3 Binuclear Transition Metal Complex Systems U L R I C H T. M U E L L E R - W E S T E R H O F F
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Physical Sciences Dept., IBM Research Laboratory, San Jose, Calif. 95193
The search for organic materials with high electrical con ductivity has led to the highly anisotropic charge transfer complexes of the TTF-TCNQ type (one-dimensional metals) as an interim goal. Intermolecular exchange interactions between π-electron systems are limited but may be increased by use of planar complexes of transition metals in either the donor or the acceptor part of the charge transfer system. There is ample precedent for extended interaction in square planar inorganic complexes, but simple mononuclear organometallic compounds of this type cannot be conductive since they form only dimers. However, binuclear planar com plexes offer all the needed advantages to synthesize stable anisotropic conductors: their particular combination of intra -and intermolecular interactions may prevent the distortion (Peierls instability) that leads to metal-to-insulator transition in other one-dimensional materials.
O o m e recent w o r k of o u r g r o u p has c o n c e r n e d the synthesis of b i n u c l e a r , ^ c o m p l e t e l y p l a n a r t r a n s i t i o n m e t a l complexes a n d t h e i r r e a c t i o n w i t h d o n o r or a c c e p t o r m o l e c u l e s to g i v e charge transfer c o m p l e x systems w i t h strong i n t r a m o l e c u l a r as w e l l as i n t e r m o l e c u l a r interactions. W e o r i g i n a l l y b e c a m e i n t e r e s t e d i n s u c h systems s e v e r a l years ago b e c a u s e of t h e as s u m p t i o n that t h e y w o u l d h a v e u n u s u a l p h y s i c a l p r o p e r t i e s . T h i s a s s u m p t i o n is b a s e d o n o u r — c e r t a i n l y not c o m p l e t e — u n d e r s t a n d i n g of t h e s t r u c t u r a l r e q u i r e m e n t s o n the m o l e c u l a r l e v e l n e e d e d to p r o d u c e the o p t i c a l a n d e l e c t r o n i c p h e n o m e n a that are associated w i t h h i g h l y a n i s o t r o p i c m o l e c u l a r crystals of o r g a n i c c h a r g e transfer complexes ( i n c l u d i n g also salts of the t e t r a c y a n o q u i n o d i m e t h a n e ( T C N Q ) r a d i c a l a n i o n ) ( J , 2, 3) of i n o r g a n i c m i x e d v a l e n c e systems ( s u c h as K C P , K P t ( C N ) 0 . 3 B r • 3 H 0 , c o m m o n l y c a l l e d K r o g m a n n s salt) (4,5) a n d of o u r o w n m i x e d v a l e n c e f e r r o c e n o p h a n e complexes (6). A n a r t i c l e o n t h e d e s i g n of o r g a n i c metals a p p e a r e d r e c e n t l y (7). I n o r d e r to present a l o g i c a l r e a s o n 2
2
31 In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
4
32
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
i n g for o u r assumptions, w e p r e f a c e this d i s c u s s i o n w i t h a b r i e f a n d g e n e r a l i n t r o d u c t i o n to the p e c u l i a r i t i e s of o r g a n i c c o n d u c t o r s . Conductivity E l e c t r i c a l c o n d u c t i v i t y a (expressed i n Q ^ c m "
1
a n d defined as the
r e c i p r o c a l of the e x p e r i m e n t a l l y d e t e r m i n a b l e r e s i s t i v i t y p of a s a m p l e ) is p r i n c i p a l l y a f u n c t i o n of the p r o d u c t of t w o m a i n v a r i a b l e s : the c o n c e n t r a t i o n of c h a r g e carriers n a n d t h e i r m o b i l i t y fi. T h i s statement h o l d s for all n o r m a l conductors, excluding superconductors
(a i n f i n i t e ) . W i t h the
latter, a l l e l e c t r i c a l r e s i s t i v i t y vanishes b e l o w a c e r t a i n c r i t i c a l t e m p e r a ture T
c
w h e r e a different c o n d u c t i v i t y m e c h a n i s m takes over.
