e charge p o t e n t i a l i n the v i c i n i t y o f t h e m o l e c u l a r o r b i t a l w i l l s h i f t t h e i o n i z a t i o n energy b y the e l e c t r o static interaction. The s i g n i f i c a n c e o f t h e s e b o n d i n g and charge d i s t r i b u t i o n f e a t u r e s i n the p h o t o e l e c t r o n s p e c t r a o f t r a n s i t i o n m e t a l complexes i s apparent i n the l i g a n d a d d i t i v i t y (16-19) and c o r e - v a l e n c e c o r r e l a t i o n (19-20) p r i n c i p l e s . These p r i n c i p l e s have r e c e n t l y been e s t a b l i s h e d t o c l a r i f y the r e l a t i o n s h i p s between i o n i z a t i o n s and m o l e c u l a r e l e c t r o n i c s t r u c t u r e , s t a b i l i t y , and r e a c t i v i t y . L i g a n d a d d i t i v i t y s u g g e s t s t h a t s u c c e s s i v e l i g a n d replacements i n t r a n s i t i o n m e t a l complexes r e s u l t i n r e p r o d u c i b l e and s y s t e m a t i c ( a d d i t i v e ) s h i f t s o f a l l v a l e n c e and c o r e i o n i z a t i o n e n e r g i e s . The p r i n c i p l e i s c l e a r l y i l l u s t r a t e d w i t h the m e t a l d i o n i z a t i o n p o t e n t i a l s o f a s e r i e s o f MAg_ B complexes i n w h i c h s u c c e s s i v e s u b s t i t u t i o n s o f l i g a n d s A w i t h Β may Be compared. I t h a s been shown t h a t a m e t a l - b a s e d i o n i z a t i o n p o t e n t i a l o f any o f these complexes (E ( i ) ) i s e m p i r i c a l l y p r e d i c t e d with the f o l l o w i n g r e l a t i o n : n

n

E (i) n

- E

Q

+ m(i) AEg(i) + η A E ( i ) Q

The f i r s t term r e p r e s e n t s t h e i o n i z a t i o n p o t e n t i a l o f the unsubs t i t u t e d MA complex, and c o n t a i n s t h e i n h e r e n t s t a b i l i t y o f t h e atomic o r b i t a l s c o m p r i s i n g the m e t a l d m o l e c u l a r o r b i t a l . The second term r e p r e s e n t s the b o n d i n g o r o v e r l a p c o n t r i b u t i o n t o t h e i o n i z a t i o n energy s h i f t . The t h i r d term r e p r e s e n t s the change i n charge poten t i a l c o n t r i b u t i o n t o t h e o r b i t a l i o n i z a t i o n energy. The c o n s t a n t A E g ( i ) i s t h e d i f f e r e n c e i n bonding s t a b i l i z a t i o n between the two l i g a n d s A and B, and A E ( i ) i s t h e d i f f e r e n c e i n charge p o t e n t i a l p r o v i d e d b y A and B. Thus a r e l a t i v e b o n d i n g s t a b i l i z a t i o n and charge p o t e n t i a l term can be c h a r a c t e r i z e d f o r each type o f l i g a n d bound t o t h e t r a n s i t i o n m e t a l c e n t e r . I o n i z a t i o n s t u d i e s o f c o r e o r b i t a l s p r o v i d e i n f o r m a t i o n com plementary t o valence i o n i z a t i o n s t u d i e s . While valence i o n i z a t i o n e n e r g i e s i n c l u d e b o t h c o n t r i b u t i o n s from charge d i s t r i b u t i o n ( A E ( i ) ) and b o n d i n g o r h y p e r c o n j u g a t i v e ( A E ( i ) ) e f f e c t s , t h e c o r e i o n i z a t i o n s i n c l u d e t h e charge d i s t r i b u t i o n c o n t r i b u t i o n , A E ( k ) ( k r e p r e s e n t i n g a core o r b i t a l ) , b u t not the o v e r l a p bonding c o n t r i b u t i o n . Core i o n i z a t i o n e n e r g i e s can be used as an e x p e r i m e n t a l i n d i c a t i o n o f fi

Q

0

g

Q



272

a t o m i c c h a r g e s (21-23) and c o m p a r i s o n o f a c o r e s h i f t w i t h a v a l e n c e s h i f t can be u s e d t o s e p a r a t e out the b o n d i n g c o n t r i b u t i o n t o a v a l e n c e i o n i z a t i o n energy s h i f t . J o l l y ' s c o r e - v a l e n c e i o n i z a t i o n c o r r e l a t i o n thus complements l i g a n d a d d i t i v i t y by r e l a t i n g c o r e i o n i z a t i o n energy s h i f t s and v a l e n c e l o c a l i z e d o r b i t a l i o n i z a t i o n energy s h i f t s w i t h a s i m p l e r e l a t i o n : AE (i)/AE (k)


