Coordinatively Unsaturated Metal Carbonyls in the Gas Phase via

Chapter 5

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 19, 2018 | https://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch005

Coordinatively Unsaturated Metal Carbonyls in the Gas Phase via Time-Resolved Infrared Spectroscopy 1

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Tom Seder , Andrew Ouderkirk , Stephen Church , and Eric Weitz Department of Chemistry, Northwestern University, Evanston, IL 60201 The spectroscopy, reaction kinetics, and photophysics of coordinatively unsaturated metal carbonyls generated in the gas phase via UV photolysis are probed via transient infrared spectroscopy. The parent compounds that have been used to generate coordinatively unsaturated species are Fe(CO) , Cr(CO) and Mn (CO) . In contrast to what is observed in solution phase, photolysis of these compounds produces a variety of coordinatively unsaturated photoproducts. The rate constants for addition of CO to Fe(CO) (x=2,3,4), Cr(CO) (x=2,3,4,5) and Mn (CO) are reported as is the rate constant for the reaction of two Mn(CO) radicals to form Mn (CO) . The reasons for differences in magnitudes of the measured rate constants are discussed in terms of spin conservation and the nature of the reaction: whether i t is an addition reaction or a displacement reaction. Spectra of a l l of the above species have been recorded and absorption peaks are assigned to specific vibrational modes. The spectra are generally compatible with structures for these species deduced from matrix isolation studies of the compounds. Coordinatively unsaturated fragments are observed to be formed with significant internal excitation. This observation and the trend toward an increase in the degree of coordinative unsaturation of the photofragments with increasing photolysis energy allows for the formulation of a proposed mechanism for photodissociation in these compounds. 5

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Current address: Physical Chemistry Department, General Motors Research Laboratories, Warren, MI 48090 Current address: 3M, 3M Center Road, Central Research, Building 208-01-01, Process Technology, St. Paul, MN 55144 Current address: Max Planck Institut für Strahlenchemie, D-4330, Mülheim a.d. Rühr, Federal Republic of Germany

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0097-6156/87/0333-0081 $06.00/0 © 1987 American Chemical Society

Suslick; High-Energy Processes in Organometallic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY

Over the l a s t decade the s p e c t r o s c o p y , p h o t o c h e m i s t r y and r e a c t i v i t y o f m e t a l c a r b o n y l s has been a s u b j e c t o f i n t e n s e i n t e r e s t . As a r e s u l t o f t h i s r e s e a r c h i t has been found t h a t m e t a l c a r b o n y l s undergo a wide range o f f a c i l e p h o t o c h e m i c a l r e a c t i o n s [ 1 , 2 ] . However, the pathways f o r t h e s e r e a c t i o n s , p a r t i c u l a r l y i n the gas phase, have been o n l y ; = r t i a l l y c h a r a c t e r i z e d . I n a wide v a r i e t y o f these r e a c t i o n s , c o o r d i n a t i v e l y unsaturated, h i g h l y r e a c t i v e metal c a r b o n y l s a r e p r o d u c e d [1-18]. The p r o d u c t s o f many o f these p h o t o c h e m i c a l r e a c t i o n s a c t as e f f i c i e n t c a t a l y s t s . F o r example, F e ( C 0 ) 5 can be u s e d t o g e n e r a t e an e f f i c i e n t p h o t o c a t a l y s t f o r a l k e n e i s o m e r i z a t i o n , h y d r o g e n a t i o n , and h y d r o s i l a t i o n r e a c t i o n s [19-23]. T u r n o v e r numbers as h i g h as 3000 have been o b s e r v e d f o r F e ( C 0 ) 5 i n d u c e d p h o t o c a t a l y s i s [22]. However, i n many c a t a l y t i c a l l y a c t i v e systems, the a c t i v e i n t e r m e d i a t e has n o t been d e f i n i t i v e l y determined. Indeed, i t i s o n l y r e c e n t l y t h a t s i g n i f i c a n t p r o g r e s s has b e e n made i n t h i s a r e a [20-23]. Much o f the d i f f i c u l t y i n c h a r a c t e r i z i n g e i t h e r the m e t a l c a r b o n y l p h o t o p r o d u c t s or r e a c t i o n i n t e r m e d i a t e s stems from t h e i r exceedingly high r e a c t i v i t y . F o r example, i t has been shown t h a t Cr(C0>5 c o o r d i n a t e s a h y d r o c a r b o n s o l v e n t w i t h i n a few p i c o s e c o n d s a f t e r i t has been p r o d u c e d [24]. However, c o o r d i n a t i v e l y u n s a t u r a t e d m e t a l c a r b o n y l s have been s p e c t r o s c o p i c a l l y o b s e r v e d i n a v a r i e t y o f e l e g a n t s t u d i e s i n v o l v i n g p h o t o l y s i s i n i n e r t gas m a t r i c e s , l i q u i d s , and h y d r o c a r b o n g l a s s e s [1,2,25-30]. Via p r o l o n g e d p h o t o l y s i s i n m a t r i c e s , i n i t i a l p h o t o p r o d u c t s can be i n d u c e d t o l o s e a d d i t i o n a l l i g a n d s l e a d i n g to the p r o d u c t i o n o f a v a r i e t y of c o o r d i n a t i v e l y unsaturated species. Spectroscopic s t u d i e s o f t h e s e s p e c i e s have been v e r y v a l u a b l e i n d e t e r m i n i n g i n f o r m a t i o n on s t r u c t u r e and b o n d i n g i n t h i s c l a s s o f compounds. However, m a t r i x i s o l a t i o n s t u d i e s have t h e i r l i m i t a t i o n s . Because o f the n a t u r e o f the t e c h n i q u e i t i s d i f f i c u l t to o b t a i n k i n e t i c information. Performing studies i n " l i q u i d matrices", s o l u t i o n s of compounds i n n o b l e gas l i q u i d s , a l l e v i a t e s t h i s p r o b l e m t o some e x t e n t b u t t h i s t e c h n i q u e i s a l s o l i m i t e d i n terms o f the s o l v e n t s and temperature ranges t h a t are a c c e s s i b l e [26]. In a d d i t i o n , m a t r i x s t u d i e s have always r a i s e d the s p e c t r e o f " m a t r i x e f f e c t s " a l t e r i n g the geometry o f m a t r i x i s o l a t e d m o l e c u l e s v e r s u s m o l e c u l e s i n the gas phase or even i n s o l u t i o n . D e s p i t e the c o n s i d e r a b l e amount o f i n f o r m a t i o n t h a t has been g a r n e r e d from more t r a d i t i o n a l methods o f s t u d y i t i s c l e a r l y d e s i r a b l e to be a b l e to g e n e r a t e , s p e c t r o s c o p i c a l l y c h a r a c t e r i z e and f o l l o w the r e a c t i o n k i n e t i c s o f c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s i n r e a l time. Since d e s i r e d timescales f o r r e a c t i o n w i l l t y p i c a l l y be i n the m i c r o s e c o n d to s u b - m i c r o s e c o n d range, a system w i t h a r a p i d time r e s p o n s e w i l l be r e q u i r e d . T r a n s i e n t a b s o r p t i o n systems e m p l o y i n g a v i s i b l e or UV probe which meet t h i s c r i t e r i o n have been d e v e l o p e d and have p r o v i d e d v a l u a b l e i n f o r m a t i o n f o r m e t a l c a r b o n y l systems [14,15,27]. However, s i n c e m e t a l c a r b o n y l s a r e e x t r e m e l y p h o t o l a b i l e and t h e i r U V - v i s i b l e a b s o r p t i o n s p e c t r a a r e n o t v e r y s t r u c t u r e s e n s i t i v e , the p r e f e r r e d c h o i c e f o r a s p e c t r o s c o p i c probe i s time r e s o l v e d i n f r a r e d s p e c t r o s c o p y . Unfortunately, infrared d e t e c t o r s are enormously l e s s s e n s i t i v e and s i g n i f i c a n t l y slower