T h e gen-
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e r a l l y a c c e p t e d B a r d e e n - C o o p e r - S c h r i e f f e r t h e o r y ( 8 ) of s u p e r c o n d u c t i v i t y is b a s e d o n the existence b e l o w T of C o o p e r P a i r s of electrons w h i c h c
travel w i t h a common
momentum
a n d w h i c h therefore
are n o t
sub-
ject to t h e same s c a t t e r i n g processes as single electrons, w h i c h l i m i t the c o n d u c t i v i t y of n o r m a l metals.
F r e e electrons are the carriers i n
metals l i k e C u a n d A g , a n d , since t h e i r p r o b a b i l i t y f o r collisions w i t h scatterers increases w i t h t e m p e r a t u r e , f o r metals a ~
l/T.
T h i s means
t h a t e v e n i f the c a r r i e r c o n c e n t r a t i o n r e m a i n s a p p r o x i m a t e l y constant, t h e i r net ( f o r w a r d )
m o b i l i t y a n d therefore also n • fi decrease w i t h i n -
c r e a s i n g T. T h e o p p o s i t e is true for other c o n d u c t i v e m a t e r i a l s l i k e s e m i conductors.
I t is o b v i o u s t h a t w h e n a c e r t a i n a c t i v a t i o n e n e r g y is r e -
q u i r e d for the h o p p i n g of a n e l e c t r o n f r o m one site to the next, the m o b i l i t y is p r o p o r t i o n a l to T; therefore, w h e n n = both n and
const, a ~
T.
When
are v a r i a b l e , a m a x i m u m i n a m a y result at some i n t e r m e d i -
ate t e m p e r a t u r e . M o s t o r g a n i c a n d o r g a n o m e t a l l i c m a t e r i a l s are c l o s e d s h e l l systems a n d as s u c h are insulators because there are n o carriers unless i m p u r i t i e s o r i n j e c t e d carriers are present.
E v e n t h e n , m o b i l i t i e s are u s u a l l y l o w ,
since t h e i n d i v i d u a l m o l e c u l e s i n the s o l i d are s p a c e d r e l a t i v e l y f a r apart. It therefore seems necessary to f u l f i l l b o t h of t w o c o n d i t i o n s i n o r d e r to create c o n d u c t i v e
organics or o r g a n o m e t a l l i c s .
T h e s e are ( a )
carriers
m u s t b e g e n e r a t e d b y t r a n s f o r m i n g the u s u a l c l o s e d s h e l l ( a l l electrons p a i r e d ) m a t e r i a l s i n t o o p e n s h e l l ( u n p a i r e d s p i n ) free r a d i c a l or m i x e d v a l e n c e systems; a n d ( b ) the o v e r l a p b e t w e e n these species ( a n d w i t h i t t h e c a r r i e r m o b i l i t y ) m u s t b e i n c r e a s e d b y dense p a c k i n g i n the c r y s t a l ( s i n c e p a r a l l e l o v e r l a p of ir systems is most efficient, dense p a c k i n g i n l i n e a r stacks is the most advantageous
a r r a n g e m e n t for c r e a t i n g h i g h
mobilities). T h e i n t e r m o l e c u l a r s p a c i n g , i n a d d i t i o n to b e i n g close, m u s t also b e u n i f o r m a l o n g t h e entire stack w i t h i n the c r y s t a l . A n y d e v i a t i o n f r o m e q u i d i s t a n c e w i l l l e a d to the o p e n i n g of a g a p at t h e F e r m i surface so t h a t a s e m i c o n d u c t o r or i n s u l a t o r r a t h e r t h a n a m e t a l w i l l b e f o r m e d . I n 1955,
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
3.