Q

Q

-

0.8

V a l u e s s u b s t a n t i a l l y d i f f e r i n g from 0.8 i n d i c a t e a b o n d i n g ( o r a n t i b o n d i n g ) i n t e r a c t i o n p r o v i d i n g an a d d i t i o n a l s h i f t t o the v a l e n c e ionization. L i g a n d and m e t a l r e p l a c e m e n t s i n s e r i e s o f r e l a t e d m o l e c u l e s r e p r e s e n t p e r t u r b a t i o n s o f the b a s i c e l e c t r o n i c s t r u c t u r e t h a t a l l o w d e t e r m i n a t i o n o f the r e l a t i v e E Q , AEg, and A E Q f a c t o r s f o r the i o n i z a t i o n s . Knowledge o f the range o f these f a c t o r s f o r g i v e n s u b s t i t u t i o n s a l s o a i d s assignment o f the i o n i z a t i o n s and i n t e r p r e t a t i o n o f the e l e c t r o n d i s t r i b u t i o n and b o n d i n g o f new m o l e c u l e s . For example, the e f f e c t s o f c y c l o p e n t a d i e n y l r i n g m e t h y l a t i o n , e.g. c y c l o p e n t a d i e n y l (Cp) v s . p e n t a m e t h y l c y c l o p e n t a d i e n y l (Cp*), are u t i l i z e d widely i n organometallic chemistry. The e l e c t r o n i c c o n t r i b u t i o n s t o t h i s e f f e c t have been c h a r a c t e r i z e d i n d e t a i l by the p h o t o e l e c t r o n s p e c t r o s c o p y o f sandwich and h a l f - s a n d w i c h complexes.(24.25) The e l e c t r o n i c e f f e c t o f r i n g m e t h y l a t i o n f o r the cyclopentadienylmanganese t r i c a r b o n y l s i s shown i n the s p e c t r a o f F i g u r e 2. H e l / H e l l i n t e n s i t y comparisons i n d i c a t e t h a t the i o n i z a t i o n s a s s o c i a t e d w i t h the p r e d o m i n a n t l y m e t a l " t 2 g " e l e c t r o n s o c c u r a t l o w e s t i o n i z a t i o n energy on the r i g h t o f the f i g u r e . These i o n i z a t i o n s s h i f t t o lower energy w i t h each r i n g m e t h y l a t i o n because o f i n c r e a s i n g charge dona t i o n t o the m e t a l from the r i n g . The s l i g h t l y s p l i t i o n i z a t i o n band a t 9-10 eV c o r r e l a t e s w i t h the c y c l o p e n t a d i e n y l e^' r i n g π o r b i t a l s . The s h i f t o f t h i s band w i t h r i n g m e t h y l a t i o n i s t w i c e the s h i f t o f the m e t a l band, a g a i n i d e n t i f y i n g i t as h a v i n g the predominant r i n g e" c h a r a c t e r . The s l i g h t s p l i t t i n g o f t h i s i o n i z a t i o n f e a t u r e f o l l o w s from the symmetry r e d u c t i o n o f the e^' o r b i t a l s o f f r e e Cp when c o o r d i n a t e d t o the M n ( C 0 ) fragment. T h i s s h o u l d e r i s c h a r a c t e r i s t i c f o r these c y c l o p e n t a d i e n y l i o n i z a t i o n s o f h a l f - s a n d w i c h complexes. The l a r g e band growing i n w i t h r i n g m e t h y l a t i o n a t around 11 eV c o r r e s p o n d s t o i o n i z a t i o n o f the e c o m b i n a t i o n (C3v symmetry) o f the C-H σ bonds o f the r i n g m e t h y l s . ( 2 1 ) The o v e r l a p o f t h i s C-H σ bond symmetry o r b i t a l w i t h the c y c l o p e n t a d i e n y l e^' o r b i t a l i s a f i l l e d - f i l l e d o r b i t a l i n t e r a c t i o n t h a t produces the l a r g e s h i f t o f the e ^ - b a s e d i o n i z a t i o n s between Cp and Cp* analogues. Thus a l a r g e p o r t i o n o f the d e s t a b i l i z a t i o n o f the c y c l o p e n t a d i e n y l v a l e n c e π i o n i z a t i o n s w i t h m e t h y l a t i o n i s due t o o r b i t a l o v e r l a p e f f e c t s r a t h e r t h a n t o i n d u c t i v e e f f e c t s as commonly d e s c r i b e d . T h i s i s shown e x p e r i m e n t a l l y by the comparison o f the c o r e and v a l e n c e i o n i z a t i o n shifts.(25) 1

3

1

The s p e c t r a o f d i f f e r e n t m o l e c u l e s t h a t are r e l a t e d by s u b s t i t u t i o n o f atoms down a group o f the p e r i o d i c t a b l e i s a n o t h e r common example where l i g a n d r e p l a c e m e n t s can y i e l d s i g n i f i c a n t e l e c t r o n i c s t r u c t u r e i n f o r m a t i o n . For i n s t a n c e , two m o l e c u l e s may be the same e x c e p t f o r the r e p l a c e m e n t o f a c h l o r i n e atom by a bromine atom, o r the r e p l a c e m e n t o f an oxygen by a s u l f u r . The f i r s t e f f e c t o b s e r v e d from such a r e p l a c e m e n t i s the a l t e r e d s t a b i l i t y o f the i o n i z a t i o n s caused by the d i f f e r e n t s t a b i l i t y ( e l e c t r o n e g a t i v i t y ) o f the atomic orbitals. I n c o r p o r a t i o n o f a t h i r d row t r a n s i t i o n m e t a l o r heavy



11.


