A. G. Pinkus Bay lo r Un ive rs i t y

W a c o , T e x a s 7 6 7 0 3

Magnesocene: Structure and Bonding

Covalent or ionic?

A l t h o u g h t h e m o s t c o m m o n s tate o f c o o r d i n a t i o n for m a g -n e s i u m i n i t s c o m p o u n d s is f o u r (1), examples o f t h r e e - ( 2 ) , f i v e - ( 3 ) , s i x - ( 4 ) , a n d e i g h t - c o o r d i n a t i o n (5) have also been r e p o r t e d . A n u n u s u a l t y p e o f b o n d i n g f o r m a g n e s i u m w h i c h does n o t f i t i n t o a n y o f these categories occurs i n magneso -cene. 1 Magnesocene is a m e m b e r o f t h e genera l class o f o r -g a n o m e t a l l i c c o m p o u n d s k n o w n as meta l locenes (6 ) o r " s a n d w i c h " c o m p o u n d s o f w h i c h t h e f i r s t e x a m p l e t o be rec -ognized is t h e w e l l - k n o w n ferrocene. A l t h o u g h i t is g e n e r a l l y agreed t h a t b o n d i n g i n meta l locenes o f t r a n s i t i o n metis a p -pears t o be p r e d o m i n a n t l y covalent , t h e n a t u r e o f b o n d i n g i n magnesocene is c u r r e n t l y i n d i s p u t e . T h i s p a p e r w i l l p r e s e n t a bref r ev i ew o f t h e s t r u c t u r e s a n d b o n d i n g p r o p o s e d f o r magnesocene l e a d i n g u p t o t h e m o s t r e cent w o r k .

A l t h o u g h magnesocene was f i r s t p r e p a r e d (7) b y a d d i t i o n o f cyc lopentadiene t o e t h y l m a g n e s i u m b r o m i d e a n d r e m o v i n g e ther so lvent i n vacuo , i t can also be p r e p a r e d (8) b y t h e r e -a c t i o n o f m e t a l l i c m a g n e s i u m w i t h c y c l o p e n t a d i e n e ( e q n . (D) .

2 C 6 H 6 + M g - H 2 + M g ( C 5 H 5 ) 2 (1) Magnesocene is a use fu l intermedate for t h e p r e p a r a t i o n (9) o f o ther metal locenes e i ther i n t h e m o l t e n state or i n s o l u t i o n (eqn . (2 ) ) . T h e h i g h free energy o f f o r m a t i o n o f M g C l 2 f a v o r s t h e meta thes i s .

M g ( C 5 H 5 ) 2 + M C 1 2 - M ( C 5 H 5 ) 2 + M g C l 2 (2) Magnesocene has been r e p o r t e d (10) t o have been p r e p a r e d i n a rose-colored m o d i f i c a t i o n i n a d d i t i o n t o the b e t t e r - k n o w n f o r m .

Weiss a n d F i s h e r (11) d e t e r m i n e d t h a t M g ( C 5 H 5 ) 2 i n t h e c r y s t a l l i n e s tate h a d a s a n d w i c h - l i k e s t r u c t u r e a n d p r o p o s e d weak cova lent d 2 s p 3 h y b r i d i z a t i o n o v e r l a p p i n g t h e i o n i c e lectrostatic b i n d i n g between t h e m e t a l a n d cyc l opentad ieny l ions. Based on magnetic , spectral , a n d chemica l invest igat ions , W i l k i n s o n , C o t t o n , a n d B i r m i n g h a m (12) conc luded t h a t ionic b o n d i n g was present; t h e t w o cyc lopentadienide anions w o u l d be expec ted on e l e c t ros ta t i c g r o u n d s t o a l i g n themse lves o n oppos i t e sides o f t h e cat in w i t h p a r a l l e l p lanes a n d t o v e r y c losely resemble ferrocene . T h e concept o f i o n i c b o n d i n g i n M g ( C s H 5 ) 2 was f u r t h e r e l a b o r a t e d b y C o t t o n a n d R e y n o l d s (13). L i p p i n c o t t , X a v i e r , a n d Steele (14) s t u d i e d t h e i n f r a r e d a n d R a m a n spectra o f s o l u t i o n s a n d t h e s o l i d s tate a n d i n c o m p a r i s o n w i t h s tud ies o n ferrocene c o n f i r m e d t h e s a n d -w i c h - t y p e s t r u c t u r e . H o w e v e r , t h e y c o n c l u d e d t h a t t h e i r r e -su l t s were n o t i n a g r e e m e n t w i t h t h e i on i c b o n d i n g c o n c e p t b u t were b e t t e r e x p l a i n e d b y cova lent r i n g - t o - m e t a l b o n d i n g w h i c h was, however , w e a k e r t h a n t h e m e t a l - t o - r i n g b o n d i n g i n ferrocene . F r o m a gas-phase electrn d i f f r a c t i o n s t u d y , H a a l a n d , et al. (15,16), d e t e r m i n e d t h a t t h e best a g r e e m e n t between calculated a n d e x p e r i m e n t a l intens i t ies was ob ta ined b y assuming ecl ipsed C5H5 r ings (Drt s y m m e t r y ) (see f i g u r e ) . H o w e v e r , a m o d e l w i t h s taggered s y m m e t r y (Z) 5 ( ) c o u l d n o t

T 2 . 0 0 8

1 . 4 2 3 A

Structure of magnesocene (15, 16).

s i de view ( r i n g s p e r p e n d i c u l a r )

be conclusively r u l e d o u t . T h e y also conc luded t h a t there was no s ign i f i cant d e v i a t i o n f r o m p l a n a r i t y for the C5H5 r ings a n d t h a t magnesocene is best r e g a r d e d as a cova lent r a t h e r t h a n a n i on i c c o m p o u n d .