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t h a n p h o t o t u b e s , thus time r e s o l v e d i n f r a r e d t e c h n i q u e s have h i s t o r i c a l l y been p l a g u e d by a l a c k o f speed and/or s e n s i t i v i t y . These problems can be somewhat overcome by a s t u d y o f r e a c t i o n s i n s o l u t i o n where much g r e a t e r d e n s i t i e s a r e p o s s i b l e t h a n i n the gas phase and f a s t b i m o l e c u l a r r e a c t i o n a r e d i f f u s i o n l i m i t e d [1,28,29]. However, s i n c e c o o r d i n a t i v e l y u n s a t u r a t e d m e t a l c a r b o n y l s have shown a g r e a t a f f i n i t y f o r c o o r d i n a t i n g s o l v e n t we f e l t t h a t the a p p r o p r i a t e p l a c e t o b e g i n a s t u d y o f the s p e c t r o s c o p y and k i n e t i c s o f t h e s e s p e c i e s would be i n a phase where t h e r e i s no s o l v e n t ; the gas phase. I n the gas phase, the o b s e r v e d spectrum i s e x p e c t e d t o be t h a t o f the "naked" c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s and r e a c t i o n s o f t h e s e s p e c i e s w i t h added l i g a n d s a r e a d d i t i o n r e a c t i o n s r a t h e r than displacement r e a c t i o n s . However, s i n c e many o f the s a t u r a t e d m e t a l c a r b o n y l s have l i m i t e d v a p o r p r e s s u r e s , the gas phase p l a c e s a d d i t i o n a l c o n s t r a i n t s on the s e n s i t i v i t y o f the t r a n s i e n t spectroscopy apparatus. N e v e r t h e l e s s , we were a b l e to d e v e l o p a t r a n s i e n t a b s o r p t i o n a p p a r a t u s i n v o l v i n g IR probe r a d i a t i o n t h a t i s s u i t a b l e f o r gas phase s t u d i e s , as have a number o f o t h e r groups e i t h e r c o i n c i d e n t w i t h o r subsequent t o our work [ 1 ] . I n the remainder o f t h i s a r t i c l e we w i l l d i s c u s s the a p p a r a t u s and the r e s u l t s o f our s t u d i e s on t h r e e p r o t o t y p i c a l m e t a l c a r b o n y l s p e c i e s ; F e ( C 0 ) 5 , Cr(CO)g and Mn2(C0)^QThe d i s c u s s i o n i n t h i s a r t i c l e w i l l c e n t e r on the n a t u r e o f the p h o t o l y t i c a l l y g e n e r a t e d c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s , t h e i r k i n e t i c b e h a v i o r and p h o t o p h y s i c a l i n f o r m a t i o n r e g a r d i n g t h e s e species. T h i s l a t t e r i n f o r m a t i o n has e n a b l e d us t o comment on the mechanism f o r p h o t o d i s s o c i a t i o n i n t h e s e systems. S i n c e most o f the r e s u l t s t h a t w i l l be d i s c u s s e d have been p r e s e n t e d e l s e w h e r e [3-10], we w i l l c o n c e n t r a t e on a p r e s e n t a t i o n o f d a t a t h a t i l l u s t r a t e s the most i m p o r t a n t f e a t u r e s t h a t have come out o f our r e s e a r c h and d i r e c t l y r e l a t e d r e s e a r c h r e g a r d i n g the k i n e t i c s , p h o t o p h y s i c s and photochemistry of c o o r d i n a t i v e l y unsaturated metal carbonyls. Experimental The a p p a r a t u s u s e d f o r our t r a n s i e n t a b s o r p t i o n measurements has been d e s c r i b e d i n d e t a i l elsewhere [3-10]. B r i e f l y , the o u t p u t o f an excimer l a s e r o p e r a t i n g on e i t h e r XeF, K r F o r A r F makes a s i n g l e pass t h r o u g h the p h o t o l y s i s c e l l a f t e r b e i n g d i r e c t e d t h r o u g h a c y l i n d r i c a l BaF2 l e n s which produces a more homogeneous beam. The low ( l e s s t h a n 5mj/cm^) energy p u l s e s a r e made t o f i l l the e n t i r e c e l l volume. T h i s procedure i s necessary to a v o i d s p u r i o u s s i g n a l s r e s u l t i n g from b o t h temperature and p h o t o p r o d u c t i n h o m o g e n e i t i e s . The g l a s s p h o t o l y s i s c e l l has a r a d i u s o f .75 cm and an a c t i v e l e n g t h o f 10 cm. The C a F windows o f the c e l l a r e p r o t e c t e d from p h o t o p r o d u c t s by a c u r t a i n o f r a r e gas which f l o w s o v e r the windows and out the exhaust p o r t s w i t h o u t m i x i n g w i t h the sample g a s e s . A f t e r p a s s i n g t h r o u g h computer c o n t r o l l a b l e f l o w c o n t r o l l e r s and a water j a c k e t e d column, which can be u s e d f o r temperature c o n t r o l , t h e sample gases e n t e r the c e l l t h r o u g h a c e n t r a l l y l o c a t e d p o r t and a r e pumped out o f two s y m m e t r i c a l l y l o c a t e d exhaust p o r t s . P h o t o p r o d u c t s a r e o b s e r v e d t o be d e p o s i t e d o n l y i n the r e g i o n between the two exhaust p o r t s . The sample gases a r e f l o w e d a t a r a t e such t h a t they a r e r e p l a c e d between the 1Hz excimer l a s e r 2



