8 Novel Cluster Interactions in Metalloboranes
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NORMAN N. GREENWOOD University of Leeds, Department of Inorganic and Structural Chemistry, Leeds LS2 9JT, England
Metal atoms have fewer valence electrons than orbitals available for bonding and in this they resemble boron. The consequences of this idea are examined and it is shown that many metals with electronegativities in the range 1.6-2.4 (B = 2.0) can subrogate boron atoms as vertices in poly hedral clusters. Such metalloboranes are often much more stable than the parent boranes or borane anions. Not only can metals mimic boron in known cluster geometries but the flexibility thus introduced can lead to novel and previously unsuspected cluster geometries. The construction of macropolyhedral clusters containing 17-20 vertices is also described. R a l p h R u d o l p h made m a j o r c o n t r i b u t i o n s t o o u r u n d e r s t a n d i n g o f t h e s t r u c t u r e and b o n d i n g o f p o l y h e d r a l c l u s t e r compounds a n d he h a d a n a b i d i n g i n t e r e s t i n d e v e l o p i n g a r a t i o n a l e w h i c h w o u l d e n a b l e t h e s t r u c t u r e o f i n d i v i d u a l compounds t o be s y s t e m a t i z e d and r e l a t e d t o e a c h o t h e r . He i n d e p e n d e n t l y a r r i v e d a t a method o f c o u n t i n g s k e l e t a l e l e c t r o n s w h i c h i s now g e n e r a l l y r e f e r r e d t o as Wade's R u l e s , a n d t h i s h a s h a d a d e c i s i v e i n f l u e n c e o n o u r g e n e r a l p e r c e p t i o n s o f p o l y h e d r a l c l u s t e r compounds. R e l a t e d t o t h i s was h i s p r e o c c u p a t i o n w i t h t h e p r o b l e m o f h e t e r o a t o m s s u c h a s s u l f u r , and t h e number o f e l e c t r o n s w h i c h s u c h atoms c o n t r i b u t e to the heteroborane c l u s t e r s . You w i l l r e c a l l t h a t b o r a n e s a r e now c l a s s i f i e d i n t o v a r i o u s s e r i e s a s summarized i n T a b l e I . The f o r m u l a e and s t r u c t u r e s c a n be r a t i o n a l i z e d o n t h e b a s i s o f t h e number o f s k e l e t a l e l e c t r o n s a v a i l a b l e f o r b o n d i n g and e a c h Β atom i s c o n s i d e r e d t o c o n t r i b u t e 2 e l e c t r o n s i n a d d i t i o n t o t h e one u s e d t o f o r m a t e r m i n a l B - H bond. S u p e r n u m e r a r y Η atoms f o r m B-H -B b r i d g e s o r c o m p r i s e t h e endo-U atom i n a BH2 g r o u p : B H H g j ; t h e y o c c u r i n t h e "open f a c e " o f s t r u c t u r e s from which B H groups a r e n o t i o n a l l y m i s s i n g t
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0097-6156/83/0232-0125 $06.00/0 © 1983 American Chemical Society
Cowley; Rings, Clusters, and Polymers of the Main Group Elements ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
126
RINGS, CLUSTERS,
Table I .
AND
POLYMERS
B o r a n e S t r u c t u r e s - Wade's R u l e s
Series
Parent Formula
Skeletal Electrons
C l u s t e r Geometry ( d e l t a h e d r o n = closed t r i a n g u l a t e d polyhedron)
preoloso-
Β Η η η
2η
(n-1)vertexed deltahedron p l u s 1 c a p p i n g BH
closo-
Β Η η η+2 Β Η , η η+4
2η+2
n-vertexed deltahedron
2η+4
(n+1)deltahedron
(n o c c u p i e d )
2η+6
(n+2)deltahedron
(n o c c u p i e d )
2η+8
(n+3)deltahedron
(n o c c u p i e d )
nidoarachnohypho-
Β
η
η+6
Η
ο η+ο
η
conjuncto-
Α
Β Η η m
two o r more c l u s t e r s j o i n e d by B-B o r BBB b o n d s , o r by s h a r i n g common v e r t i c e s , e d g e s , o r f a c e s
from the parent c l o s e d t r i a n g u l a t e d p o l y h e d r a l s t r u c t u r e i n which a l l v e r t i c e s are o c c u p i e d . I n g e n e r a l the s t r u c t u r e s r e f l e c t the f a c t t h a t b o r o n has one l e s s v a l e n c e e l e c t r o n t h a n a t o m i c o r b i t a l s a v a i l a b l e f o r bonding. Most m e t a l s a l s o have f e w e r v a l e n c e e l e c t r o n s t h a n o r b i t a l s a v a i l a b l e f o r b o n d i n g and some y e a r s ago we i n i t i a t e d a r e s e a r c h p r o g r a m t o d e l i n e a t e t h e e x t e n t t o w h i c h m e t a l s c o u l d a c t as p o l y h e d r a l v e r t i c e s i n métalloborane complexes. I t t u r n s out t h a t v i r t u a l l y a l l m e t a l s except t h e most e l e c t r o p o s i t i v e ones i n Groups I - I I I o f t h e P e r i o d i c T a b l e c a n a c t i n t h i s way as h o n o r a r y b o r o n atoms. Indeed, the m e t a l l o b o r a n e s a r e f r e q u e n t l y v e r y much more s t a b l e t h a n t h e c o r r e s p o n d i n g p a r e n t b o r a n e s and b o r a n e a n i o n s . T h r e e e x a m p l e s f r o m o u r own r e c e n t work w i l l s u f f i c e t o show how c o o r d i n a t e d m e t a l atoms c a n s u b r o g a t e BH u n i t s i n closo-, nido- and avaohno-decaborane. Thus, the b r i g h t r e d n i c k e l a oloso-decaborane eloso-[(PMe Ph) (I-N1B9H7CI2-2,4)] has b e e n s y n t h e s i z e d 0 ) i n w h i c h t h e B m o i e t y i s η^-bonded t o a c a p p i n g L2N1 g r o u p ( F i g u r e 1 ) . The s t r u c t u r a l r e l a t i o n s h i p t o olosoB10H10 "" i s o b v i o u s a n d , as t h e m e t a l c e n t r e f o r m a l l y r e p l a c e s a BH "" g r o u p w h i c h c o n t r i b u t e s 4e t o t h e c l u s t e r , i t c a n be c o n s i d e r e d as N i - ^ . We f i n d , i n g e n e r a l , t h a t p o l y h a p t o b o r a n e l i g a n d s a r e v e r y good a t s t a b i l i z i n g formal high oxidation s t a t e s of m e t a l s . N e x t we c a n m e n t i o n a nido-analogue, n£