273


atoms such as i o d i n e a l s o i n t r o d u c e s s p i n - o r b i t s p l i t t i n g s t h a t a s s i s t i d e n t i f i c a t i o n o f the p r i m a r y atomic c h a r a c t e r o f the i o n i z a t i o n s and the e x t e n t o f m i x i n g w i t h o t h e r o r b i t a l s . These p r i n c i p l e s , a l o n g w i t h a few o t h e r s such as changes i n i o n i z a t i o n c r o s s s e c t i o n s w i t h s o u r c e energy and the o b s e r v a t i o n o f v i b r a t i o n a l f i n e s t r u c t u r e mentioned i n the experiment s e c t i o n , o f t e n a r e s u f f i c i e n t t o o v e r d e t e r m i n e t h e assignment and i n t e r p r e t a t i o n o f the e l e c t r o n i c s t r u c t u r e . That i s , more t h a n one e x p e r i m e n t a l o b s e r v a t i o n l e a d s t o a given c o n c l u s i o n . T h e o r e t i c a l c a l c u l a t i o n s a r e then not necessary f o r d r a w i n g t h e s e c o n c l u s i o n s from the e x p e r i m e n t , and the e x p e r i m e n t can be a t r u e independent and u n b i a s e d t e s t o f the t h e o r i e s and models o f o r g a n o m e t a l l i c c h e m i s t r y . The f o r m a l r e l a t i o n s h i p between e x p e r i m e n t a l i o n i z a t i o n e n e r g i e s and t h e o r e t i c a l l y c a l c u l a t e d o r b i t a l e i g e n v a l u e s i s g i v e n b y Koopmans' theorem.(26) Koopmans' theorem shows t h a t t h e a d d i t i o n a l t h e o r e t i c a l c o n t r i b u t i o n s o f e l e c t r o n r e l a x a t i o n and e l e c t r o n c o r r e l a t i o n t h a t appear as terms i n ab i n i t i o o r b i t a l a p p r o x i m a t i o n s a r e neglected i n equating o r b i t a l eigenvalues t o i o n i z a t i o n energies. Both o f t h e s e c o n t r i b u t i o n s are known t o be l a r g e f o r t r a n s i t i o n m e t a l s p e c i e s . E x p e r i m e n t a l l y , e l e c t r o n r e l a x a t i o n energy d i f f e r e n c e s between atoms w i t h i n the same m o l e c u l e o r between m o l e c u l e s w i t h d i f f e r i n g atoms c a n a l s o p r o v i d e i m p o r t a n t b o n d i n g and e l e c t r o n i c s t r u c t u r e i n f o r m a t i o n . F o r example, t h e l a r g e d i f f e r e n c e i n r e l a x a t i o n energy between Co and Rh e x p l a i n s many o f the c h e m i c a l d i f f e r e n c e s t h a t are m a n i f e s t e d i n the e x c i t e d ( p o s i t i v e i o n ) s t a t e , whereas ground s t a t e p r o p e r t i e s f o r analogous Co and Rh complexes a r e nearly identical.(27.28) To i l l u s t r a t e t o t h e r e a d e r the p r i n c i p l e s i n t r o d u c e d i n t h i s s e c t i o n i n the most d i r e c t and p r a c t i c a l way, we p r e s e n t some case s t u d i e s t h a t compare the b o n d i n g p r o p e r t i e s o f d i f f e r e n t l i g a n d s t o the same m e t a l c e n t e r , and t h a t compare the b o n d i n g p r o p e r t i e s o f d i f f e r e n t m e t a l f u n c t i o n a l groups t o t h e same l i g a n d . I t i s hoped t h a t t h e s e e x p e r i m e n t a l r e s u l t s w i l l g i v e t h e r e a d e r an a p p r e c i a t i o n o f the types o f d e t a i l e d e l e c t r o n i c s t r u c t u r e i n f o r m a t i o n a v a i l a b l e from p h o t o e l e c t r o n spectroscopy. Case Study:

Bonding C a p a b i l i t i e s o f L i g a n d s

T h i s s e c t i o n w i l l demonstrate t h e r o l e o f p h o t o e l e c t r o n spectroscopy i n c h a r a c t e r i z i n g the r e l a t i v e b o n d i n g c a p a b i l i t i e s o f l i g a n d s . The f o u r l i g a n d s t o be d i s c u s s e d a r e CO, P R , C H , and C R . These a r e q u i t e g e n e r a l i n o r g a n o t r a n s i t i o n m e t a l c h e m i s t r y , and knowledge o f t h e i r b o n d i n g c a p a b i l i t i e s has been the f o c u s o f many i n v e s t i g a t i o n s u t i l i z i n g a v a r i e t y o f techniques. The v a l e n c e p h o t o e l e c t r o n spec t r o s c o p y o f the l i g a n d s as f r e e m o l e c u l e s p r o v i d e s t h e f i r s t i n d i c a t i o n o f the b o n d i n g c a p a b i l i t i e s o f t h e s e l i g a n d s t o m e t a l complexes. The i o n i z a t i o n e n e r g i e s o f the h i g h e s t o c c u p i e d m o l e c u l a r o r b i t a l s f o r t h e s e s p e c i e s , w h i c h w i l l a c t as t h e e l e c t r o n donors t o t h e m e t a l s , a r e summarized i n T a b l e I . The o r d e r o f e l e c t r o n r i c h n e s s as d e f i n e d i n the f i r s t s e c t i o n i s PMe >C Et >C H >C0. The donor a b i l i t i e s o f t h e s e l i g a n d s w i l l l a r g e l y f o l l o w the same t r e n d as l o n g as o v e r l a p w i t h the m e t a l o r b i t a l s i s s i m i l a r . A more i n t e r e s t i n g f e a t u r e o f many o r g a n o m e t a l l i c l i g a n d s , however, i s t h e i r π-acceptor c a p a b i l i t i e s . The v a l e n c e s p e c t r a o f the f r e e l i g a n d s do n o t p r o v i d e any d i r e c t i n f o r m a t i o n on the empty π o r b i t a l s . The π-acceptor c a p a b i l i t y of a ligand i s experimentally characterized i n p h o t o e l e c t r o n s p e c t r o s c o p y b y o b s e r v i n g the e f f e c t on the i o n i z a t i o n s o f o c c u p i e d m e t a l donor o r b i t a l s i n t e r a c t i n g w i t h the l i g a n d a c c e p t o r 3