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pulses. The sample gas m i x t u r e c o n t a i n s a s m a l l q u a n t i t y o f m e t a l c a r b o n y l p a r e n t (always l e s s t h a n 200 m t o r r and t y p i c a l l y l e s s t h a n 30 m t o r r ) t o w h i c h i s added v a r i a b l e q u a n t i t i e s o f i n e r t gas and/or r e a c t a n t gases. The i n e r t gas i n c r e a s e s the o v e r a l l h e a t c a p a c i t y o f the c e l l w h i c h r e s u l t s i n an a t t e n u a t i o n o f p o t e n t i a l shock waves i n d u c e d v i a the UV p u l s e . I t a l s o a c t s t o d i m i n i s h the r a t e o f d i f f u s i o n o f m a t e r i a l out o f the r e g i o n o f the probe beam and a c t s as a t h i r d body i n r e c o m b i n a t i o n r e a c t i o n s . F o r the l a t t e r purpose, the r a r e gas o r r e a c t a n t gas p r e s s u r e i s always k e p t h i g h enough so t h a t t h i r d o r d e r r e c o m b i n a t i o n r e a c t i o n s a r e i n a pseudo second o r d e r regime. Thus the r a t e c o n s t a n t s we measure do n o t depend on the p r e s s u r e o f added gas and we do n o t see c u r v a t u r e i n our p l o t s o f r a t e o f r e a c t i o n v e r s u s added CO [ 3 - 1 0 ] . The t r a n s i e n t s p e c i e s p r o d u c e d v i a UV p h o t o l y s i s a r e m o n i t o r e d v i a the o u t p u t o f a h o m e - b u i l t , l i q u i d n i t r o g e n c o o l e d , l i n e t u n a b l e , c a r b o n monoxide l a s e r . This l a s e r i s capable of operating on low quantum number v i b r a t i o n a l t r a n s i t i o n s o f CO i n c l u d i n g 1-0. The c.w. i n f r a r e d beam, a f t e r making a double pass t h r o u g h the f l o w c e l l f i l l s the e n t i r e a r e a o f the element o f an i n d i u m a n t i m o n i d e detector. F o r wavelength d e t e r m i n a t i o n , the IR beam i s s p l i t and a p o r t i o n i s p a s s e d t h r o u g h a 0. 5m monochromator e q u i p p e d w i t h a 10 /xm g r a t i n g and c a l i b r a t e d f o r use i n second o r d e r v i a a HeNe l a s e r . To o b t a i n maximum l i n e a r i t y and d e t e c t i v i t y , the p h o t o v o l t a i c i n d i u m a n t i m o n i d e d e t e c t o r was e q u i p p e d w i t h a v a r i a b l e back b i a s i n g c i r c u i t which a l l o w s o p e r a t i o n o f the d e t e c t o r a t the o r i g i n o f the i - v c u r v e o f the d i o d e . The o u t p u t o f the d e t e c t o r i s a m p l i f i e d , f e d t h r o u g h a u n i t y g a i n b u f f e r a m p l i f i e r and u l t i m a t e l y d i g i t i z e d w i t h a B i o m a t i o n 8100 t r a n s i e n t d i g i t i z e r . T y p i c a l l y 64 waveforms a r e a v e r a g e d v i a s i m p l e a d d i t i o n u s i n g a N i c o l e t 1170 s i g n a l a v e r a g e r . The r e s u l t i n g s i g n a l s a r e s t o r e d on a Nova/4 minicomputer which i s i n communication w i t h a H a r r i s super-minicomputer. The H a r r i s computer i s u s e d f o r s i g n a l a n a l y s i s v i a a n o n - l i n e a r l e a s t squares r o u t i n e . The measured r e s p o n s e time o f the d e t e c t o r and a s s o c i a t e d e l e c t r o n i c s i s 35 n s e c . T r a n s i e n t s p e c t r a were c o n s t r u c t e d by r e c o r d i n g t r a n s i e n t s a t d e s i r e d w a v e l e n g t h s , a l l o f which were n o r m a l i z e d by the probe energy, and h a v i n g the computer assemble s p e c t r a by c o n n e c t e d p o i n t s on each t r a n s i e n t a t a common d e l a y time f o l l o w i n g the p h o t o l y s i s pulse. K i n e t i c i n f o r m a t i o n was o b t a i n e d by m o n i t o r i n g t r a n s i e n t s a t the d e s i r e d wavelengths as a f u n c t i o n o f r e a c t a n t gas p r e s s u r e and/or c e l l t e m p e r a t u r e . Unless otherwise s t a t e d a l l experiments were c a r r i e d out a t 21 ± 1° C. The p a r e n t m e t a l c a r b o n y l s were o b t a i n e d from A l p h a C h e m i c a l s at s t a t e d p u r i t i e s o f >98%, 98%, and 95% f o r Fe, Cr and Mn, respectively. I n a l l c a s e s v o l a t i l e i m p u r i t i e s were removed b e f o r e use. CO and r a r e gases were o b t a i n e d from Matheson a t s t a t e d p u r i t i e s o f >99.99+% and were u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . Results F i g u r e 1 d e p i c t s the time r e s o l v e d spectrum g e n e r a t e d by p h o t o l y s i s of 30mtorr o f Fe(C0)5 w i t h a K r F excimer l a s e r . As w i t h a l l t r a n s i e n t a b s o r p t i o n e x p e r i m e n t s a major p o t e n t i a l p r o b l e m i n v o l v e s a s s i g n i n g the o b s e r v e d a b s o r p t i o n s t o s p e c i f i c s p e c i e s . F o r the



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SEDER ETAL.

85


Fe(C0)5 system we were s i g n i f i c a n t l y a i d e d by p r e v i o u s m a t r i x work on t h i s system w h i c h a s s i g n e d the i n f r a r e d a b s o r p t i o n s o f F e ( C 0 ) 4 and Fe(C0)3 and by c h e m i c a l t r a p p i n g s t u d i e s , w h i c h p r o v i d e d us w i t h i n f o r m a t i o n on which p h o t o f r a g m e n t s a r e p r o d u c e d a t v a r i o u s p h o t o l y s i s w a v e l e n g t h s [17,30,31]. However, i t i s u n d e s i r a b l e t o have t o r e l y on the p r i o r e x i s t e n c e o f m a t r i x and/or c h e m i c a l t r a p p i n g d a t a f o r assignments o f a b s o r p t i o n s . Thus we d e v e l o p e d a r a t h e r s t r a i g h t f o r w a r d p r o c e d u r e which we r e f e r to as a " k i n e t i c b o o t s t r a p " p r o c e d u r e which a l l o w s us to determine the n a t u r e of s p e c i f i c a b s o r p t i o n bands. T h i s procedure i s i l l u s t r a t e d i n f i g u r e 2 which shows the same s p e c t r a l r e g i o n as f i g u r e 1 b u t f o r K r F l a s e r p h o t o l y s i s o f Fe(C0)5 i n the p r e s e n c e o f a l a r g e e x c e s s o f CO. On t h i s time s c a l e the added CO has a l r e a d y r e a c t e d w i t h lower fragments t o r e g e n e r a t e Fe(C0)4, which can be o b s e r v e d t o f u r t h e r r e a c t w i t h CO on the t i m e s c a l e t h a t i s d e p i c t e d , t o g e n e r a t e Fe(C0)5 [8]. T h i s " k i n e t i c b o o t s t r a p " p r o c e d u r e w i l l be f u r t h e r i l l u s t r a t e d w i t h i t s a p p l i c a t i o n t o the Cr(C0)£ and Mn2(C0)^Q systems ( v i d e infra). U s i n g e x i s t i n g m a t r i x d a t a , c h e m i c a l t r a p p i n g d a t a and our " k i n e t i c b o o t s t r a p " p r o c e d u r e , we have been a b l e t o a s s i g n the a b s o r p t i o n d e s i g n a t e d a,b,c i n f i g u r e 1 t o F e ( C 0 ) where x = 4,3,2 r e s p e c t i v e l y . F e a t u r e d i s due t o d e p l e t i o n o f p a r e n t and f e a t u r e e, as w i l l be d i s c u s s e d i n more d e t a i l , i s due t o v i b r a t i o n a l l y e x c i t e d CO formed i n the p h o t o d i s s o c i a t i o n p r o c e s s [ 3 , 8 ] . x

Once s p e c i f i c a b s o r p t i o n f e a t u r e s a r e a s s i g n e d , k i n e t i c s t u d i e s can be p e r f o r m e d v i a t u n i n g the probe l a s e r t o a f r e q u e n c y a b s o r b e d by the fragment whose r e a c t i o n k i n e t i c s a r e o f i n t e r e s t . Ideally, i t i s a l s o d e s i r a b l e t o measure the r a t e o f f o r m a t i o n o f the r e a c t i o n p r o d u c t and t o v e r i f y t h a t t h e s e two r a t e s c o r r e l a t e w i t h each o t h e r . T h i s has been done f o r the F e ( C 0 ) system w i t h added CO where the r e a c t i o n can be s c h e m a t i c a l l y d e p i c t e d as x

Fe(C0)

x

+ CO

>

Fe(C0)

x + 1

(1)

