9 Complex Compounds with a Large Charge Separation JOSEPH CHATT
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School of Molecular Sciences, University of Sussex, Brighton BN1 9QJ, U.K.
Asymmetrical coordination compounds can have very high electric dipole moments. Covalent compoundswithmo ments of up to 14.8 Debye units are known. The charge separation in the coordinate bonds to the metal from the ligand atoms of so-called good electron-donor neutral ligands is at least as important in many complex com pounds as that of the bonds from the metal to the anionic ligands. It is shown how existing knowledge could be used to tailor covalent molecules with very high dipole moments, perhaps as much as 20 Debye units. *"ρΐιβ p u r p o s e of this r e v i e w is to d r a w attention to the l a r g e c h a r g e A
s e p a r a t i o n w h i c h occurs i n some c o o r d i n a t i o n c o m p o u n d s a n d to its
source.
T h e r e v i e w is the result of the author's e x p e r i e n c e i n the use of
d i p o l e m o m e n t measurements to d e t e r m i n e the c o n f i g u r a t i o n of c o m p l e x compounds
before unambiguous
spectroscopic
means
were available.
H i s r e s e a r c h g r o u p has m e a s u r e d a great n u m b e r of d i p o l e m o m e n t s d u r i n g the past 38 years, yet f e w c o o r d i n a t i o n chemists a p p e a r to b e a w a r e of the l a r g e d i p o l e m o m e n t s w h i c h c a n b e d e v e l o p e d i n a s s y m e t r i c c o v a l e n t c o m p o u n d s as a r e s u l t of the presence of c o o r d i n a t e b o n d s . I n the p r e - P a u l i n g era, one w a s l e d to b e l i e v e t h a t t h e m a i n s e p a r a t i o n of c h a r g e i n a m o l e c u l e of a c o m p l e x c o m p o u n d w a s associated w i t h t h e c o o r d i n a t e b o n d s b e t w e e n the m e t a l a n d t h e l i g a n d atoms of
the
s o - c a l l e d u n c h a r g e d l i g a n d s , r a t h e r t h a n i n the b o n d s f r o m t h e m e t a l to t h e f o r m a l l y a n i o n i c l i g a n d s . F o r m a n y years t h i s a s s u m p t i o n s e r v e d w e l l i n a s s i g n i n g configurations to t e r t i a r y p h o s p h i n e complexes
of
V I I a n d V I I I m e t a l h a l i d e s o n t h e basis of t h e i r d i p o l e m o m e n t s .
Groups Many
years a n d a c o n s i d e r a b l e n u m b e r of m e a s u r e d d i p o l e m o m e n t s l a t e r , t h e a u t h o r is s t i l l c o n v i n c e d t h a t the c h a r g e s e p a r a t i o n i n the b o n d b e t w e e n a m e t a l a n d a l i g a t i n g a t o m of a g o o d d o n o r l i g a n d is at least as great as 95 King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
96
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
t h a t b e t w e e n the m e t a l a n d a n a n i o n i c l i g a n d . S o m e r e c e n t X P S m e a s u r e m e n t s ( 1 ) s u p p o r t this c o n t e n t i o n a n d i n d i c a t e , i n c o n j u n c t i o n w i t h d i p o l e m o m e n t s , that i n t e r t i a r y p h o s p h i n e complexes of the t y p e [ M C l ( P R ) ] n
t h e m e t a l is a b o u t n e u t r a l .
3
n
T h e c h l o r i n e a t o m w o u l d c a r r y a c h a r g e of
a b o u t —0.3 e a n d the p h o s p h o r u s a b o u t + 0 . 3
e.
T h i s k n o w l e d g e c a n b e u s e d to t a i l o r h i g h l y d i p o l a r m o l e c u l e s t h a t are s o l u b l e i n o r g a n i c solvents or f u s i b l e w i t h o u t change.