MUELLER-WESTERHOFF
Peierls (9)
Binuclear
Transition
Metal
33
Complexes
p r e d i c t e d t h a t n o s t r i c t l y o n e - d i m e n s i o n a l m e t a l c o u l d exist
b e c a u s e there w o u l d a l w a y s b e a d i s t o r t e d state of l o w e r e n e r g y
with
p a i r s , ( o r , m o r e g e n e r a l , m u l t i p l e s ) of the b a s i c u n i t ( F i g u r e 1 ) .
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Density of States
Density of States
Figure 1. The Peierls instability. A lattice distortion opens a gap, and a lower energy state is reached which has a completely filled valence band; the distorted state is insulating. A c r y s t a l c o m p o s e d of s u c h densely p a c k e d i n d e p e n d e n t stacks of p l a n a r m o l e c u l e s m u s t necessarily h a v e u n i q u e a n i s o t r o p i c T h e r e are i n d e e d a n u m b e r of p u r e l y o r g a n i c a n d also
properties.
organometallic
m a t e r i a l s w h i c h f u l f i l l the a b o v e r e q u i r e m e n t s a n d w h i c h seem to b e oned i m e n s i o n a l metals. A t present, these m a t e r i a l s p r o v i d e e x c i t e m e n t a n d s t i m u l u s to p h y s i c i s t s since t h e y a l l o w t h e s t u d y of p h y s i c s i n one d i m e n sion (anisotropic electrical, optical, a n d magnetic properties)
a n d also
offer the o n l y p r o m i s i n g p o s s i b i l i t y of o b t a i n i n g h i g h t e m p e r a t u r e s u p e r conductors.
F o r technical applications, anisotropic properties m a y be a
p r o b l e m as w e l l as a p a r t i c u l a r l y u s e f u l asset. I n T T F - T C N Q , a n o r g a n i c c h a r g e transfer salt w h i c h at present is t h e most p r o m i n e n t e x a m p l e of a n o r g a n i c m e t a l l i c s y s t e m (2,3),
because
of the suggestion (3)
fluctuations,
a n elec-
t r o n is t r a n s f e r r e d f r o m the s t r o n g d o n o r T T F to the s t r o n g
acceptor
that i t shows s u p e r c o n d u c t i n g
T C N Q t h e r e b y c r e a t i n g a r a d i c a l c a t i o n - r a d i c a l a n i o n salt: b o t h p a r t n e r s b e c o m e o p e n s h e l l systems. I n a d d i t i o n , b o t h the d o n o r a n d the a c c e p t o r moieties f o r m i n d e p e n d e n t stacks (10)
a l l o w i n g the t r a n s p o r t of electrons
a n d holes i n the respective c o l u m n s . T h e exact a m o u n t of charge transfer, a l t h o u g h i t is one of the most c r u c i a l q u a n t i t i e s , is s t i l l d i s p u t e d (11,
12)
T T F - T C N Q has a m a x i m u m c o n d u c t i v i t y at a b o u t 6 0 ° K w h i c h is m o r e p r o n o u n c e d i n some samples t h a n i n others (3).
A l l samples, h o w e v e r ,
b e c o m e i n s u l a t i n g b e l o w 50 °K. T h e c o n c e n t r a t i o n of free spins as d e t e r m i n e d b y E S R does not h a v e s u c h a m a x i m u m : i t declines m o n o t o n i c a l l y
In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
34
INORGANIC
COMPOUNDS WITH
as the t e m p e r a t u r e is l o w e r e d (12).
UNUSUAL PROPERTIES
T h i s w o u l d i n d i c a t e that e i t h e r the
c a r r i e r m o b i l i t y has a v e r y s h a r p a n d so f a r u n e x p l a i n a b l e m a x i m u m , or t h a t i n d e e d there are s u p e r c o n d u c t i v e TCNQ
and related one-dimensional
fluctuation conductors
at 60 °K.