A t y p i c a l example o f the d a t a i s shown i n f i g u r e 3 where the r a t e o f r e a c t i o n o f Fe(CO)3 w i t h added CO i s d e p i c t e d . Data i s p r e s e n t e d f o r b o t h the l o s s o f Fe(CO)3 and the r e g e n e r a t i o n o f Fe(C0)4. Data f o r r e a c t i o n o f Fe(CO)3 and the o t h e r F e ( C 0 ) s p e c i e s i s p r e s e n t e d i n t a b l e I [3,4,8]. S i m i l a r d a t a have been o b t a i n e d f o r the C r ( C 0 ) g system and a r e a l s o p r e s e n t e d i n t a b l e I [5,9,12,13]. Time r e s o l v e d s p e c t r a f o r t h i s system a r e shown i n f i g u r e 4. S p e c t r a a r e p r e s e n t e d f o r XeF, K r F and A r F p h o t o l y s i s . T h i s system a f f o r d s an e x c e l l e n t example o f the a p p l i c a t i o n o f our k i n e t i c p r o c e d u r e f o r a s s i g n i n g a b s o r p t i o n s to s p e c i f i c c o o r d i n a t i v e l y u n s a t u r a t e d p h o t o f r a g m e n t s . Observation o f f i g u r e 4 c l e a r l y i n d i c a t e s t h a t as p h o t o l y s i s energy i s i n c r e a s e d a d d i t i o n a l a b s o r p t i o n bands appear, p r i m a r i l y a t lower f r e q u e n c y t h a n t h o s e o b s e r v e d on lower energy p h o t o l y s i s . Both matrix data and c h e m i c a l t r a p p i n g d a t a s t r o n g l y imply t h a t the p r i m a r y s p e c i e s p r o d u c e d on XeF l a s e r p h o t o l y s i s i s C r ( C 0 ) 5 [16,32]. Figure 5 i l l u s t r a t e s how o t h e r a b s o r p t i o n s can be a s s i g n e d v i a our k i n e t i c procedure. T h i s f i g u r e d e p i c t s the change i n the a b s o r p t i o n spectrum on a d d i t i o n o f CO t o a sample o f Cr(CO)g t h a t has been p h o t o l y z e d w i t h a K r F excimer l a s e r . From t h i s f i g u r e i t can be x


86


— X e F


KrF

2060 "

2022

1984

1946

1908

Wavenumbers

F i g u r e 1. T r a n s i e n t IR s p e c t r a f o l l o w i n g p h o t o l y s i s o f Fe(C0>5 w i t h XeF and K r F l a s e r r a d i a t i o n . T r a c e s a r e t a k e n ~1 /xsec a f t e r photolysis. I n a d d i t i o n t o Fe(C0>5 (30 m t o r r f o r K r F , 200 m t o r r f o r XeF) t h e p h o t o l y s i s c e l l c o n t a i n e d 5 t o r r A r . The symbols are d e f i n e d i n the t e x t .

F i g u r e 2. T r a n s i e n t IR s p e c t r a f o l l o w i n g K r F p h o t o l y s i s o f 30 m t o r r o f F e ( C 0 ) + 100 t o r r CO. The spectrum (-2060-1920 cm" ) i s d e p i c t e d o v e r a 5 /is time range which h a s been segmented i n t o 10 e q u a l time i n t e r v a l s , t h e f i r s t t h r e e o f which a r e d e s i g n a t e d . The arrows i n d i c a t e t h e now p a r t i a l l y r e s o l v e d A^ and modes o f Fe(C0)4. (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 8. C o p y r i g h t 1986 A m e r i c a n I n s t i t u t e o f P h y s i c s . ) 1

5


5.

87


SEDER ET AL.

7


a

J*

C O P R E S S U R E (Torr)

F i g u r e 3. P l o t o f t h e pseudo f i r s t o r d e r r a t e f o r r e a c t i o n o f Fe(C0>3 d i s a p p e a r a n c e o f Fe(C0)3 a t 1954 cm" (a) and t h e r a t e o f appearance o f Fe(C0>4 a t 1984 cm" (•) a r e p l o t t e d a g a i n s t p r e s s u r e o f added CO. (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 8. C o p y r i g h t 1986 A m e r i c a n I n s t i t u t e of Physics.) w

i

t

h

c o

T

h

e

r

a

t

e

o

f

1

1

CO

a

U

H

2050

2000 1950 1900 Wavenumbers

1850

F i g u r e 4. The t r a n s i e n t a b s o r p t i o n spectrum o f C r ( C 0 ) g 0.5 msec f o l l o w i n g a) XeF, b) K r F and c) A r F p h o t o l y s i s o f gas phase Cr(C0)5. The arrows i n a) i n d i c a t e t h e C r ( C 0 ) 5 a b s o r p t i o n s . (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 9. C o p y r i g h t 1986 American Chemical S o c i e t y . )


H I G H - E N E R G Y P R O C E S S E S IN O R G A N O M E T A L L I C C H E M I S T R Y

88

Table I Summary o f t h e B i m o l e c u l a r Rate C o n s t a n t s Recombination Reactions Spin Allowed

f o r M(CO) -CO x

Gas Phase Rate C o n s t a n t (10" cnAnol-T-s" ) 1 3


Cr(CO)

5

Cr(C0) Cr(CO)

4

3

Fe(C0)

4

+ CO - C r ( C 0 )

6

Y

1..5, >

+ CO - C r ( C 0 )

5

Y

2,.4, >

+ CO

Cr(C0)

+ CO - F e ( C O )

4

5

Fe(CO)

3

+ CO -> F e ( C 0 )

Fe(CO)

2

+ CO -+ F e ( C O )

4

3

1

a

e

2.2,

D

a

e

2.6, 14.0°

b

e

Y

1..8

N

0,.003

Y

1,.2

d

Y

1 .7

d

d

aRef. 5. R e f . 12. R e f . 13. R e f s . 3, 8. R e f . 9.

b

c

d

e

seen t h a t the i n i t i a l a b s o r p t i o n spectrum on K r F l a s e r p h o t o l y s i s e v o l v e s i n t o a spectrum v e r y s i m i l a r t o t h a t on XeF l a s e r photolysis. T h i s spectrum c o n t i n u e s t o e v o l v e w i t h t h e C r ( C 0 ) 5 a b s o r p t i o n s d i s a p p e a r i n g and C r ( C 0 ) g b e i n g r e g e n e r a t e d [ 9 ] . Measurements o f t h e r a t e o f change o f t h e a b s o r p t i o n o f d i f f e r e n t peaks i n t h e K r F and XeF s p e c t r a f u r t h e r i n d i c a t e s t h a t t h e major peaks i n each s p e c t r a a l l e v o l v e a t t h e same r a t e . This indicates t h a t t h e r e i s p r i m a r i l y one c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s p r o d u c e d a t each o f t h e s e wavelengths. S i n c e t h e r a t e o f l o s s o f the s p e c i e s p r o d u c e d on K r F l a s e r p h o t o l y s i s i s t h e same as the r a t e o f p r o d u c t i o n o f C r ( C 0 ) 5 and C r ( C 0 ) 5 i s p r o d u c e d w i t h o u t an i n d u c t i o n time, t h i s s t r o n g l y i m p l i e s t h a t by f a r t h e major p r o d u c t o f K r F laser photolysis i s Cr(C0) . This i s c o n s i s t e n t with chemical t r a p p i n g data [16]. I n a d d i t i o n , the f a c t t h a t the l o s s o f Cr(C0)5 on r e a c t i o n w i t h CO l e a d s t o r e g e n e r a t i o n o f Cr(C0)£ a t the same r a t e as C r ( C 0 ) 5 i s l o s t i s f u r t h e r c o n f i r m a t i o n o f o u r a s s i g n m e n t s . A s i m i l a r p r o c e d u r e a l l o w s us t o a s s i g n t h e a d d i t i o n a l peaks p r o d u c e d on A r F p h o t o l y s i s as b e i n g due t o Cr(C0>3 and Cr(C0>2 [ 9 ] . The f r e q u e n c i e s o f t h e gas phase a b s o r p t i o n s f o r t h e s e c o o r d i n a t i v e l y u n s a t u r a t e d fragments i s p r e s e n t e d i n t a b l e I I . Data f o r t h e r a t e s o f r e a c t i o n o f each o f t h e s e s p e c i e s w i t h CO i s summarized i n table I. 4

