Electronic and Magnetic Properties of Linear Chain Complexes

11 Electronic and Magnetic Properties o f Linear Chain

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Complexes Derived from Biscyclopentadienyl Titanium(III) and o f the Infinite

RMX

Linear

3

Chain Complexes D.

SEKUTOWSKI,

R.

JUNGST,

and G .

D.

STUCKY

M a t e r i a l s Research L a b o r a t o r y , U n i v e r s i t y o f Illinois, U r b a n a , Ill. 61801

Introduction T h e f o c u s o f this p a p e r will b e o n the results of experimental studies of the structural and electronic properties o f two types o f linear chain systems containing m e t a l atoms:

and

The results we h a v e obtained for the biscyclopentadienyl c o m p l e x e s , w h i c h are recent a n d of a substantially m o r e preliminary nature, will be described first. A part o f o u r earlier work on the infinite linear tribridged chain systems will t h e n be described partly by way of introduction to the following two p a p e r s by Holt and McPherson on their studies of these materials. Biscyclopentadienyl

Titanium

Derivatives

T h e r e d u c t i o n c h e m i s t r y o f T i ( l V ) h a l i d e s was f i r s t e x t e n s i v e l y s t u d i e d as p a r t o f t h e development o f Z i e g l e r - N a t t a c a t a l y s t s i n t h e l a t e 1950 s. I t was recognized t h a t an important feature o f t i t a n i u m ( i l l ) chemistry i s t h e a b i l i t y of T i ( l l l ) t o a c t as a Lewis 1

142

Interrante; Extended Interactions between Metal Ions ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

11.

SEKUTOwsKi E T A L .

Linear

Chain

143

Complexes

a c i d , a n d one c a n o b t a i n a v a r i e t y o f c o m p l e x e s d e p e n d i n g upon t h e s o l v e n t , r e d u c i n g agent and a n i o n . For example, t h e r e d u c t i o n o f C p T i C l w i t h Zn p r o c e e d s ac cording to: 2

2Cp TiCl

+

2

ZnCl

2 C

} or

2Cp TiCl 2

+

2

2

H

i

e

[Cp TiCl] ZnCl 2

2

2

Zn DME

[Cp TiCl] 2

+

2

ZnCl

[Cp Ti(DME)] [Zn Cl ] 2

2

2

2

6

[Cp TiCl] 2

DME

=

+ 2

dimethoxyethane

T h e 2:1 c o m p l e x was f i r s t r e p o r t e d b y S a l z m a n n i n 1968 (JL). T h e 1:1 complex has not been p r e v i o u s l y r e p o r t e d a n d was o b t a i n e d b y a d d i t i o n o f d i m e t h o x y e t h a n e t o a s o l u t i o n o f t h e 2:1 complex. The s t r u c t u r a l c h e m i s t r y o f t h e s e mixed m e t a l T i ( l l l ) compounds i s i n t e r e s t i n g . The m o l e c u l a r s t r u c t u r e o f t h e 1:1 z i n c c o m p l e x a s p r e p a r e d w i t h d i m e t h o x y e t h a n e was r e c e n t l y d e t e r m i n e d b y u s a n d i s shown i n f i g u r e 1. I t contains [Cp Ti(DME)] c a t i o n s and the f i r s t example o f t h e Z n C l " a n i o n , w h i c h i s i s o e l e c tronic with Ga Cl . I t w i l l be i m p o r t a n t t o o u r l a t e r d i s c u s s i o n t o h a v e some i d e a o f w h a t one m i g h t e x p e c t f o r the i n t e r m o l e c u l a r magnetic i n t e r a c t i o n s of b i s cyclopentadienyl d systems. We h a v e e x a m i n e d t h e mag netic s u s c e p t i b i l i t y of [Cp Ti(DME)] [Zn Cl ]·C H to τ.5°Κ and found t h a t t h e i n t e r m o l e c u l a r exchange i s l e s s t h a n 1 cm"" . The m o l e c u l a r s t r u c t u r e o f t h e dibenzene s o l v a t e o f t h e 2:1 compound was d e t e r m i n e d b y u s a n d i n d e p e n d e n t l y b y V o n k (_2). O u r c o o r d i n a t e s a r e shown i n f i g u r e 2. The p a r a m a g n e t i c m o l e c u l e s a r e s e p a r a t e d from each o t h e r i n t h e l a t t i c e by benzene m o l e c u l e s . T h e Z n - C l - T i a n g l e i s 90° w i t h i n e x p e r i m e n t a l e r r o r , w h i l e t h e C l - T i - C l a n g l e i s 8 l . 9 ( l ) . The T i - Z n - T i a n g l e i s 173.2(l)° and t h e T i - T i d i s t a n c e i s 6 820(5)S. An obvious q u e s t i o n o f i n t e r e s t i s whether or not t h e r e i s any e v i d e n c e o f exchange c o u p l i n g between t h e two d m e t a l atoms w h i c h a r e s e p a r a t e d by t h e diamagn e t i c z i n c atom. T h e r o t a t i o n o f t h e two C p T i g r o u p s t h r o u g h 90° w i t h r e s p e c t t o e a c h o t h e r s u g g e s t s t h a t any exchange pathway t h r o u g h t h e z i n c atom w h i c h i n volves zinc o r b i t a l s which are orthogonal with respect t o a 90° r o t a t i o n a b o u t t h e l o n g o r t h o g o n a l a x i s o f t h e 2