3

2

2

2

4

2

2

2

4



274

T a b l e I . V a l e n c e I o n i z a t i o n E n e r g i e s f o r L i g a n d st Ligand CO PMe C H C Et 3

2

2

I o n i z a t i o n P o t e n t i a l . eV 14.01 8.58 10.51 9.35

Ionization 5(7 lone p a i r π bond π bond

4

2

'Data f o r CO and C H from r e f e r e n c e 2, d a t a f o r PMe 19, and d a t a f o r C E t from r e f e r e n c e 36. 2

4


2

from r e f e r e n c e

3

2

o r b i t a l s . Thus t h e π-acceptor a b i l i t i e s o f two l i g a n d s c a n be com p a r e d i f complexes a r e p r e p a r e d i n w h i c h those two l i g a n d s a r e bound t o a common m e t a l fragment o r t e m p l a t e . The [CpMn(C0) ] fragment d i s p l a y s many o f t h e a t t r a c t i v e f e a t u r e s o f a good t e m p l a t e . Perhaps most i m p o r t a n t i s t h e f a c t t h a t a l a r g e number o f s u b s t i t u e n t d e r i v a t i v e s have been p r e p a r e d and c h a r a c t e r i z e d . F o r i n s t a n c e , t h e s p e c t r a o f CpMn(C0) , C p M n ( C 0 ) ( P M e ) , C p M n ( C O ) ( C H ) , and C p M n ( C 0 ) ( C E t ) c a n be com pared t o r e v e a l the r e l a t i v e bonding c a p a b i l i t i e s o f these l i g a n d s . The e l e c t r o n i c s t r u c t u r e and d o n o r / a c c e p t o r c a p a b i l i t i e s o f t h i s fragment have been t h o r o u g h l y c h a r a c t e r i z e d . ( 2 9 - 3 3 ) The c y c l o p e n t a d i e n y l r i n g c a n be p e r t u r b e d w i t h r i n g m e t h y l a t i o n s t o s h i f t v a l e n c e i o n i z a t i o n s , and t h e s h i f t o f t h e r i n g i o n i z a t i o n s w i t h l i g a n d s u b s t i t u t i o n s c a n r e v e a l a d d i t i o n a l charge and π d e r e a l i z a t i o n e f f e c t s . CpMn(C0) L compounds may be c o n s i d e r e d pseudo-octahe d r a l w i t h the c y c l o p e n t a d i e n y l r i n g occupying three c o o r d i n a t i o n s i t e s . A c o n v e n i e n t c o o r d i n a t e system f o r u n d e r s t a n d i n g t h e o r b i t a l i n t e r a c t i o n s i n CpMn(C0) L p l a c e s L a l o n g t h e ζ a x i s , w i t h t h e χ a x i s b i s e c t i n g t h e two c a r b o n y l s as shown i n F i g u r e 3. The o r b i t a l con t o u r s shown a r e f o r c a l c u l a t e d e i g e n v e c t o r s o f t h e [CpMn(C0)] fragment, i . e . n o t i n c l u d i n g L. The LUMO o f t h e fragment i s t h e 3a' o r b i t a l , which i s h i g h i n d z c h a r a c t e r . This i s the σ acceptor o r b i t a l o f t h e m e t a l f o r a l i g a n d on t h e v a c a n t ζ a x i s c o o r d i n a t i o n site. The t h r e e 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 t h e l a ' , a", and 2a' shown i n F i g u r e 3. The 2a' and a" b o t h p o s s e s s π symmetry w i t h r e s p e c t t o t h e z - a x i s c o o r d i n a t i o n s i t e , and b o t h c a n a c t as π donors. The 2a' i s l a r g e l y dxz i n c h a r a c t e r . The a" i s t h e HOMO ( d y z ) . L i g a n d s w i t h s i n g l e π donor o r b i t a l s (e.g. o l e f i n s ) t e n d t o a l i g n f o r i n t e r a c t i o n w i t h t h e a".(29.30) The l a ' ( d x - y ) i n t e r a c t s w i t h t h e two i n - p l a n e c a r b o n y l s and has a p r i m a r i l y S symmetry i n t e r a c t i o n w i t h t h e v a c a n t c o o r d i n a t i o n s i t e . The s h i f t i n i o n i z a t i o n energy o f t h e l a ' o r b i t a l w i t h l i g a n d s u b s t i t u t i o n on t h e ζ a x i s l a r g e l y r e f l e c t s t h e t o t a l change i n charge p o t e n t i a l . 2