The time r e s o l v e d s p e c t r a p r o d u c e d on excimer l a s e r p h o t o l y s i s o f Mn2(CO)io a r e shown i n f i g u r e 6. Note t h a t as i n t h e case o f i r o n p e n t a c a r b o n y l and chromium h e x a c a r b o n y l p h o t o l y s i s , t h e r e i s a d i s t i n c t i n c r e a s e i n the a m p l i t u d e o f the lower f r e q u e n c y a b s o r p t i o n bands as t h e p h o t o l y s i s energy i n c r e a s e s . By comparison w i t h t h e f r e q u e n c y o f m a t r i x i s o l a t e d and s o l u t i o n phase Mn(C0)5, the band a t -1996 cm" i s a s s i g n e d t o t h e gas phase Mn(C0)5 r a d i c a l [ 3 3 ] . This 1



5.

SEDER ET AL.


2050

2000

1950

1900

89

1850

Wavenumbers

F i g u r e 5. T r a n s i e n t time r e s o l v e d spectrum f o l l o w i n g K r F p h o t o l y s i s o f C r ( C 0 ) g w i t h 5.0 t o r r A r and 0.5 t o r r CO. The spectrum i s d i s p l a y e d o v e r a 10 fis range which i s segmented i n t o 10 e q u a l time i n t e r v a l s . The f i r s t 3 i n t e r v a l s a r e l a b e l l e d . (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 9. C o p y r i g h t 1986 American Chemical S o c i e t y . ) (*i(C0)j In C II 6

2048

2006

12

1964

1922 ' 1890

Wavenumbers F i g u r e 6. T r a n s i e n t time r e s o l v e d s p e c t r a f o l l o w i n g excimer l a s e r p h o t o l y s i s o f Mn2(CO)xop o s i t i o n o f t h e Mn(C0>5 a b s o r p t i o n i n C g H ^ s o l u t i o n i s i n d i c a t e d by an arrow. The i n s e t a t t h e bottom i s an e x t e n s i o n o f t h e A r F spectrum. T

n

e



90

Table II I n f r a r e d A b s o r p t i o n s o f M a t r i x I s o l a t e d and Fe(C0) + Cr(C0) x

-


1995

4

a

Assignment

2000

1988

Fe(C0)

3

B

-2042

(b)

Al

1950

E

1957

Bl

1945

Al

3

d

Fe(C0)

2

1905

Cr(C0)

5

2093.4

Cr(C0)

4

3

Cr(C0)

2

c

2v

31

c

3v

31

c

2v

c

4v

2

1920 Al

1965.4

1980

E

1936.1

1948

Al

1938

C

Bl

1932

1957

Al

1891

1920

B

1867

1880

E

1903

Ref.

Bl 1985

1935.6

Cr(C0)

Symmetry

Al

1973

c

Fe(C0)

Phase

1

F r e q u e n c y (cm - -) Ar M a t r i x Gas P h a s e

Fe(C0)

Gas

x

e

2 v

39

40

2

C3v

32c 32c

1914

(a) A p p r o x i m a t e v a l u e s from r e f e r e n c e s 8 and 9. (b) Not o b s e r v e d due t o o v e r l a p w i t h "hot" CO a b s o r p t i o n s see text. (c) T e n t a t i v e assignment o f the bands o b s e r v e d upon A r F l a s e r p h o t o l y s i s t o the e x c i t e d s i n g l e t s t a t e F e ( C 0 ) . See r e f e r e n c e 8. (d) T e n t a t i v e l y a s s i g n e d as F e ( C 0 ) i n r e f e r e n c e 32c. (e) C H matrix. 3

2

4


5.

Coordinaîively

SEDER ET A L .

Unsaturated Metal Carbonyb

91

a s s i g n m e n t i s f u r t h e r c o n f i r m e d by k i n e t i c s t u d i e s . Observation of the r a t e o f decay o f t h i s band i n d i c a t e s t h a t i t s decay i s s e c o n d o r d e r (see f i g u r e 7 ) . From t h i s p l o t , I / I measurements, and an e s t i m a t e d v a l u e f o r ε, the Mn(C0)5 a b s o r p t i o n c o e f f i c i e n t , b a s e d on the s t r e n g t h o f the CO a b s o r p t i o n bands i n Mn(C0)5Cl [33], a v a l u e f o r the r a t e c o n s t a n t f o r e q u a t i o n (2) Q

Mn(CO)

5

+ Mn(C0>5

>

Mn (CO) 2

(2)

1 0


1 0

1

has been d e t e r m i n e d to be (2.7 ± 0.6) χ 1 0 i mole'^-s" [6,10]. E r r o r l i m i t s r e f e r to e x p e r i m e n t a l u n c e r t a i n t i e s and do not i n c l u d e u n c e r t a i n t i e s i n the c h o i c e o f ε. F i g u r e 8 d i s p l a y s the t r a n s i e n t a b s o r p t i o n s p e c t r u m f o r Fe(C0)5 p h o t o l y z e d by a K r F l a s e r on a s h o r t e r t i m e s c a l e t h a n t h a t d i s p l a y e d i n f i g u r e 1. Note t h a t on t h i s t i m e s c a l e the a b s o r p t i o n s a s c r i b e d t o the F e ( C 0 ) f e a t u r e s do not a l l d e v e l o p a t the same r a t e [ 3 , 8 ] . The a b s o r p t i o n band a s s i g n e d to Fe(C0>2 c l e a r l y a p p e a r s more r a p i d l y t h a n the a b s o r p t i o n band f o r Fe(C0)3 w h i c h i n t u r n a p p e a r s more r a p i d l y t h a n the a b s o r p t i o n f e a t u r e f o r F e ( C 0 ) 4 . The more r a p i d appearance o f a b s o r p t i o n bands b e l o n g i n g to the more h i g h l y c o o r d i n a t i v e l y u n s a t u r a t e d photofragments i s a g e n e r a l f e a t u r e o b s e r v e d i n a l l the p h o t o l y s i s e x p e r i m e n t s we have c o n d u c t e d to date. I t s r a m i f i c a t i o n s i n terms o f the p h o t o p h y s i c s o f the p r o c e s s e s we have s t u d i e d w i l l be d i s c u s s e d i n more d e t a i l i n the next s e c t i o n . I t i s i n t e r e s t i n g to n o t e t h a t the f a c t t h a t more h i g h l y c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s appear more p r o m p t l y t h a n l e s s c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s and w i t h l e s s s h i f t i n g o f the a b s o r p t i o n l i n e to h i g h e r energy can be u s e d as a f u r t h e r a i d i n the a s s i g n m e n t o f a b s o r p t i o n bands to s p e c i f i c s p e c i e s [ 9 ] . x