2

2

2

6

6

1

2

2

2

6

e

2

1

#

1

2


6

144

EXTENDED

Figure

2.

INTERACTIONS

Molecular structure of [Cp TiCÏ] ZnCl Solvent molecules not shown. É

É

t

BETWEEN

METAL

· 2C H . R

6


IONS

11.


Linear

Chain

145

Complexes

m o l e c u l e must be f e r r o m a g n e t i c . Thus, exchange t h r o u g h ρ , ρ or s p h y b r i d o r b i t a l s on t h e z i n c atom would re sult i n ferromagnetic coupling. T o o u r k n o w l e d g e , t h e r e a r e no k n o w n e x a m p l e s o f l i n e a r o r n e a r l y l i n e a r t r i n u c l e a r m e t a l l i c systems i n w h i c h 1,3 m a g n e t i c e x c h a n g e h a s b e e n d e t e c t e d t h r o u g h a d i a m a g n e t i c m e t a l atom. S i n n (3) has r e p o r t e d on t h e m a g n e t i c i n t e r a c t i o n s o f a number o f t r i n u c l e a r s y s t e m s o f t h e t y p e shown b e l o w i n w h i c h t h e a p p r o x i m a t e a r rangement o f t h e m e t a l atoms i s a n i s o s c e l e s t r i a n g l e 3

f

w h e r e M = Cu a n d M i s a v a r i e t y of metals including Mg. No 1,3 m a g n e t i c c o u p l i n g was d e t e c t e d , a l t h o u g h i t s h o u l d be p o i n t e d o u t t h a t t h e m a g n e t i c d a t a were measured o n l y t o l i q u i d n i t r o g e n t e m p e r a t u r e and t h e a b s e n c e o f m a g n e t i c e x c h a n g e was b a s e d o n t h e m a g n i t u d e o f t h e W e i s s c o n s t a n t , Θ. S t u d i e s o f the tempera t u r e dependence o f t h e magnetic s u s c e p t i b i l i t y o f Ni (acac) h a v e shown t h a t a n a n t i f e r r o m a g n e t i c ex change between t h e t e r m i n a l n i c k e l atoms v i a t h e p a r a m a g n e t i c n i c k e l ( I I ) c e n t r a l atom i s n e c e s s a r y t o f i t the experimental data Q). T h e 2:1 t i t a n i u m - z i n c c o m p l e x e s d o i n f a c t e x h i b i t a n t i f e r r o m a g n e t i c b e h a v i o r a s shown b y t h e mag n e t i c s u s c e p t i b i l i t y d a t a ( f i g u r e 3) f o r t h e compound 3

6

[Cp TiBr] ZnBr *2C H 2

2

These d a t a were f i t by the s i n g l e t - t r i p l e t system:

X

m

= |^(τ?θ) t

1

+

x

/

3

2

Van

e x

P

6

6

Vleck

expression f o r a

(-2JAT)]'

1

The d a t a were measured a t a m a g n e t i c f i e l d g a u s s w i t h a Zeeman e n e r g y o f ^ 1 cm" . 1

+ Να o f 10 Κ In Table


1

146

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

2.00 r

Figure 3. Temperature depend ence of molar susceptibility and effective moment per titanium (BM) for lCp TiBr] ZnBr, · 2C H . Diamagnetic correction of —483 X lOr c.g.s. was applied. Theoretical curves cakuhted using: J = —15.66 cm , g = 1.94, θ 1.37°K, Να = 260 Χ lOr c.g.s. g

t

6

6

6

1

6

300

MO DIAGRAMS* FOR M(h -C H ) CI 5

5

0

ΠΊ4ρ

Tv4p

Figure 4. Molecular orbital di agrams for d° and d biscyclopentadienyl systems 1

Ti

d

v

V

cr

* ARROWS DESIGNATE NUMBER OF ELECTRONS IN HOMO


5

2

2

11.