That T T F -
b e c o m e i n s u l a t i n g at
l o w e r t e m p e r a t u r e s has b e e n a t t r i b u t e d to the P e i e r l s i n s t a b i l i t y , b u t other m e c h a n i s m s w e r e also p r o p o s e d .
A n a p p a r e n t e x c e p t i o n to this r u l e is
the T C N Q c o m p l e x of b i s - t r i m e t h y l e n e - t e t r a s e l e n o - f u l v a l e n e t h a t has b e e n prepared a n d studied b y C o w a n a n d co-workers
it m a i n t a i n s c o n -
(13);
s i d e r a b l e c o n d u c t i v i t y e v e n at 1.8 °K. T h e c o n d u c t i o n m e c h a n i s m a n d the n a t u r e of the m e t a l - i n s u l a t o r transitions i n o r g a n i c c h a r g e transfer c o m p o u n d s w i t h h i g h a n i s o t r o p i c s
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h a v e b e e n w i d e l y discussed. N u m e r o u s p u b l i c a t i o n s o n T T F — T C N Q a n d r e l a t e d salts a p p e a r e d d u r i n g the past three years; i t w o u l d b e i n a p p r o p r i a t e to discuss t h e m i n this context.
The unambiguous quantita-
t i v e d e t e r m i n a t i o n of c o n d u c t i v i t y i n a n i s o t r o p i c m e d i a is at present a task f r o u g h t w i t h m a n y p i t f a l l s . A n a d d e d d i f f i c u l t y i n assessing exactly the absolute m a x i m u m of the c o n d u c t i v i t y (e.g. i n single crystals of T T F T C N Q ) is the v a r i a t i o n i n s a m p l e p u r i t y a n d c r y s t a l p e r f e c t i o n . c r y s t a l defect amounts to a n i n t e r r u p t i o n i n the c o n d u c t i v e
Each
one-dimen-
s i o n a l s t r a n d , a n d , since e a c h j u m p b y a n e l e c t r o n to a n adjacent s t r a n d of these h i g h l y a n i s o t r o p i c m a t e r i a l s ( a n i s o t r o p y ratios of the c o n d u c t i v i t y as h i g h as 1 0 h a v e b e e n f o u n d ) r e q u i r e s c o n s i d e r a b l e energy, the
conduc-
tive properties
smallest
5
of these m a t e r i a l s are sensitive to
even
the
a m o u n t s of i m p u r i t i e s . O b s e r v a t i o n s w e r e s i m i l a r for i n o r g a n i c m i x e d v a l e n c e m a t e r i a l s w i t h l i n e a r stacks of t r a n s i t i o n m e t a l c o m p l e x ions s u c h as K C P (14,15,16,
17,
K C P is a P t ( I I - I V ) m i x e d v a l e n c e system t h a t is p r e p a r e d b y the
18,19).
p a r t i a l o x i d a t i o n of K P t ( C N ) 2
4
w i t h bromine. T h e most interesting con-
s e q u e n c e of this o x i d a t i o n is t h e s h r i n k i n g of the i n t e r p l a n a r spacings of the square p l a n a r P t ( C N )
4
2
' u n i t s f r o m 3.35 to 2.88 A , so that there is a
s h a r p l y i n c r e a s e d o v e r l a p of the p l a t i n u m 5d
2
z
AO's.
The initial
d
z
b a n d w i d e n s c o n s i d e r a b l y a n d , b e c a u s e of the r e m o v a l of electrons f r o m the d
l e v e l u p o n o x i d a t i o n , this b a n d is o n l y p a r t i a l l y filled, thus g i v i n g
2
z
rise to m e t a l l i c p r o p e r t i e s a l o n g the s t a c k i n g axis ( F i g u r e 2 ) . o n a n absolute scale the v a l u e of a (about
Although
500 O ^ c m " ) is n o t e x t r a o r d i 1
n a r y w h e n c o m p a r e d w i t h t h a t of t y p i c a l metals l i k e C u or A g (