Discussion Kinetics. I n s p e c t i n g t a b l e I, i t can be s e e n t h a t the r a t e o f r e a c t i o n f o r CO a d d i t i o n to a l l F e ( C 0 ) and C r ( C 0 ) s p e c i e s i s o f the same o r d e r o f magnitude e x c e p t f o r the r a t e o f r e a c t i o n o f Fe(C0>4 w i t h CO. Why i s t h i s r e a c t i o n d i f f e r e n t t h a n a l l the o t h e r r e a c t i o n s ? The answer to t h i s q u e s t i o n can be f o u n d i n s t u d i e s o f the e l e c t r o n i c s t r u c t u r e o f F e ( C 0 ) 4 . The ground s t a t e o f F e ( C 0 ) 4 i s a t r i p l e t whereas the ground s t a t e o f Fe(C0>5 i s a s i n g l e t [34,35]. Thus the a d d i t i o n r e a c t i o n o f CO to Fe(C0>4 i s s p i n f o r b i d d e n . This has f u r t h e r i m p l i c a t i o n s f o r the F e ( C 0 ) system. If a spin forbid den r e a c t i o n i s e x p e c t e d t o be s i g n i f i c a n t l y s l o w e r t h a n a s p i n c o n s e r v i n g r e a c t i o n , t h e n the ground s t a t e s o f F e ( C 0 ) 3 and Fe(C0>2 are a l s o t r i p l e t s . T h i s p r e d i c t i o n has been p r e v i o u s l y made f o r F e ( C 0 ) 3 and we have p o s t u l a t e d , b a s e d on our k i n e t i c d a t a , t h a t F e ( C 0 ) 2 has a t r i p l e t ground s t a t e [8], Note t h a t i n the C r ( C 0 ) system, the r a t e s o f r e a c t i o n o f a l l the c o o r d i n a t i v e l y u n s a t u r a t e d fragments w i t h CO are v e r y s i m i l a r . T h i s i s i n a c c o r d w i t h what i s known about the e l e c t r o n i c s t r u c t u r e o f the C r ( C 0 ) system: the ground s t a t e o f each o f the c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s are e x p e c t e d to be s i n g l e t s t a t e s [ 9 ] . One c o u l d ask f u r t h e r whether the r e l a t i v e l y slow r a t e o f r e a c t i o n o f Fe(C0>4 w i t h CO m a n i f e s t s i t s e l f i n the a c t i v a t i o n energy or p r e e x p o n e n t i a l . To answer t h i s q u e s t i o n , we have x

x

x

x

x


92


100 80

_o

60


- 40

20

0.4

0.8

Time (msec.)

F i g u r e 7. The r a t e o f l o s s o f Mn(C0>5 i s p l o t t e d as a s e c o n d o r d e r decay. The d a t a were o b t a i n e d a t 2004.3 c m . The i n s e t i s a t r a n s i e n t waveform a t t h i s f r e q u e n c y w h i c h c o v e r s a 2 ms time range. - 1

I

2050

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I

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, I

2031

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I

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2012

I

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,11

1993

111

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1974

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ι

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1955

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ι

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WAVENUMBERS F i g u r e 8. T r a n s i e n t a b s o r p t i o n spectrum r e s u l t i n g from K r F p h o t o l y s i s o f 20 m t o r r o f Fe(C0>5 i n 5 t o r r A r . (Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 3. C o p y r i g h t 1986 A m e r i c a n I n s t i t u t e of Physics.)


5.

SEDER ET AL.

93


p e r f o r m e d t e m p e r a t u r e dependent s t u d i e s o f the r e a c t i o n s i n the F e ( C O ) system o v e r the l i m i t e d temperature range o f 10-55°C. W i t h i n e x p e r i m e n t a l e r r o r , over t h i s temperature range, we do n o t see any change i n the r a t e o f r e a c t i o n o f any o f the F e ( C O ) s p e c i e s w i t h CO [8]. T h i s i m p l i e s t h a t the a c t i v a t i o n energy f o r each r e a c t i o n i s 4 on r e a c t i o n w i t h CO i s n o t v a s t l y d i f f e r e n t t h a n the change i n geometry t h a t o c c u r s on r e a c t i o n o f Cr(C0>5 w i t h CO, i t i s u n l i k e l y t h a t t h i s type o f geometry change i s a major f a c t o r i n the slowness o f the r e a c t i o n o f Fe(C0)4 w i t h CO. Thus the e f f e c t o f the change i n s p i n m a n i f e s t s i t s e l f i n the preexponential. A f u r t h e r i n t e r e s t i n g d i f f e r e n c e i n k i n e t i c b e h a v i o r can be o b s e r v e d i n the Mn2(CO)io system. For t h i s system the r a t e c o n s t a n t f o r the r e a c t i o n o f Mn2(CO)9 w i t h CO has been measured as (2.4 ± 0.8)x 10^ 1 m o l e " I s " I w h i c h i s v e r y s i m i l a r t o the r a t e c o n s t a n t f o r t h i s r e a c t i o n measured i n s o l u t i o n [6,10,33a]. T h i s i s an a d d i t i o n a l o r d e r o f magnitude s l o w e r t h a n even the r e a c t i o n o f Fe(C0)4 w i t h CO. Why i s t h i s ? Mn2(C0)o. has been s t u d i e d i n the m a t r i x and has been f o u n d t o have a s t r u c t u r e w i t h a b r i d g i n g CO l o c a t e d between the two Mn m e t a l c e n t e r s . However, s i n c e the CO i s n o t symmetrically l o c a t e d r e l a t i v e to the two m e t a l c e n t e r s , i t has been d e s i g n a t e d as a s e m i - b r i d g i n g " CO group [33a] . S i n c e t h i s CO s h a r e s e l e c t r o n d e n s i t y between the two m e t a l c e n t e r s , f o r m a l l y each Mn atom can a t t a i n an 18 e l e c t r o n c o u n t . Thus the r e a c t i o n o f CO w i t h Mn2(CO)9 c o u l d b e t t e r be v i e w e d as a d i s p l a c e m e n t r e a c t i o n r a t h e r t h a n an a d d i t i o n r e a c t i o n o f CO to a c o o r d i n a t i v e l y u n s a t u r a t e d compound. Viewed i n t h i s l i g h t i t i s n o t s u r p r i s i n g t h a t t h i s r e a c t i o n i s much s l o w e r t h a n a d d i t i o n r e a c t i o n s t o c o o r d i n a t i v e l y u n s a t u r a t e d compounds. We have a l s o measured the r a t e c o n s t a n t f o r the a s s o c i a t i o n r e a c t i o n o f two Mn(C0)5 r a d i c a l s g e n e r a t e d on p h o t o l y s i s o f Mn2(CO)ioW i t h a p p r o p r i a t e assumptions r e g a r d i n g the absorption c o e f f i c i e n t f o r Mn(C0)5, the r a t e c o n s t a n t f o r t h i s r e a c t i o n was d e t e r m i n e d t o be (2.7 ± 0.6) χ 1 0 1 mole" s " [6,10]. This i s c o m p a t i b l e w i t h the d i f f u s i o n l i m i t e d r a t e c o n s t a n t f o r t h i s r e a c t i o n t h a t has been measured i n s o l u t i o n and i s w i t h i n an o r d e r o f magnitude o f a gas k i n e t i c r a t e c o n s t a n t as would be e x p e c t e d f o r an e s s e n t i a l l y u n a c t i v a t e d r a d i c a l - r a d i c a l a s s o c i a t i o n r e a c t i o n [33a]. x


x

11

1 0

1

1

P h o t o p h y s i c s and P h o t o c h e m i s t r y . F i g u r e 8 i l l u s t r a t e s what has been f o u n d t o be a g e n e r a l f e a t u r e i n the p r o d u c t i o n o f c o o r d i n a t i v e l y u n s a t u r a t e d m e t a l c a r b o n y l s i n the gas phase : t h e y are n o r m a l l y formed w i t h i n t e r n a l e x c i t a t i o n . T y p i c a l l y , s p e c i e s t h a t l o s e the l e a s t l i g a n d s are formed w i t h most i n t e r n a l e x c i t a t i o n . As the i n t e r n a l e x c i t a t i o n r e l a x e s the a b s o r p t i o n s narrow and s h i f t toward higher frequency. T y p i c a l l y , the a b s o r p t i o n s o f the s p e c i e s formed w i t h the l a r g e s t amount o f i n t e r n a l e x c i t a t i o n w i l l take the l o n g e s t time t o a p p r o a c h t h e i r f i n a l p o s i t i o n and shape. This occurs since the most h i g h l y e x c i t e d s p e c i e s must l o s e the most i n t e r n a l energy v i a c o l l i s i o n a l r e l a x a t i o n processes. T h i s b e h a v i o r has been o b s e r v e d i n a l l m e t a l c a r b o n y l systems s t u d i e d t o d a t e w i t h an