M o r e r e c e n t l y , A l e k s a n y a n a n d coworkers (17) o b t a i n e d R a m a n a n d i n f r a r e d spectra on t h e c o m p o u n d . R a m a n results were f o r so l id a n d m e l t f o r m s a n d i n c l u d e d d e p o l a r i z a t i o n m e a s u r e m e n t s w h i l e i n f r a r e d d a t a f o r t h e s o l i d covered t h e range 400-3200 c m - 1 . T h e i r analys is o f t h e d a t a l ed t o t h e conclus in t h a t t h e M g - C p b o n d is m a i n l y i on i c .

Literature Cited (1) Bai lar , J . C , Jr. in " T h e Chemistry of the Coordinat ion Compounds," (Editor: Bai lar ,

J . C , J r . ) , Re inho ld , New Y o r k , 1956, p. 243. (2) For a recent review: P inkus , A. G., Coord. Chem. Rev., 25, 173 (1978). (3) Stucky, G., and Rundle , R. E., J . Amer. Chem. Soc, 86, 4821 (1964); Va l l ino , M . , Or-

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(6) Reviews dealing in p a r t w i t h various aspects of metallocenes (excluding ferrocenes): (a) Pauson, P. L . , " Cyclopentadienyl M e t a l Compounds" in "Organometal l i c Chemis t ry " , (Editor: Zeiss, H . ) , Re inho ld , New Y o r k , 1960, Chap. 7, pp . 346-379; (b) Zeiss, H . , "Arene Complexes o f the T r a n s i t i o n M e t a l s " i n ibid., Chap. 8, pp. 380-425. (c) B irmingham, C. M - , "Synthesis of Cyclopentadienyl M e t a l Compounds," in "Advances in Organometall ic C h e m i s t r y , " Vo l . 2, (Editors : Stone, F . G. A. , and West, R.) Academic Press, New Y o r k , 1964, pp . 365-413; (d) C h u r c h i l l , M . R., and Masn, R., " T h e Structural Chemistry of Organo-Transition Meta l Complexes: Some Recent Developments ," ibid., vo l . 5, 1967, pp. 93-135; (e) Cais, M . , and L u p i n , M . S., "Mass Spectra of Metallocenes & Related Compounds , " ibid., vol . 8, 1970, pp. 211-333; (f) Slocum, D. W., and E r n s t , C. R., "E lec t ron i c Effects in Metallocenes and Certa in Related Systems," ibid., vo l . 10, 1972, pp. 79-114; (g) Bruce , M . I . , "Organo -Trans i t i on M e t a l Chemis t ry -A Guide to the L i t e r a t u r e 1950-1970," ibid., vol . 10, pp. 273-346; (h) Rubezhov, A . Z. and G u b i n , S. P., " L i g a n d S u b s t i t u t i o n in T r a n s i t i o n M e t a l x-Complexes," ibid., vol . 10, pp. 347-417; (i) Bruce, M . I . , " T h e L i terature of Organo-Transit ion M e t a l Chemistry 1971," ibid., vol . 11,1973, pp. 447 ff ; (j) Bruce, M . I . , " T h e L i t e r a t u r e o f Organo T r a n s i t i o n M e t a l Chemistry 1972," ibid., vol . 12, 1974, pp. 379-407; (k) S i l v e r t h o r n , W. E., "Arene T r a n s i t i o n M e t a l Chemis t ry , " i b i d . , vo l . 13, 1975, pp. 47-137; (1) Razuvaev, G. A. , and Latyaeva, V . N . , "Covalent Organometall ic Compounds of the T r a n s i t i o n M e t a l s , " Russ. Chem. Reo. 34, 251-267 (1965); Uspekhi Khim., pp . 585 ff; (m) Rybinskaya, M . I . , and Korneva , L . M . , "E lec t roph i l i c S u b s t i t u t i o n i n Cyclopentadienide An ions , " ibid., 40,247-255 (1971); Uspekhi Khim, pp . 444 ff; (n) Leonova, E. V . , and Kochetkova, N . S., ibid., 42,278-292 (1973); Uspekhi Khim., pp. 615 ff; (o) Braterman , P. S., and Cross, R. J . , "Organo-Trans i t ion-meta l complexes: Stab i l i ty , Reactivity and Orb i ta l Correlations," Chem. Soc. Reo., 2, 271 (1973); (p) H u n t , C. B. , Educ. Chem., 14,110 (1977).

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(1956). (13) Cot ton , F. A., and Reynolds, L . J . , J . Amer. Chem. Soc, 80, 269 (1958). (14) L i p p i n c o t t , E. R., Xavier , J . , and Steele, D.,J.Amer. Chem. Soc., 83, 2262 (1961). (15) Haaland, A., Lusztyk , J . , Novak, D. P., B r u n v o l l , J . , and Starowieyski, K . B. , J . Chem.

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ganometal. Chem., 129, 139 (1977).

lChemical Abstracts nomenclature, from 1974 to the present date for this compound is bis(r) 5-2,4-cyclopentadien-l-yl)magnesium. I n earlier references the compound is variously named: bis(cyclopen-tadienyl)magnesium, (also bis-cyclopentadienylmagnesium), dicy-clopentadienylmagnesium, di-ir-cyclopentadienylmagnesium and magnesium cyclopentadienide. I n the present paper, this compound w i l l be referred to by its t r iv ia l ame, magnesocene, for space-saving brevity.

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