3

2

3

2

2

4

2

2

2

2

2

2

2

2

2

V a l e n c e I o n i z a t i o n Data and Band Assignments. C l o s e up H e l (5-11 eV) s p e c t r a f o r Cp*Mn(C0) ( 2 5 ) , Cp*Mn(C0) (PMe ) ( 3 4 ) , C p * M n ( C 0 ) ( C H ) ( 3 5 ) , and C p * M n ( C 0 ) ( C E t ) (36) a r e d i s p l a y e d i n F i g u r e 4. The " p a r e n t " t r i c a r b o n y l complex d i s p l a y s i o n i z a t i o n s w h i c h c o r r e l a t e w i t h t h e m e t a l v a l e n c e d o r b i t a l s and t h e c y c l o p e n t a d i e n y l r i n g e " o r b i t a l s , as d i s c u s s e d e a r l i e r . The low energy i o n i z a t i o n band r e p r e s e n t s t h e p r e d o m i n a n t l y m e t a l d e l e c t r o n s and t h e v e r y s m a l l b r o a d e n i n g o f t h i s i o n i z a t i o n envelope i n t h e t r i c a r b o n y l complex s u p p o r t s t h e v i e w o f t h e s e m o l e c u l e s as p s e u d o - o c t a h e d r a l w i t h a n e a r l y degenerate " t 2 g " s e t o f m e t a l d e l e c t r o n s . The a d d i t i o n a l v a l e n c e i o n i z a t i o n s o f t h e PMe , C H , and C E t complexes (9-10 eV) c o r r e l a t e w i t h t h e donor o r b i t a l s o f t h e s e l i g a n d s . The donor 3

2

2

2

3

2

2

4

2

1

6

3

2

4

2

2


11.



275

Ionization Energy (eV) 1

£

14

12

1

1

10 1

8

6

,

,


lA Λ

A

6

8

10

B

12

14

Electron Kinetic Energy (eV) 5

F i g u r e 2. H e l f u l l (6-16 eV) s p e c t r a f o r (A) (η -C H )Mn(C0) , (B) ( t 7 - C H C H ) M n ( C 0 ) 3 , and (C) ( r ? - C (CH ) ) M n ( C 0 ) . 5

5

5

3

5

6

4

3

3

5

3

5

3

ζ

5

F i g u r e 3. 3 - d i m e n s i o n a l o r b i t a l c o n t o u r s o f t h e [(rç C H ) M n ( C 0 ) ] fragment w i t h r e s p e c t t o a v a c a n t l i g a n d on t h e za x i s , w i t h t h e dominant m e t a l o r b i t a l c o n t r i b u t i o n shown n e x t t o the c o n t o u r s . The 3a' c o r r e l a t e s w i t h the d z σ a c c e p t o r , t h e a" c o r r e l a t e s w i t h t h e dyz π donor, t h e 2a' c o r r e l a t e s w i t h t h e dxz π donor, and t h e l a ' c o r r e l a t e s w i t h t h e d x - y 8 donor. s

5

2

2

2

2



Ionization Energy (eV)


11

10

9

8

7

6

F i g u r e 4. H e l c l o s e u p s p e c t r a f o r (A) Cp*Mn(C0) , C p * M n ( C 0 ) ( P M e ) , (C) C p * M n ( C 0 ) ( C H ) , and (D) Cp*Mn(C0) (C Et ). Cp* = r ; - C ( C H ) . 3

2

3

2

2

(B)

4

5

2

2

2

5

3

5


11.



277

o r b i t a l o f CO i s more s t a b l e and i o n i z e s i n t h e envelope from 12-16 eV. Note t h a t u s i n g t h e p e n t a m e t h y l c y c l o p e n t a d i e n y l (Cp*) l i g a n d r a t h e r t h a n t h e u n s u b s t i t u t e d Cp l i g a n d e n a b l e s t h e c l e a n s e p a r a t i o n o f t h e Cp e " i o n i z a t i o n s from l i g a n d donor i o n i z a t i o n s o f these complexes. (35.36) The v a l e n c e i o n i z a t i o n s o f CpMn(C0) , Cp*Mn(C0) , C p * M n ( C 0 ) ( P M e ) , C p * M n ( C O ) ( C H ) , and C p * M n ( C O ) ( C E t ) a r e c h a r a c t e r i z e d by t h e i r v e r t i c a l i o n i z a t i o n p o t e n t i a l s , band shapes, and r e l a t i v e a r e a s i n T a b l e I I . Independent h a l f - w i d t h s a r e u s e d t o model t h e h i g h and l o w energy s i d e s o f t h e asymmetric G a u s s i a n peaks. V a l e n c e i o n i z a t i o n bands a r e b r o a d e r on t h e h i g h b i n d i n g energy s i d e because o f t h e i n c r e a s i n g s l o p e o f t h e p o t e n t i a l w e l l t o t h e h i g h e r e x c i t e d v i b r a t i o n a l l e v e l s o f the p o s t i v e ion.(31) The d i f f e r e n c e s i n i o n i z a t i o n p o t e n t i a l s a r e t h e most i n f o r m a t i v e d a t a , and w i l l be discussed i n d e t a i l i n the A n a l y s i s s e c t i o n . The deeper c o r e i o n i z a t i o n e n e r g i e s o f these complexes a r e o b t a i n e d w i t h t h e x - r a y i o n i z a t i o n source. Table I I I l i s t s the core i o n i z a t i o n d a t a f o r CpMn(C0) , Cp*Mn(C0) , CpMn(CO) (PMe ), C p M n ( C 0 ) ( C H ) , and C p M n ( C O ) ( C M e ) . I n t h i s case t h e emphasis i s p l a c e d on t h e u n m e t h y l a t e d c y c l o p e n t a d i e n y l r i n g compounds because o f cleaner s e p a r a t i o n o f the i o n i z a t i o n s i n the carbon I s i o n i z a t i o n r e g i o n , where t h e c a r b o n I s i o n i z a t i o n s o f t h e c a r b o n y l s a r e sepa r a t e d from t h e c a r b o n I s i o n i z a t i o n s o f t h e c y c l o p e n t a d i e n y l r i n g and PMe . F o r r e f e r e n c e , t h e d a t a f o r b o t h CpMn(C0) and Cp*Mn(C0) have been l i s t e d i n T a b l e s I I and I I I . Core s p e c t r a a r e d i s p l a y e d i n F i g u r e 5 f o r t h e manganese 2 p , oxygen I s , c a r b o n I s , and phosphorus 2p i o n i z a t i o n r e g i o n s o f C p M n ( C 0 ) ( P M e ) . x