Linear

Chain

Complexes

147

are the r e s u l t s of a s i m i l a r a n a l y s i s f o r the c h l o r i d e and f o r t h e d i m e r i c s p e c i e s ; [ C p T i X ] , X = CI, B r as r e p o r t e d b y C o u t t s , W a i l e s and M a r t i n (5). Martin (6) 2

Table

2

1 x

J(cm" ) [Cp TiCl] ZnCl -2C H

e

[Cp TiBr] ZnBr «2C H

6

2

2

2

2

[Cp TiBr]

2

2

2

2

e

6

J

l

J

B r

=

1.75

/ J ,C I =

I.60

*

/

J

C l

-78 •125

a

From r e f e r e n c e

-8.93

-15.66

a

[Cp TiCl]

a.

2

2

5.

has p o i n t e d out t h a t f o r a s u p e r e x c h a n g e mechanism, t h e t r a n s f e r i n t e g r a l and hence t h e exchange c o u p l i n g s h o u l d become l a r g e r a s t h e e l e c t r o n e g a t i v i t y o f t h e a n i o n d e c r e a s e s a n d t h i s a p p e a r s t o be t h e c a s e f o r t h e t r i m e t a l l i c systems as w e l l . However, as i n d i c a t e d below, t h i s s i m i l a r i t y may v e r y w e l l be f o r t u i t o u s a n d , i n f a c t , t h e r e i s e v i d e n c e t h a t t h e exchange mechanisms i n t h e b i n u c l e a r and t r i n u c l e a r systems a r e q u i t e different. F o r t u n a t e l y , t h e r e i s some e x p e r i m e n t a l a n d t h e o r e c t i c a l information a v a i l a b l e concerning the e l e c t r o n i c d i s t r i b u t i o n about t h e m e t a l atom i n b i s c y c l o p e n t a dienyl d systems w h i c h l e a d s t o a p l a u s i b l e mechanism f o r the observed antiferromagnetic coupling. D a h l and P e t e r s e n ( 7 ) 5 ( 8 ) have r e c e n t l y d e m o n s t r a t e d by s i n g l e c r y s t a l e s r e x p e r i m e n t s and p h o t o e l e c t r o n spectroscopy that the unpaired e l e c t r o n i n the d system, C p V C l , i s p r i m a r i l y i n a molecular o r b i t a l of a symmetry w h i c h i s 3-5 ev below t h e l o w e s t u n o c c u p i e d molecular o r b i t a l ( f i g u r e 4). The u n p a i r e d e l e c t r o n d e n s i t y i s p r i m a r i l y i n a metal o r b i t a l which i s perpendicular t o t h e t w o f o l d symmetry a x i s o f C p V C l and i n t h e p l a n e o f t h e VC1 group. A s m a l l e r amount o f u n p a i r e d d e n s i t y i s i n a d o r b i t a l which l i e s along the b i s e c t o r of t h e C l - V - C l g r o u p ( T a b l e 2). T h e same o r d e r i n g o f l e v e l s was o b t a i n e d t h e o r e t i c a l l y f o r C p T i C l , a n d i t seems l i k e l y t h a t a s i m i l a r e l e c t r o n i c d i s t r i b u t i o n about t h e t i t a n i u m atom w i l l p r e v a i l i n [ C p T i X ] Z n X (X = C I , B r ) . I n f i g u r e 5* two a n t i f e r r o m a g n e t i c e x c h a n g e p a t h ways a r e shown w h i c h a r e c o n s i s t e n t w i t h t h e s y m m e t r y o f t h e m o l e c u l e and w i t h t h e t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s o f D a h l and P e t e r s e n . B o t h a r e o f symmetry b i n the p o i n t group D . A d o r b i t a l can a l s o 1

1

2

2

x

2

2

2

2

2

2

2

2

h


2

2

Interrante; Extended Interactions between Metal Ions ACS Symposium Series; American Chemical Society: Washington, DC, 1974. 3.3

67.β

Cp

4.9

69.5

M

% Orbital

(a

y

22.3

z

0.3

0.4

ei (Humo) (Homo)

4.4

2

6.1

e

L. D a h l and J . P e t e r s e n P r i v a t e Communication

0.2

0.3

8-2

symmetry)

0.8

p

x

1-6.1

Cl

2

0.9

Character

Table

SEKUTOWSKi E T A L .

