21 Stereochemical Description and N o t a t i o n for Coordination Systems 1
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THOMAS E. SLOAN Chemical Abstracts Service, P.O. Box 3012, Columbus, OH 43210 DARYLE H. BUSCH Department of Chemistry, The Ohio State University, Columbus, OH 43210
The terminology and notation that have been used to de scribe coordination compounds have been derived with one no table exception from the terms and symbols developed to describe the stereochemistry of carbon compounds. The terms cis, trans; endo, exo; dextro, d, D, ( + ); and levo, l, L (-) a l l have been used to describe the stereochemistry of coor dination compounds in a close analogy with organic compounds (see Figure 1). As the descriptions of the chemistry and structures of coordination systems have become more varied and complex, the meanings of these terms have become less precise, as in the example of a cis or trans tricarbonyl octahedral compound (see Figure 2). The terms fac and mer were coined to indicate the facial and meridional disposition of substituted octahedral structures. Geometric isomers of linear quadridentate ligands on octa hedral compounds are recognized to exist in one trans and two cis forms (see Figure 3). The cis compounds are generally referred to as the α and β forms. In these examples we can see that the terminology developed to denote the relatively simple tetrahedral and planar carbon stereochemistry is not adequate when applied to the stereochemistry of octahedral compounds. And when we consider that there are eighteen well-defined geo metries of mononuclear complexes for coordination numbers 4 to 9 with literally tens of thousands of possible isomeric config urations, then i t is not difficult to comprehend the need for notations that are systematic and developed within the basic requirements and boundary limits that are unique to coor dination chemistry. The first systematic designation of the stereochemistry of coordination compounds was developed by Werner. Werner num bered the ligand sites on the coordination polyhedra as shown in Figure 4. 1
To whom correspondence should be addressed. 0-8412-0538-8/80/47-119-397$05.50/0 © 1980 American Chemical Society
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
STEREOCHEMISTRY OF TRANSITION
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398
Figure 1.
METALS
The commonly used stereochemical terms and symbols in inorganic and organic chemistry
CO CiJ-iCO M
I
ciL-Hco Figure 2. Stereochemical terms for tris(unidentate) octahedral complexes
CO H C N |-,C0 M 5
5
0
r
CH-IP(CH ) 3
ί ^' cis, fac
ι CI t r a n s
m
e
r
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
(/) c
s
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21.
SLOAN
AND
BUSCH
Coordination
Systems
399
M u e t t e r t i e s and W r i g h t have s u g g e s t e d n u m b e r i n g schemes f o r c o o r d i n a t i o n p o l y h e d r a w i t h up t o 20 c o o r d i n a t i o n s i t e s ( 1 ) . IUPAC a d o p t e d a p o l y h e d r a l n u m b e r i n g s y s t e m t h a t was f i r s t s u g g e s t e d b y McDonnel and P a s t e r n a c k ( 2 , 3) ( s e e F i g u r e 4 ) . The McDonnel method f o r p o l y h e d r a l n u m b e r i n g s u b s t i t u t e s l o w e r c a s e Roman c h a r a c t e r s t o d e s i g n a t e t h e c o o r d i n a t i o n p o s i t i o n s . This was done t o e l i m i n a t e a n y c o n f u s i o n t h a t w o u l d o c c u r between t h e s u b s t i t u t i o n n u m b e r i n g w i t h i n a l i g a n d and t h e n u m b e r i n g o f t h e c o o r d i n a t i o n p o s i t i o n . McDonnel a l s o s u g g e s t e d t h e a d o p t i o n o f a c l a s s s y m b o l f o r use i n c i p h e r c o d i n g t o i n d i c a t e t h e g e o m e t r y o f t h e c e n t r a l atom c o o r d i n a t i o n s i t e . A l l methods f o r n u m b e r i n g o f c o o r d i n a t i o n p o l y h e d r a a r e s i m i l a r and do p r o v i d e unambiguous n o m e n c l a t u r e t h a t i s s t r u c t u r a l l y e x a c t , p r o v i d i n g t h e g e o m e t r y o f t h e c e n t r a l atom c o o r d i n a t i o n s p h e r e i s e i t h e r r e a d i l y a p p a r e n t o r remembered. Locant numbering c o n v e n t i o n s share s i m i l a r handicaps. These h a n d i c a p s become a p p a r e n t when t h e l o c a n t s a r e p u t t o u s e (see Table I below).