3


2

3

2

2

2

3

2

2

4

3

4

3

2

2

2

2

2

3

2

3

3

3

3 / 2

2

3

A n a l y s i s . W i t h t h i s g e n e r a l assignment o f t h e i o n i z a t i o n s o f these complexes, i t i s now p o s s i b l e t o examine t h e i n f o r m a t i o n t h a t these i o n i z a t i o n s provide f o r the r e l a t i v e bonding c a p a b i l i t i e s o f the d i f f e r e n t l i g a n d s . The most d r a m a t i c d i f f e r e n c e s a r e i n t h e m e t a l i o n i z a t i o n r e g i o n s . The t 2 g s e t o f m e t a l o r b i t a l s i n t h e pseudoo c t a h e d r a l t r i c a r b o n y l ( F i g u r e 4A) a r e o n l y s l i g h t l y s p l i t ( m o s t l y as a consequence o f t h e l o c a l symmetry o f t h e M n ( C 0 ) p o r t i o n o f t h e m o l e c u l e ) i n t o an a + e s e t . These a r e i n t h e r e l a t i v e a r e a r a t i o o f 1:2. I n t e r a c t i o n s between t h e t h i r d ( z - a x i s ) CO and t h e [CpMn(C0) ] fragments a" (HOMO) and 2a" donor o r b i t a l s a r e shown below. 3

2

I t s h o u l d be n o t e d f o r comparison w i t h t h e b o n d i n g o f o l e f i n s and alkynes t h a t both metal o r b i t a l s a r e s t a b i l i z e d by donation i n t o empty c a r b o n y l a c c e p t o r o r b i t a l s . When t h e i n c o m i n g l i g a n d i s a phosphine, t h e l i g a n d " a c c e p t o r " o r b i t a l s a r e t h e v e r y h i g h - l y i n g phosphorus d l e v e l s , and t h e i n t e r a c t i o n i s e x p e c t e d t o be weak. I n f a c t , t h e weakness o f t h i s i n t e r a c t i o n allows experimental inference o f the e l e c t r o n i c s t r u c t u r e o f the [CpMn(C0) ] fragment. F i g u r e 6 i s an i o n i z a t i o n c o r r e l a t i o n diagram ( s i m i l a r t o a m o l e c u l a r o r b i t a l energy diagram, b u t w i t h 2



278

T a b l e I I . V a l e n c e I o n i z a t i o n E n e r g i e s o f [Cp]Mn(C0) L 2

Ionization Label


Complex

Complexes Relative Area

Half-Widths,eV

Ionization Energy, eV

W

H

W

L

Ml M2 Cpl Cp2

7. 97 8. 33 9. 84 10. 25

0.65 0.65 0.58 0.58

0.38 0.38 0.33 0.33

1.00 0.58 0.91 0.46

Ml M2 Cpl Cp2

7.,46 7.,82 8..72 9..09

0.55 0.55 0.67 0.67

0.39 0.39 0.25 0.25

1.00 0.58 1.26 0.37

Cp*Mn(C0) (PMe )

Ml M2 Cpl Cp2 PI

6 .60 7 .01 8 .11 8 .51 9 .49

0.63 0.63 0.73 0.73 0.74

0.50 0.30 0.38 0.28 0.30

1.00 0.58 1.40 0.35 0.88

Cp*Mn(C0) (C H )

Ml M2 Cpl Cp2 LI

6 .94 7 .34 8 .48 8 .81 9 .67

0.61 0.61 0.42 0.42 0.71

0.32 0.32 0.28 0.28 0.40

1.00 1.60 1.55 0.88 1.19

Ml M2 Cpl Cp2 LI

6 .40 7 .22 8 .31 8 .67 9 .35

0.53 0.58 0.46 0.46 0.77

0.34 0.53 0.37 0.37 0.46

1.00 1.77 1.62 1.05 3.31

CpMn(C0)

3

Cp*Mn(C0)

3

2

3

2

2

4

Cp*Mn(C0) (C Et ) 2

2

2

Data f o r CpMn(C0) and Cp*Mn(C0) from r e f e r e n c e 25, d a t a f o r Cp*Mn(C0) (PMe ) from r e f e r e n c e 34, d a t a f o r C p * M n ( C 0 ) ( C H ) from r e f e r e n c e 35, and d a t a f o r C p * M n ( C 0 ) ( C E t ) from r e f e r e n c e 36. 3

2

3

3

2

2

2

2

4

2


Table I I I .