Linear

Chain

Complexes

x

Figure

5.

Exchange pathways for tallic Ti(III) complexes

trime-

0.40 r 0.35 h

ooo '

I

1

OO

'

600

'

'

I

1200

I

180.0

1

1

1

240.0

1

300.0

Degrees Kelvin Figure 6. Experimental data (A) and theoretical curve for tem perature dependence of molar susceptibility of [Cp TiCl] BeCk ' 2C H . Parameters used in curve generation: J = —6.89 cm' , g = 1.91, θ = 1.87°K, Να = 260 Χ ΙΟ c.g.s. t

6

1

2

G

6


150

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

be u s e d r a t h e r t h a n a ρ o r b i t a l g i v i n g a m o l e c u l a r orbital of a symmetry ( D . ) . We n o t e i n p a s s i n g t h a t the combination a t t h e bottom o f t h e f i g u r e which i n v o l v e s t h e more h i g h l y p o p u l a t e d d o r b i t a l (d i n the C n o t a t i o n o f Dahl and Petersen) i s only p o s s i b l e i f a n s o r d o r b i t a l i s a v a i l a b l e on t h e c e n t r a l atom. I n order t o determine i ft h epresence o f a d o r b i t a l i s e s s e n t i a l f o r t h e presence o f magnetic exchange i n t h i s s y s t e m , t h e compound [ C p T i C l ] B e C l * 2 C H was p r e p a r e d and r e c r y s t a l l i z e d from a benzene s o l u t i o n . I t i s isos t r u c t u r a l w i t h t h e z i n c compounds a n d h a s a t e m p e r a t u r e d e p e n d e n t m a g n e t i c s u s c e p t i b i l i t y , w h i c h i s shown in^figure_6 The value o f t h e coupling constant (-6.89 cm" ) i s comparable t o t h a t o f t h e z i n c c h l o r i d e compound. I t appears t h a t t h e a v a i l a b i l i t y o f d o r b i t a l s o n t h e c e n t r a l m e t a l atom i s n o t e s s e n t i a l f o r 1,3 e x c h a n g e . A s l i g h t l y d i f f e r e n t t y p e o f one d i m e n s i o n a l c h a i n system i s o b t a i n e d w i t h l a r g e r metal i o n s i n t h e central position. With Mn(ll) i ntetrahydrofuran ( T H F ) , t h e s t r u c t u r e , a s s h o w n i n f i g u r e 7, c o n t a i n s a s i x c o o r d i n a t e c e n t r a l m e t a l atom. The spin state cor r e l a t i o n diagram f o r a d - d - d s y s t e m ( f i g u r e 8) s u g g e s t s two p o s s i b l e ground s t a t e s f o r < 1* i.e., forJ < J . Here, S represents the t o t a l s p i n s t a t e o f t h e s y s t e m a n d S* i s t h e s p i n s t a t e o b t a i n e d b y c o u p l i n g t h e 1,3 a t o m s . J has been abbrevi a t e d as J . T h e (3/2, l ) ground s t a t e w i l l be o b t a i n e d i n t h e above range f o r a l l J < 0, w h i l e t h e (7/2,1) s t a t e w i l l be l o w e s t i n e n e r g y f o r J > 0. T h e e x perimental μ curve f o r [ C p T i C l ] M n C l ( T H F ) i s shown i n f i g u r e 9 . The value o f M extrapolates t o ^ ~ 3 . 8 BM a t 0 ° K w h i c h i s w i t h i n e x p e r i m e n t a l e r r o r o f t h e s p i n o n l y v a l u e o f 3 . 8 7 BM f o r t h r e e u n p a i r e d e l e c t r o n s and a q u a r t e t ground s t a t e , i . e . , J must be n e g a t i v e w i t h a n t i f e r r o m a g n e t i c c o u p l i n g between t h e t i t a n i u m a n d manganese atoms. I t was p o s s i b l e t o o b t a i n an approximate value f o r J o f -o c m " b y f i t t i n g the experimental data, but J c o u l d n o t be determined w i t h any a c c u r a c y s i n c e o n l y s m a l l changes i n t h e c a l c u l a t e d magnetic s u s c e p t i b i l i t i e s over a narrow temperature range r e s u l t e d from l a r g e changes i n J . Zero f i e l d s p l i t t i n g a t t h e d i o n has been neglected i n t h e above c o n s i d e r a t i o n s . P o l y c r y s t a l l i n e e s r m e a s u r e m e n t s h a v e b e e n made a n d i n d i c a t e a z e r o f i e l d s p l i t t i n g o f 0.1 c m " f o r t h i s m a t e r i a l . Further studies o f c^-c^-d coupling arei n progress. I n a d d i t i o n t o t h eb r i d g i n g groups and t h e c e n t r a l m e t a l atom, a n o t h e r p a r a m e t e r w h i c h c a n i n f l u e n c e t h e exchange c o u p l i n g i n b i s c y c l o p e n t a d i e n y l d complexes x