Table I.
Stereochemical numbering for diamminebis-(ethylenediamine) platinum a^-diammine-c^c/e-bislethylenediamine) platinum a/?-diammine-ccY,ef-bis(ethylenediamine) platinum (+)x-ab-diammine-cûf e/ -bis(ethylenediamine)platinum /
r
rac-a/j-diammine-Cû^ef-bisiethylenediamine) platinum
The most s i g n i f i c a n t h a n d i c a p s a s s o c i a t e d w i t h d i r e c t i o n a l l y s p e c i f i c l o c a n t s are the i n a b i l i t y t o d i s t i n g u i s h enantiomeric p a i r s o f compounds d i r e c t l y f r o m t h e n o t a t i o n a n d t h e a r b i t r a r y and o f t e n c o m p l i c a t e d h i e r a r c h i c a l r u l e s t o d e t e r m i n e what l i gands a r e t o b e a s s o c i a t e d w i t h w h i c h l o c a n t s . A n o t h e r comp l e x i t y o f l o c a n t n o t a t i o n s t h a t i s sometimes o v e r l o o k e d i s t h e need f o r a d d i t i o n a l s y m b o l s o r o t h e r n o t a t i o n t o i n d i c a t e t h a t t h e r e i s l e s s i n f o r m a t i o n meant t h a n i s e x p r e s s e d i n t h e name w i t h l o c a n t s . O f t e n t h e g e o m e t r i c c o n f i g u r a t i o n o f a compound i s known but n o t t h e a b s o l u t e c o n f i g u r a t i o n . Thus t h e X i n t h e t h i r d name i s n e c e s s a r y i n t h e l o c a n t n o t a t i o n b e c a u s e i t i s n o t p o s s i b l e t o draw and number t h e cis c h i r a l s t r u c t u r e a m b i g uously. S i m i l a r l y , vac i s t h e recommended t e r m t o i n d i c a t e a mixture o f enantiomers. The need f o r a r a t i o n a l s t e r e o c h e m i c a l n o t a t i o n has become i n c r e a s i n g l y acute since the establishment o f the f i r s t a b s o l u t e c o n f i g u r a t i o n by B i j v o e t i n 1951 ( 4 ) . The i m p o r t a n c e o f t h e s t e r e o c o n f i g u r a t i o n i n l i v i n g s y s t e m s and t h e i n f l u e n c e o f s t e r e o c h e m i c a l c o n f i g u r a t i o n on t h e c o u r s e o f r e a c t i o n s a l s o
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
STEREOCHEMISTRY OF TRANSITION METALS
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400
reinforces this position. The s y s t e m a t i c n o t a t i o n t h a t h a s g a i n e d t h e g r e a t e s t a c c e p t a n c e i s t h e n o t a t i o n p r o p o s e d by Cahn, I n g o l d , a n d P r e l o g (5). The C a h n - I n g o l d - P r e l o g ( C I P ) n o t a t i o n i s b a s e d on a p r i o r i t y r a n k i n g o f l i g a n d s i n a t h r e e dimensional molecular representation. This ranking procedure i s c a l l e d t h e CIP Sequence R u l e . The CIP Sequence R u l e i s u s e d to s p e c i f y the a b s o l u t e c o n f i g u r a t i o n o f t e t r a h e d r a l carbon atoms i n o r g a n i c compounds, i s t h e b a s i s o f t h e Ε,Z notation p r o p o s e d by B l a c k w o o d et al. t o s p e c i f y t h e i s o m e r i s m o f t h e l i g a n d s a b o u t a d o u b l e bond (6), a n d i s t h e r a n k i n g employed by Brown et al. i n t h e CHEMICAL ABSTRACTS n o t a t i o n t h a t s p e c i f i e s t h e c o m p l e t e s t e r e o c h e m i s t r y o f c o o r d i n a t i o n compounds (7, 8 ). Ligand
Indexing