Core I o n i z a t i o n s f o r [Cp]Mn(CO) L 2


Complex 3

646.76 538.85 291.76 292.76

1.3 1.6 1.3 1.3

Μη 3 d 0 Is C I s - Cp C I s - CO

646.08 538.37 290.71 292.35

1.3 1.6 1.6 1.3

Μη 3 d 0 Is ' C I s - Cp/PMe C I s - CO Ρ 2ρ

645.36(4) 537.53(4) 290.27(3) 291.67(6) 136.54(4)

1.3 1.6 1.6 1.6 1.8

645.93(4) 528.19(2) 290.67(8) 292.23(8) 289.9(2)

1.5 1.7 1.7 1.7 1.7

645.92(7) 537.97(3) 290.6(1) 292.0(1) 289.6(2)

1.5 1.7 1.8 1.8 1.8

s 2

3

7

CpMn(C0) (PMe ) 2

3

s/ 2

3 / 2

CpMn(C0) (C H ) 2

2

Μη 3 d 0 Is ' C I s - Cp C I s - CO C Is - C H

4

5 / 2

2

CpMn(C0) (C Me ) 2

2

4

Mn 3 d 0 Is C I s - Cp C I s - CO C I s - C Me

2

FWHM.eV

Μη 3 d 0 Is ' C I s - Cp C I s - CO 5 / 2

Cp*Mn(C0)

Complexes

Ionization Potential.eV

Ionization

CpMn(CO)

279


11. LICHTENBERGER ET AL.

6/ 2 7

2

2

3

D a t a f o r CpMn(C0) and Cp*Mn(C0) from r e f e r e n c e 25, d a t a f o r C p M n ( C 0 ) ( P M e ) from r e f e r e n c e 34, d a t a f o r C p M n ( C O ) ( C H ) and C p M n ( C 0 ) ( C M e ) from r e f e r e n c e 36. 3

2

2

3

2

3

2

2

4

2



3

2

0 1s

542

2

3

2

534

2

2

295

5

5

E n e r g y (ev)

5

2

3

3

285 Ρ 2p

140

F i g u r e 5. Core XPS i o n i z a t i o n s f o r (η -C H )Mn(C0) (PMe ). The Μη 2 p / , oxygen I s , c a r b o n I s , and phosphorus 2p r e g i o n s a r e d i s p l a y e d . The c a r b o n I s f e a t u r e i s d e c o n v o l v e d w i t h two peaks because two d i s t i n c t i v e forms o f c a r b o n (CO and Cp/PMe ) a r e o b s e r v e d i n t h i s complex. The phosphorus 2p i o n i z a t i o n i s s p i n orbit s p l i t into a P / and Ρ χ / d o u b l e t .

641

Ionization



eV

141

B\

5

σ -

2

3

CpMn(CO) 3

3

|CpMn(CO)2l 2

PMe

3

2

3

lone pair

fragment

- - - -|3d|

3

CpMn(CO) (PMe )

F i g u r e 6. I o n i z a t i o n c o r r e l a t i o n s f o r CO, CpMn(C0) , C p M n ( C 0 ) ( P M e ) , and PMe . L e v e l s f o r t h e [CpMn(CO) ] are e s t i m a t e d b a s e d on t h e i o n i z a t i o n c o r r e l a t i o n s .

CO


h8

eV

x

00

•δ

ο

Ο

5

H >

1

» Ο w

W

Η W

η


282

e x p e r i m e n t a l i o n i z a t i o n e n e r g i e s ) f o r [CpMn(C0) ] i n t e r a c t i n g w i t h CO and PMe t o form CpMn(C0) and CpMn(C0) (PMe ) . The s p l i t t i n g o f t h e o c c u p i e d m e t a l l e v e l s o f [CpMn(C0) ] i s t h e same as o b s e r v e d f o r t h e phosphine complex s i n c e t h e r e i s no s i g n i f i c a n t phosphine o v e r l a p i n t e r a c t i o n w i t h t h e s e o r b i t a l s . The i o n i z a t i o n energy s h i f t s o f t h e fragment m e t a l l e v e l s due t o t h e charge p o t e n t i a l p r o v i d e d by t h e phosphine l i g a n d i s d i r e c t l y r e f l e c t e d i n t h e i o n i z a t i o n energy o f the l a ' . I n a d d i t i o n t o t h e l a ' i o n i z a t i o n energy, t h e c o r e i o n i z a t i o n d a t a o f T a b l e I I I and t h e v a l e n c e c y c l o p e n t a d i e n y l π o r b i t a l i o n i z a t i o n p o t e n t i a l s a l l e x p e r i m e n t a l l y show t h e charge p o t e n t i a l d i f f e r e n c e between CO and PMe . F o r example, t h e d i f f e r e n c e i n c o r e (Mn 2 p . ) i o n i z a t i o n p o t e n t i a l s between CpMn(C0) and CpMn(CO0 (PMe ) o f -1.40 eV i n d i c a t e s about a 0.10-0.15 e l e c t r o n (22) p o t e n t i a l d i f f e r e n c e a t the metal center. This i s p a r t l y the r e s u l t o f t h e l o s s o f π b a c k b o n d i n g s t a b i l i z a t i o n concomitant w i t h t h e replacement o f CO w i t h PMe , and p a r t l y due t o t h e e l e c t r o n r i c h n e s s (and σ d o n a t i o n ) o f PMe compared t o CO. The π b a c k b o n d i n g d i f f e r e n c e between CO and PMe i s shown by t h e s p l i t t i n g o f t h e p r e d o m i n a n t l y m e t a l v a l e n c e i o n i z a t i o n bands. The h i g h e r i o n i z a t i o n energy band o f t h e phosphine complex c o r r e s p o n d s t o t h e one m e t a l o r b i t a l b a c k b o n d i n g w i t h two c a r b o n y l s , and t h e lower energy band c o r r e s p o n d s t o t h e two m e t a l o r b i t a l s i n t e r a c t i n g w i t h o n l y one c a r b o n y l and t h e phosphine. T h i s s p l i t t i n g i s 0.41 eV, l e s s t h a n o n e - t h i r d o f the core s h i f t . E t h y l e n e p r o v i d e s a d i f f e r e n t s i t u a t i o n because i t h a s o n l y one π a c c e p t o r o r b i t a l . The e t h y l e n e ' s p r e f e r r e d o r i e n t a t i o n has t h i s a c c e p t o r o r b i t a l i n t e r a c t i n g w i t h t h e [CpMn(C0) ] a" o r b i t a l . There 2