2

2

2

2

2

2

e

6

#

1

1

5

1

IJ13/J12I

f

1

3

1

2

1

f

f

1

2

1

2

2

2

2

2

2

e f f

1

2

1

1

1

2

3

1

5

1

1

1


3


Figure

7.

Molecular

Linear

Chain

Complexes

structure of [Cp TiCl] MnCh(THF) . soids shown at 50% probability level. t

g

2

Thermal

ellip-

Figure 8. Spin state correlation diagram for (P-cP-d systems. S' is total spin state, S* is spin state obtained by coupling only the terminal paramagnetic centers, J is the exchange integral between terminal and central metals, and ] is exchange integral between terminal metals. 1

u


152

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

i s the c y c l o p e n t a d i e n y l r i n g . The m a g n e t i c s u s c e p t i b i l i t y data f o r [ C p T i C l ] shows a s m a l l a n o m a l y j u s t above the t r a n s i t i o n temperature which M a r t i n suggests may be due t o t h r e e d i m e n s i o n a l c o o p e r a t i v e i n t e r a c t i o n s (5)*(9). The most l i k e l y mechanism f o r t h e t h r e e dimensional c o o p e r a t i v e i n t e r a c t i o n would i n v o l v e the hindered r o t a t i o n of the c y c l o p e n t a d i e n y l groups. Do c o o p e r a t i v e p h e n o m e n a o f t h i s n a t u r e h a v e a n y e f f e c t on m a g n e t i c exchange i n t h i s m o l e c u l e o r i n t h e t r i nuclear molecules? When t h e m o n o - m e t h y l s u b s t i t u t e d derivative, [(MeCp) TiCl] i s e x a m i n e d , one f i n d s t h e r e s u l t s shown i n f i g u r e 10 f o r t h e m a g n e t i c s u s c e p t i bility. T h e r e i s more t h a n a t w o f o l d i n c r e a s e i n t h e v a l u e of J t o ^166 cm"" . No s t r u c t u r a l d a t a h a v e b e e n r e p o r t e d f o r [ C p T i C l ] , however, a model b a s e d on t h e structural results for [Cp TiCl ] Zn (figure l l ) s t r o n g l y s u g g e s t s t h a t t h e c y c l o p e n t a d i e n y l r i n g s may be c o u p l e d v i a i n t r a m o l e c u l a r i n t e r a c t i o n s b e t w e e n c y c l o p e n t a d i e n y l r i n g s o n t h e same t i t a n i u m a t o m a n d b e t w e e n c y c l o p e n t a d i e n y l r i n g s on t h e d i f f e r e n t t i t a n i u m atoms w i t h i n a d i m e r . We w o u l d s u g g e s t t h a t t h e mean T i - T i d i s t a n c e w i t h i n a d i m e r f o r t h e v a r i o u s rotameric isomers which are obtained a t h i g h temperat u r e w i l l p r o b a b l y be g r e a t e r t h a n t h a t f o r t h e g r o u n d s t a t e low t e m p e r a t u r e r o t a m e r i c i s o m e r . I n t h e compound [ ( M e C p ) T i C l ] , t h e m e t h y l group " l o c k s i n " t h e ground s t a t e isomer, i . e . , r a i s e s the p o t e n t i a l b a r r i e r to r o t a t i o n . T h i s h a s some i n t e r e s t i n g i m p l i c a t i o n s . I t s u g g e s t s , f o r example, t h a t the o r d e r o f Br I ^ C1 y M a r t i n (5) f o r [ C p T i X ] c o m p l e x e s i s aue t o t h e a n o m a l o u s b e h a v i o r o f X = C I and not X = I , i . e . , t h e i n c r e a s i n g m a g n e t i c exchange does not f o l l o w d e c r e a s i n g e l e c t r o n e g a t i v i t y as noted above, but i n s t e a d f o l l o w s a decrease i n the T i - T i distance. The p r e d o m i n a n t mechanism f o r exchange w o u l d t h e n be a d i r e c t m e t a l - m e t a l i n t e r a c t i o n . T h i s i s cons i s t e n t with the unpaired s p i n d e n s i t y d i s t r i b u t i o n f o u n d by D a h l , w h i c h , however, does n o t e x c l u d e superexchange v i a the b r i d g i n g l i g a n d s . A d d i t i o n a l magnetic s u s c e p t i b i l i t y , h e a t c a p a c i t y , and s t r u c t u r a l s t u d i e s a r e now i n p r o g r e s s a t t h e U n i v e r s i t y o f I l l i n o i s i n o r d e r t o more c l e a r l y d e f i n e t h e a b o v e a n d r e l a t e d s y s tems. I f t h e a b o v e m o d e l i s v a l i d , one w o u l d n o t e x p e c t a s i g n i f i c a n t d i f f e r e n c e i n the magnetic exchange upon methyl s u b s t i t u t i o n i n the t r i n u c l e a r [Cp TiCl] ZnCl compounds. The m a g n e t i c s u s c e p t i b i l i t y data f o r [ ( M e C p ) T i C l ] Z n C l i s shown i n f i g u r e 12 a n d g i v e s a v a l u e o f J o f -7.3 cm"" w h i c h i s comparable to that of the unsubstituted m a t e r i a l . 2