94


i n c r e a s i n g degree o f e x c i t a t i o n o b s e r v e d i n a g i v e n photofragment as the energy o f the p h o t o l y s i s photon i n c r e a s e s . This observation i s compatible with other s t u d i e s i n v o l v i n g p h o t o d i s s o c i a t i o n of metal c a r b o n y l s [11,36]. A n o t h e r i n t e r e s t i n g o b s e r v a t i o n r e g a r d i n g the d i s s o c i a t i o n p r o c e s s i s a g e n e r a l i n c r e a s e i n the degree o f u n s a t u r a t i o n o f the p h o t o p r o d u c t s as a f u n c t i o n o f the energy o f the i n p u t photon. This b e h a v i o r i s a p p a r e n t i n e i t h e r the i r o n o r chromium system where the b r a n c h i n g r a t i o s f o r p h o t o p r o d u c t s changes d r a m a t i c a l l y w i t h i n p u t energy. T h i s i s a l s o o b s e r v e d i n the Mn2(CO)io system [10] . As shown i n f i g u r e 1 , f o r the i r o n system, a l m o s t e x c l u s i v e l y Fe(C0)3 i s formed f o r XeF l a s e r p h o t o l y s i s w h i l e f o r K r F l a s e r p h o t o l y s i s the p r o d u c t mix s h i f t s toward Fe(C0)2For ArF l a s e r p h o t o l y s i s a l m o s t e x c l u s i v e l y Fe(C0)2 i s p r o d u c e d ( n o t shown) [ 8 ] . S i m i l a r b e h a v i o r i s o b s e r v e d i n the chromium system w i t h XeF p h o t o l y s i s producing predominantly Cr(C0)5, KrF p h o t o l y s i s predominantly C r ( C 0 ) 4 and A r F p h o t o l y s i s a mix o f p r o d u c t s i n c l u d i n g C r ( C 0 ) 3 and C r ( C 0 ) 2 (see f i g u r e 4 ) . The b e h a v i o r d e s c r i b e d i n the two p r e c e d i n g p a r a g r a p h s i s q u a l i t a t i v e l y c o m p a t i b l e w i t h a s t r a i g h t f o r w a r d mechanism f o r p h o t o d i s s o c i a t i o n which a l s o r e c o n c i l e s o b s e r v e d d i f f e r e n c e s i n p r o d u c t d i s t r i b u t i o n s i n the gas phase v e r s u s condensed phases. The i n i t i a l l y a b s o r b e d photon i n i t i a t e s a p h o t o c h e m i c a l e v e n t which r e s u l t s i n l o s s o f a CO l i g a n d and the p r o d u c t i o n o f a photofragment which i s h i g h l y i n t e r n a l l y e x c i t e d . This e x c i t e d molecule i s r a p i d l y r e l a x e d i n condensed phase due t o the h i g h d e n s i t y o f surrounding c o l l i s i o n partners. Thus the n e t r e s u l t o f p h o t o l y s i s i n t h e s e and r e l a t e d systems i n condensed phase i s l o s s o f one ligand. However, i n the gas phase a d d i t i o n a l p r o c e s s e s can o c c u r . The e n e r g i z e d photofragment can go on t o f u r t h e r d i s s o c i a t e i n an RRKM l i k e p r o c e s s l e a d i n g t o m u l t i p l e p r o d u c t s [ 1 7 ] . Dissociative s t e p s a r e t e r m i n a t e d when the e x c i t e d m o l e c u l e can be c o l l i s i o n a l l y s t a b i l i z e d on the t i m e s c a l e o f the n e x t p o s s i b l e d i s s o c i a t i v e e v e n t . We a r e c u r r e n t l y w o r k i n g on f u r t h e r v e r i f y i n g t h i s h y p o t h e s i z e d p h o t o l y s i s mechanism by c a l c u l a t i o n s and a d d i t i o n a l e x p e r i m e n t s . T h i s d e s c r i p t i o n must o b v i o u s l y be m o d i f i e d when t h e r e i s the p o s s i b i l i t y f o r m u l t i p l e i n i t i a l photochemical events. Multiple i n i t i a l p h o t o c h e m i c a l e v e n t s c o u l d o c c u r i n the Fe(C0)5 and Cr(C0)£ systems due t o o v e r l a p p i n g e l e c t r o n s t a t e s and have been shown t o o c c u r i n Mn2(CO)io, where e i t h e r h o m o l y t i c bond c l e a v a g e or d e c a r b o n y l a t i o n o c c u r s i n p h o t o l y s i s [36-38]. F o r Mn2(C0)^Q» n a t u r e o f the i n i t i a l l y e x c i t e d e l e c t r o n i c s t a t e , which can v a r y w i t h wavelength, has now been c o n v i n c i n g l y shown t o i n f l u e n c e the b r a n c h i n g r a t i o f o r t h e s e two p a t h s [ 3 7 ] . T h i s l e a d s t o a w a v e l e n g t h dependence f o r p h o t o l y s i s even i n condensed phase. However, the g e n e r a l p r i n c i p l e s o f the a f o r e m e n t i o n e d d i s s o c i a t i o n mechanism can s t i l l v a l i d once the i n i t i a l p h o t o c h e m i c a l event o c c u r s . F o r the Μη2(00)^ο system i n the gas phase a change i n the r a t i o o f the two i n i t i a l p h o t o p r o d u c t s a r e o b s e r v e d as the energy o f the i n p u t photon i n c r e a s e s i n g o i n g from XeF to K r F t o A r F . Pre l i m i n a r y s t u d i e s i n d i c a t e t h a t these d i s s o c i a t i o n products are dominated by d i s s o c i a t i o n o f Mn2(CO)9 r a t h e r t h a n f u r t h e r d i s s o c i a t i o n o f Mn(C0)5. T h i s r e s u l t i s c o m p a t i b l e w i t h the o b s e r v a t i o n t


n

e


5.

SEDER ET A L .