3

3

2

3

2

3

3

2


2

3

3

3

3

3

2

i s no a c c e p t o r o r b i t a l on e t h y l e n e t h a t c a n i n t e r a c t w i t h t h e 2a'. Thus o n l y one o f t h e two donor o r b i t a l s i s s t a b i l i z e d b y t h e e t h y l e n e and t h e m e t a l i o n i z a t i o n p r o f i l e i s r e p r e s e n t e d by a 1:2 p a t t e r n o f peaks. The l e f t s i d e o f F i g u r e 7 shows t h i s i n t e r a c t i o n i n an i o n i z a t i o n c o r r e l a t i o n between t h e [CpMn(C0) ] fragment and t h e e t h y l e n e . Note t h a t t h e degeneracy i n t h e second m e t a l i o n i z a t i o n i n d i c a t e s t h a t t h e one e t h y l e n e π a c c e p t o r o r b i t a l i s as e f f e c t i v e as a s i n g l e CO π a c c e p t o r o r b i t a l a t s t a b i l i z i n g a g i v e n metal-based i o n i z a t i o n . A l s o i n t e r e s t i n g i s t h a t t h e s p l i t t i n g between t h e two m e t a l i o n i z a t i o n s i s 0.40 eV, n e a r l y i d e n t i c a l t o t h a t observed i n C p M n ( C 0 ) ( P M e ) . T h i s u n d e r s c o r e s t h e statement t h a t PMe has n e g l i g i b l e π a c c e p t o r c a p a b i l i t y . Core i o n i z a t i o n s show t h a t C H i s i n t e r m e d i a t e between CO and PMe i n terms o f t h e charge p o t e n t i a l a t the m e t a l . The a l k y n e l i g a n d has a π a c c e p t o r o r b i t a l s i m i l a r t o C H t h a t s e r v e s t o s t a b i l i z e t h e l a " . The a l k y n e i s e s p e c i a l l y i n t e r e s t i n g i n c o m p a r i s o n t o c a r b o n y l s and o l e f i n s because o f i t s d i f f e r e n t i n t e r a c t i o n w i t h t h e o t h e r p o t e n t i a l m e t a l π donor o r b i t a l , t h e 2a'. The 2a', r a t h e r t h a n b e i n g s t a b i l i z e d by a π* o r b i t a l as i n t h e case o f c a r b o n y l o r h a v i n g no l i g a n d i n t e r a c t i o n as i n t h e case o f o l e f i n s , now e x p e r i e n c e s an i n t e r a c t i o n w i t h a f i l l e d π o r b i t a l on t h e a l k y n e . 2

2

3

3

2

4

3

2


4

11. LICHTENBERGER ET AL.


283

> V

CM

V

CO cd


^ I

/

CN LU CM O

JD

'

II //

I

//

I

//

1

CM

Ο

O c s a O

Ο •μ

es

Ο) U

CS -Ρ

φ

ι

. to d) rH rH Cd cd S H 4J >—• Ο ·Η ·Η CS · rl τ-! , ΰ rH 4-1 r-l Ο Q) d) cd ο ο C > •P ο α) pu ι-Ι Q) , û g CO U 0) Cd rH ϋ Ό C cd ω >·> •P τι· Φ Ο HN H •P rH Cd ad es •H esÇJ d) TJ Λ i l o) cd ci) •Ρ ο Ρ Ρ rH 5-1 cd β W rH ο φ d) «H •Η S Cd •H rH •Ρ 4J Cd 4-1 cd CO d) ·Ρ O ι—I β ·Η 0) es Ο O rû d) M •Ρ -Ρ r-l β U W >ï o O Ο CS φ rH U Ο rH β rH JCJ O Cd 4-> β Cd 4-> ·Η ο C 4-> d) £ •H cd >*> !J S •Ρ rH JD C cd - C r-l O N /~s Ο - d) ·Η •H CS CO , β 4J 4J CO r-l -Ρ Ο Ο W ·Η ο o cd M es 4J U Ο Ό ρ, ω α) 4J ^ C

Photoelectron Spectroscopy - American Chemical Society

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