2

2

2

1

2

2

2

2

J

>

J

J

o

2

2

2

b

s

e

r

v

e

d

b

2

2

2

2

2

2

2

1


2

11.

SEKUTowsKi E T A L .

Linear

Chain

Complexes

Figure 9. Temperature dependence of effective moment (BM) per trimer of [Cp TiCl] MnCl (THF) . Diamagnetic correction of —416 X 10~ c.g.s. was applied. 2

2

2

2

6

002 h QOO « QO

ι

I

6O0

'

1

I

I

120.0

1800

I

I

24O0

1

1 3000

°K Figure 10. Temperature dependence of molar susceptibility of [(MeCp) TiCl] . Theoretical curve calculated using J = —166 cm , θ = 0.87°Κ,g = 2.06, Να = 260 Χ ΙΟ c.g.s. 2

2

1

Figure 11. Intramolecular of cyclopentadienyl rings in complexes

6

interactions [Cp TiX] 2

2


154

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

F i n a l l y , we would l i k e t o note t h a t numerous o t h e r c o m b i n a t i o n s o f m e t a l i o n s ( e . g . , z i r c o n i u m and vanadium) and b r i d g i n g group ( e . g . , - N R j -PR * -OR, -SR) a r e p o s s i b l e i n t h i s s e r i e s and a s y s t e m a t i c ex p e r i m e n t a l s u r v e y o f t h e exchange i n t h e s e compounds can be e x p e c t e d t o p r o v i d e , a t t h e v e r y l e a s t , a n em p i r i c a l u n d e r s t a n d i n g and c l a s s i f i c a t i o n o f t h e e l e c t r o n i c c o u p l i n g o f t r i a t o m i c m e t a l systems. 2