95

o r i g i n a l l y made by V a i d a t h a t t h e Mn-Mn bond s t r e n g t h i n c r e a s e s w i t h l o s s o f a d d i t i o n a l CO l i g a n d s [ 3 8 ] . A n o t h e r i n t e r e s t i n g a s p e c t o f t h e p h o t o p h y s i c s o f t h i s system i s r e v e a l e d by i n s p e c t i o n o f t h e h i g h energy r e g i o n o f f i g u r e s 1 and 4. I n these regions there are p o s i t i v e going a b s o r p t i o n s ( l a b e l e d e i n f i g u r e 1) w h i c h a r e due t o v i b r a t i o n a l l y and/or r o t a t i o n a l l y e x c i t e d CO. Thus UV p h o t o l y s i s n o t o n l y p r o d u c e s i n t e r n a l l y e x c i t e d c o o r d i n a t i v e l y u n s a t u r a t e d metal c a r b o n y l s b u t i t a l s o produces i n t e r n a l l y e x c i t e d CO as t h e o t h e r p h o t o p r o d u c t . F u r t h e r m o r e , as w i t h t h e c o o r d i n a t i v e l y u n s a t u r a t e d fragment, t h e i n t e r n a l energy o f the CO i n c r e a s e s w i t h i n c r e a s i n g p h o t o l y s i s energy. T h i s c a n be r e a d i l y o b s e r v e d i n f i g u r e 1 by a c o m p a r i s o n o f t h e shape o f t h e p a r e n t a b s o r p t i o n as a f u n c t i o n o f p h o t o l y s i s energy. The h i g h energy peak o f t h e p a r e n t a b s o r p t i o n appears weaker r e l a t i v e t o t h e low energy peak as t h e p h o t o l y s i s energy i n c r e a s e s . T h i s i s due t o i n t e r n a l l y e x c i t e d CO, w h i c h produces a p o s i t i v e a b s o r p t i o n t h a t i s s u p e r i m p o s e d on t h e p a r e n t a b s o r p t i o n . The e f f e c t i s more pronounc ed f o r K r F t h a n f o r XeF p h o t o l y s i s b e c a u s e t h e i n t e r n a l energy o f the CO i s g r e a t e r l e a d i n g i t t o absorb a t lower f r e q u e n c i e s . The e f f e c t i s even more p r o n o u n c e d f o r A r F p h o t o l y s i s ( n o t shown) where i t a c t u a l l y causes p a r t o f t h e h i g h f r e q u e n c y p a r e n t band t o appear above t h e b a s e l i n e [ 8 ] . S p e c t r o s c o p i c C o n s i d e r a t i o n s . Though s p e c t r o s c o p i c c o n s i d e r a t i o n s have n o t been emphasized i n t h i s m a n u s c r i p t , a few g e n e r a l comments a r e i n o r d e r . As seen i n f i g u r e 2, where t h e now p a r t i a l l y r e s o l v e d A]^, Έ>ι and B2 bands a r e i n d i c a t e d , t h e gas phase spectrum o f F e ( C 0 ) 4 i s c o m p a t i b l e w i t h a C 2 s t r u c t u r e , as i s t h e f a c t t h a t gas phase F e ( C 0 ) 4 has a t r i p l e t ground s t a t e [ 3 , 4 , 8 ] . T h i s i s the same s t r u c t u r e t h a t has been o b s e r v e d f o r m a t r i x i s o l a t e d F e ( C 0 ) 4 [30,31]. S i m i l a r l y , v i r t u a l l y a l l o f t h e gas phase a b s o r p t i o n f e a t u r e s t h a t we have o b s e r v e d f o r c o o r d i n a t i v e l y u n s a t u r a t e d compounds o f Fe, C r and Mn a r e c o m p a t i b l e w i t h t h e i r r e p o r t e d m a t r i x structures. T h i s i s an i m p o r t a n t o b s e r v a t i o n i n t h a t i t i m p l i e s that matrix i s o l a t e d c o o r d i n a t i v e l y unsaturated metal carbonyls are n o t s u b j e c t t o " m a t r i x e f f e c t s " and t h e s t r u c t u r e d e t e r m i n e d i n t h e m a t r i x i s v e r y l i k e l y t o be t h a t o f t h e gas phase s p e c i e s . Some s u b t l e t i e s may m o d i f y t h i s statement such as t h e e f f e c t o f c o o r d i n a t e d r a r e gas m o l e c u l e s o r o t h e r c o o r d i n a t e d m a t r i x o r g l a s s s u b s t r a t e m o l e c u l e s on t h e s t r u c t u r e . N e v e r t h e l e s s , t h i s statement i s l i k e l y t o be a c c u r a t e i n a l a r g e m a j o r i t y o f c a s e s . The o n l y p o s s i b l e e x c e p t i o n t o t h i s statement t h a t we have o b s e r v e d t o d a t e , d e a l s w i t h t h e d i f f e r e n c e i n p o s i t i o n o f t h e s e m i - b r i d g i n g CO band i n Mn2(CO)io i n t h e m a t r i x v e r s u s t h e gas phase. T h i s band i s o b s e r v e d t o be a t h i g h e r f r e q u e n c y i n t h e m a t r i x w h i c h i s c o u n t e r t o t y p i c a l b e h a v i o r [ 1 0 ] . T h i s c o u l d i n d i c a t e a change i n s t r u c t u r e i n the gas phase v e r s u s t h e m a t r i x f o r t h i s compound. However, u n t i l f u r t h e r s t u d i e s a r e complete t h e p r e v i o u s s t a t e m e n t s h o u l d be r e g a r d e d more as c o n j e c t u r e t h a n p r o v e n f a c t . V


96


Conclusions Perhaps t h e b e s t way t o sum up t h e g e n e r a l c o n c l u s i o n s r e g a r d i n g our s t u d i e s o f c o o r d i n a t i v e l y unsaturated metal carbonyls i s i n a s e r i e s o f p r o p e n s i t y r u l e s w i t h t h e u n d e r s t a n d i n g t h a t t h e s e r u l e s may be m o d i f i e d by f u t u r e s t u d i e s . The r u l e s a r e :


1)

2) 3) 4) 5)

6)

7)

A d d i t i o n r e a c t i o n s t o c o o r d i n a t i v e l y u n s a t u r a t e d compounds a r e e x p e c t e d t o be s i g n i f i c a n t l y f a s t e r t h a n s u b s t i t u t i o n reactions. S p i n c o n s e r v i n g r e a c t i o n s are s i g n i f i c a n t l y f a s t e r than s p i n disallowed reactions. S p i n a l l o w e d a d d i t i o n r e a c t i o n s have r a t e c o n s t a n t s n e a r gas kinetic. I n t h e gas phase the degree o f c o o r d i n a t i v e u n s a t u r a t i o n i n c r e a s e s w i t h i n c r e a s i n g p h o t o l y s i s energy. The n a t u r e o f t h e e l e c t r o n i c s t a t e a c c e s s e d c a n a l s o i n f l u e n c e branching r a t i o s f o r products but w i t h i n a given e l e c t r o n i c s t a t e statement 4 w i l l p r e v a i l . Both t h e c o o r d i n a t i v e l y u n s a t u r a t e d photofragment and t h e e j e c t e d CO t e n d t o be p r o d u c e d w i t h more i n t e r n a l e x c i t a t i o n as the energy o f t h e p h o t o l y s i s p h o t o n i n c r e a s e s . The s t r u c t u r e s o f gas phase c o o r d i n a t i v e l y u n s a t u r a t e d m e t a l c a r b o n y l s a r e v e r y s i m i l a r t o those o f t h e m a t r i x i s o l a t e d species.

Acknowledgments We acknowledge s u p p o r t o f t h i s work by t h e A i r F o r c e O f f i c e o f S c i e n t i f i c R e s e a r c h under c o n t r a c t #83-0372, t h e N a t i o n a l S c i e n c e F o u n d a t i o n under g r a n t #CHE 82-06976 and t h e donors o f t h e P e t r o l e u m R e s e a r c h Fund a d m i n i s t e r e d by t h e A m e r i c a n C h e m i c a l S o c i e t y under g r a n t #15163-AC3. We acknowledge many u s e f u l c o n v e r s a t i o n s w i t h Dr. M a r t y n P o l i a k o f f and P r o f . J . J . T u r n e r and thank NATO f o r a t r a v e l g r a n t w h i c h f a c i l i t a t e d t h e s e c o n v e r s a t i o n s . We a l s o thank o u r coworkers i n t h e f i e l d f o r t h e i r u s e f u l s u g g e s t i o n s and comments.

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RECEIVED

November 3, 1986


Coordinatively Unsaturated Metal Carbonyls in the Gas Phase via

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