T h i s is i l l u s -
t r a t e d b y a representative selection of c o m p o u n d s w h o s e d i p o l e m o m e n t s w e r e m e a s u r e d i n b e n z e n e s o l u t i o n . T h e p r a c t i c a l l y i m p o r t a n t factors are c i t e d . Downloaded by NORTHWESTERN UNIV on May 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch009
I n 1936 K . A . Jensen (2, 3) m e a s u r e d t h e m o m e n t s of a n u m b e r of p l a t i n u m ( I I ) d i h a l i d o complexes w i t h t e r t i a r y p h o s p h i n e s a n d other s u c h l i g a n d s . H e s h o w e d t h a t the cis complexes h a d m o m e n t s of u p to a b o u t 11.0 D e b y e units ( D ) a n d that a t o m p o l a r i z a t i o n i n c o m p l e x is m u c h h i g h e r t h a n i n o r g a n i c c o m p o u n d s .
compounds
H e also s h o w e d that a l l the
complexes of this t y p e h a d h a d t h e i r cis or trans configurations i n c o r r e c t l y assigned o n the basis of color, i n a n a l o g y w i t h the d i a m m i n e d i chloro platinum (II)
complexes.
Factors Affecting the Size of Dipole Moments in Mononuclear
Complexes
E m p i r i c a l l y there are n o w sufficient d a t a to s h o w w h i c h factors affect t h e charge s e p a r a t i o n i n c o m p l e x c o m p o u n d s . below.
T h e s e are discussed
T h e references to t h e T a b l e s list m a n y m o r e values of
dipole
m o m e n t s , a n d t h e y also cite references for others. D e p e n d e n c e of D i p o l e M o m e n t o n the M e t a l . m o m e n t s , e.g.
of the g r o u p P - M - C l
(M =
G e n e r a l l y the g r o u p
transition metal), i n
cis-
[ M C l ( P R ) ] - t y p e complexes do not d e p e n d s t r o n g l y o n the m e t a l or n
3
n
o n its o x i d a t i o n state, p r o v i d e d that o n l y g o o d d o n o r l i g a n d s , w h e t h e r n e u t r a l or a n i o n i c , are i n v o l v e d .
S u c h l i g a n d s are t e r t i a r y p h o s p h i n e s
a n d c h l o r i d e ions i n the examples i n T a b l e I. T h e d i p o l e m o m e n t s g i v e n are those of s o m e a x i a l l y s y m m e t r i c a l c o m p l e x c o m p o u n d s that c o n t a i n a v a r i e t y of metals. O n the a s s u m p t i o n t h a t the l i g a n d a t o m b o n d s are at r i g h t angles, w h i c h is not q u i t e justified, the m o m e n t s h a v e b e e n r e s o l v e d a l o n g the P - M - C l d i r e c t i o n s , a n d the i n f e r r e d g r o u p m o m e n t s are l i s t e d . T h e g r o u p m o m e n t is g e n e r a l l y 6 - 7 D ( w i t h a l i t t l e scatter outside this r a n g e ) , a n d the a l i p h a t i c p h o s p h i n e s t e n d to p r o d u c e
slightly higher
m o m e n t s t h a n the m o r e e l e c t r o n e g a t i v e a r o m a t i c p h o s p h i n e s .
T h e exact
m e t a l i n v o l v e d is o b v i o u s l y not of great significance i n d e t e r m i n i n g the group moment. D e p e n d e n c e on S o - c a l l e d E l e c t r i c a l l y N e u t r a l L i g a n d s . G o o d donors —e.g.
t e r t i a r y p h o s p h i n e s , arsines, a n d s t i b i n e s — g i v e analogous
com-
plexes w i t h a l m o s t e q u a l m o m e n t s w h e n t h e y are associated w i t h h a l i d e
King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
9.
Large
CHATT
T a b l e I.
Charge
Some M o l e c u l a r D i p o l e M o m e n t s a n d t h e P — M — C l G r o u p Moments Inferred f r o m T h e m Molecular Dipole Moment,
Compound
0
as-[RuCl (depe) ] as-[RuCl (dppe) ] cis-[OsCl (depe) ] ds-[OsCl (dppe) ] mer-[ReCl (PEt Ph) ] mer-[RhCl (PEt ) ] mer-[IrCl (PEt ) ] m-[PtCl (PEt ) ] 2
2
a
2
2
a
2
3
2
3
3
3
3
Downloaded by NORTHWESTERN UNIV on May 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch009
a
2
3
3
6
c
3
d
3
e
2
P-M-Cl Group Moment, D
D
9.8 9.5 9.3 8.3 6.3 7.0 6.9 10.9
a
2
2
2
97
Separation
7.0 6.8 6.6 5.9 6.3 7.0 6.9 7.6
Legend: depe = E t P C H C H P E t ; dppe = P h P C H C H P P h . Ref. 16. Ref. 9.