RMX

3

2

Complexes

Compounds o f t h e t y p e RMX (R = u n i p o s i t i v e c a t i o n , M = divalent t r a n s i t i o n metal c a t i o n , X = halo gen) form a l a r g e c l a s s o f one d i m e n s i o n a l svstems w i t h t h e g e n e r a l s t r u c t u r e shown i n f i g u r e 13 (10;. The one d i m e n s i o n a l p r o p e r t i e s have been b e a u t i f u l l y demon s t r a t e d by l o w t e m p e r a t u r e nmr and n e u t r o n d i f f r a c t i o n s t u d i e s ( l l ) . The f a c t t h a t one i s n o t d e a l i n g w i t h a c o m p l e t e l y i s o l a t e d one d i m e n s i o n a l c h a i n i s e v i d e n t i n t h a t t h e R group c a n be used t o "tune" t h e e l e c t r o n i c p r o p e r t i e s o f the t r a n s i t i o n metal as i l l u s t r a t e d i n T a b l e 3. Thus, one f i n d s t h a t t h e r e i s a change o f 0.141 i n t h e n i c k e l - n i c k e l d i s t a n c e g o i n g from R = ( C H ) N t o R = Κ . The c o r r e s p o n d i n g change i n 10 Dq i s 750 cm" and t h e C u r i e - W e i s s c o n s t a n t changes from 0 t o -112°K ( R b N i C l ) . There a r e few, i f any, systems i n w h i c h t h e f i r s t c o o r d i n a t i o n sphere o f t h e m e t a l atom c a n be s o s y s t e m a t i c a l l y and s u b t l y v a r i e d . An i m p o r t a n t q u e s t i o n w h i c h has become o b v i o u s t o many p e o p l e d u r i n g t h e p a s t y e a r i s , "Can one p r e d i c t i f a one d i m e n s i o n a l s t r u c t u r e s u c h a s t h a t shown i n f i g u r e 13 w i l l be o b t a i n e d ? " We have examined t h i s q u e s t i o n from a s t r i c t l y e m p i r i c a l p o i n t o f view w i t h t h e r e s u l t s shown i n f i g u r e 14 and T a b l e 4. There a r e a t l e a s t f i v e d i f f e r e n t configurât i o n s adopted b y RMX compounds ( f i g u r e 1 4 ) : p r i m i t i v e c u b i c , h e x a g o n a l , and v a r i o u s c o m b i n a t i o n s o f h e x a g o n a l (one d i m e n s i o n a l ) and p r i m i t i v e c u b i c packed s t r u c t u r e s . The p u r e l y h e x a g o n a l c o n f i g u r a t i o n i s t h e 2L, one d i m e n s i o n a l s t r u c t u r e d i s c u s s e d p r e v i o u s l y . 2L r e f e r s t o the c r y s t a l l o g r a p h i c repeat d i s t a n c e i n t h e l i n e a r c h a i n d i r e c t i o n . The r a t i o o f h e x a g o n a l ( l i n e a r ) t o c u b i c p a c k i n g i n c r e a s e s f r o m 6 L t o 4L t o 9L. Remembering t h i s , t h e n , a n e x a m i n a t i o n o f known RMX s t r u c t u r e s r e v e a l s t h a t t h e h e x a g o n a l 1-d con f i g u r a t i o n i s f a v o r e d by: ( l ) I n c r e a s e d CFSE ( c r y s t a l f i e l d s t a b i l i z a t i o n energies) 2) L a r g e r X groups 3/ L a r g e r R groups ( t o a p o i n t ; ( C 5 H 5 ) N 3

3

4

1

3

3

3

+

4



Linear

Chain

Complexes

0.32 ρ 0.28 0.24 ^ χ

0.20 I

S 0.16 25 §" 0.121

C 3O

CO

0.08 004 0.00J

OO

600

1200

1800

3000

240.0

Degrees Kelvin Figure 12. Temperature dependence of molar susceptibility of [(MeCp) TiCl] ZnCl . Theoretical curve calculated using J = -7.26cm , g = 1.92, θ = 0.69°Κ, Να = 260 Χ ΙΟ c.g.s. B

g

a

1

6

Figure 13. One-dimensional structure RMX compounds. R = (CH ),,N\ M = and X = Cl (10). 3

S

of Ni



4

3

3

CsNiBr

RbNiBr

3

(CH ) NNiBr

3

3

RbNiCl

KNiCl

3

4

CsNiCl

3

3

(CH ) NNiCl

3

3.10(1)

3.12(1)

11,300 11,300

6600

11,600

6500

6300

7100

2.91(1)

3.17(1)

(14),(15)

-112

12,400

7000

2.9^(1) 12,300

(14),(15)

-76

12,600

6800

2.96(1)

-156

(14),(15)

-101 ( 1 4 ) , ( 1 5 )

0 (13)

13,450

6350

C u r i e Weiss Θ(°Κ)

3.054(5)

- 1

15B'(cm )

-1

10Dq(cm )

Ni-Ni distance Ά

Table 3

11.


Linear

Chain

Table

Complexes

4

Summary o f S t r u c t u r e s o f C s M X

3

Compounds

F

Cl

Br

I

(1.36)

(1.81)

(1.95)

(2.16)

—

2L (16)

2L

(17)

2L (18)

V(0.87)

—

2L (19)

2L (20)

2L (18)

Cr(0.84)

—

2L (21)

2L (22)

2L (18)

(24)

2L (20)

—

2L (20)

—

Mg(0.65)

a

Mn(0.8o)

6L ( 2 3 )

Fe(0.76)

6L

(25) 2L (26)

Co(0.78)

9L

(21) 2L (28)

Ni(0.78)

2L (2g) 2L (30)

2L (31) 4L (22)

9L

Cu(0.69)

—

2L (32)

Cd(0.97)

—

6L (32)

—

—

2L (18)

Summary o f S t r u c t u r e s o f RMC1 Compounds 3

(Me) N

Cs

Rb

K

(2.60)

(1.69)

(1.48)

(1.33)

4

Vo(0.87) Cr(0.84) Mn(0.80)

a

—

2L (19)

—

—

2L (16)

—

2L (34)

2L (19) —

9L (18)

6L (35)

tetragonal

Fe(0.76)

—

2L (25)

2L (25)

—

Co(0.78)

—

2L (28)

2L (36)

—

2L (30)

2L (32)

Ni(0.78) Cu(0.69) Cd(0.97) a.