The C I P r a n k i n g o f l i g a n d s i s t e r m e d t h e CIP p r i o r i t y a n d i s determined according t o the s u b r u l e s : 0) Nearer end o f a x i s o r s i d e o f p l a n e p r e c e d e s f u r t h e r . 1) H i g h e r a t o m i c number precedes lower. 2) H i g h e r a t o m i c mass p r e c e d e s l o w e r . 3) ( s e q c i s ) p r e c i d e s Ε ( s e q t r a n s ) (9). 4) L i k e p a i r (R,R o r S,S) p r e c e d e s u n l i k e Q?_,S o r S,R). 5) R p r e c e d e s 5 . The t i n compound i n F i g u r e 5 i l l u s t r a t e s t h e CIP p r i o r i t i e s f o r l i g a n d s o f d i f f e r e n t a t o m i c number. C h l o r i n e i s a t o m i c number 17, s i l i c o n 14, n i t r o g e n 7, a n d c a r b o n 6. N o t e , t h e h i g h e r t h e p r i o r i t y , t h e l o w e r t h e CIP p r i o r i t y number. I t i s n o t uncommon t o have l i g a t i n g atoms o f t h e same a t o m i c number b e i n g compared a s shown f o r t h e c o b a l t compound i n F i g u r e 6. F o r t h e s e s t r u c t u r e s t h e CIP method p r o v i d e s a f o r m a l i s m f o r e x p l o r i n g a l i g a n d t o determine the l i g a n d o f highest p r i o r i t y , second h i g h e s t , t h i r d , e t c . I n T a b l e I I t h e CIP e x p l o r a t i o n p r o c e d u r e s a r e i l l u s trated. The l i g a n d s a r e e x p l o r e d f r o m t h e c e n t e r o f i n t e r e s t , atom by atom, u n t i l a d e t e r m i n a t i o n i s o b t a i n e d o n t h e b a s i s o f a t o m i c number o r t h e n e x t a p p r o p r i a t e c r i t e r i o n , i n t h e o r d e r listed. C o n s i d e r i n g t h e two phenoxy l i g a n d s , t h e s t r u c t u r e s a r e r e p r e s e n t e d w i t h t h e a t o m i c number a t t h e atom p o s i t i o n s w i t h t h e d o u b l e bonds expanded w i t h r e p l i c a atoms o f a t o m i c number 6 shown i n c u r v e s . The d e t e r m i n a t i o n o f p r i o r i t y i s made a t t h e f i f t h l e v e l by c o m p a r i n g o x y g e n , a t o m i c number 8, t o c a r b o n , a t o m i c number 6, i n t h e s e c o n d b r a n c h o f t h e e x p l o r ation table. F o r t h e p y r i d i n e l i g a n d , r e p l i c a atoms on t h e ortho c a r b o n p o s i t i o n s a r e a t o m i c number (6.5). T h i s r e s u l t s f r o m t h e r e s o nance o f t h e d o u b l e bonds t o t h e p y r i d i n e n i t r o g e n . The r e s o n a n c e i s a l l o w e d f o r by a v e r a g i n g c a r b o n + n i t r o g e n on e i t h e r s i d e o f t h e ortho c a r b o n s , 6+7 =6.5. A t the t h i r d 2
2
l e v e l , t h e s e q u e n c e 6.5, 6, 1 d e t e r m i n e s t h e p y r i d i n e p r i o r i t y when compared t o t h e e x p l o r a t i o n s o f t h e r e m a i n i n g l i g a n d s .
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
SLOAN
AND
Coordination
BUSCH
Systems
401
m
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A
c/'s-β
(H Figure 3. Geometric isomers of octa hedral complexes with linear quadri dentate ligands
c trans
12
α b
Ξ
4
Ξ
3
d C 4B
Figure 4. Werner and McDonnel num bering for square planar and octahedral structures
4 f 3 CH —Sn—NH 3
2
Il? 2 , *
s
(CH
3
3
Figure 5. Cahn-Ingold-Prelog ligand relative priority determined by atomic number
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
STEREOCHEMISTRY
Table II.
OF TRANSITION METALS
Ligand CIP exploration table (by atomic number) 5
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Λ
V
17
S6)
8 1^(6) t7—6^ ^6—(6) (6K| f\(6) 6
OCH,
< te)
*(β)
./I
Ne) Ne)
6 (6)
5
AT 6 (6)
β
.6 (6)^f
(e/
f\(6)
Λ
Ne)
/V)
\
-1
6
-