2L (11) —

2L ( 3 3 )

T h e numbers e a c h atom.

2L (38)

—

6L (33)

—

i n parentheses

3L (20) 4L (39)

are the ionic

radius


EXTENDED

( |+1/2 > t r a n s i t i o n of the esr s p e c t r a of the V bromine and i o d i n e com p l e x e s ( f i g u r e 1 5 ) . The s p i n H a m i l t o n i a n p a r a m e t e r s , e x c l u d i n g t h e l i g a n d h y p e r f i n e t e n s o r s , a r e shown i n T a b l e 5. The i n c r e a s e i n c o v a l e n c y C l ~ ·> B r " -> I " i s evident i n both the decrease i n the metal hyperfine c o n s t a n t s and i n t h e changes i n t h e g t e n s o r . I n f a c t , we o b s e r v e f o r t h e i o d i n e complex a v e r y u n u s u a l ex ample o f an e a r l y t r a n s i t i o n element i n a n e a r l y o c t a h e d r a l f i e l d w i t h both g and g g r e a t e r t h a n t h e f r e e e l e c t r o n g f a c t o r o f 2.0023. T h i s i s q u a l i t a t i v e l y e x p l i c a b l e i n terms o f M c G a r v e y s t h e o r y ( 4 2 ) , ( 4 3 ) o f c o v a l e n c y f o r d systems i n w h i c h t h e l i g a n d m o l e c u l a r o r b i t a l c o e f f i c i e n t s enter i n t o the expression f o r the g t e n s o r , w e i g h t e d by t h e l i g a n d s p i n o r b i t a l c o u p l i n g constant. The l a s t e x p e r i m e n t a l o b s e r v a t i o n t h a t w i l l be d e s c r i b e d i s i l l u s t r a t e d by t h e s i n g l e c r v s t a l e l e c t r o n i c a b s o r p t i o n s p e c t r u m o f C s C r C l (21) ( f i g u r e 16). The c o n c e n t r a t i o n dependence o f t h i s s p e c t r u m i n C s M g C l i s a l s o shown. Two f e a t u r e s o f i n t e r e s t i n t h e s p e c t r u m a r e ( l ) a band a t 22,000 cm" w h i c h i s not ex p l a i n e d by l i g a n d f i e l d c a l c u l a t i o n s and (2) an en hanced i n t e n s i t y o f s p i n f o r b i d d e n t r a n s i t i o n s by ap p r o x i m a t e l y an o r d e r o f magnitude. The p o l a r i z a t i o n o f t h e 22,000 cm" band i s p a r t i c u l a r l y s t r o n g and i t s t e m p e r a t u r e dependence i s not t h a t e x p e c t e d f o r a v i b r o n i c mechanism. The e x p l a n a t i o n f o r t h i s e f f e c t was f i r s t p r o p o s e d by D e x t e r (44) and l a t e r e l a b o r a t e d upon by Day (45) and o t h e r s . In i t s s i m p l e s t form ( f i g u r e 17), a s i n g l e photon r e s u l t s i n t h e f o r m a t i o n o f e i t h e r two e x c i t o n s o r an e x c i t o n and a magnon. I n b o t h c a s e s , t h e t o t a l s p i n symmetry i s c o n s e r v e d . I n C s C r C l , the former r e s u l t s i n a band a t a p p r o x i m a t e l y t w i c e 10 Dq, t h e l a t t e r r e s u l t s i n an enhanced a l l o w e d n e s s f o r t h e s p i n f o r b i d d e n q u i n t e t - t r i p l e t and q u i n t e t - s i n g l e t t r a n s i t i o n . I n summary, t h e i n f i n i t e one d i m e n s i o n a l complexes, RMX , p r o v i d e an u n u s u a l l y b r o a d and i n t e r e s t i n g c l a s s 4

3

->

2

fJ

x

1

3

3

3

1

1

3

3



h

(77°K)

(297°K)

4

l

+ 70 + 2.

4

+ 74. + 1.

B(xl0 cm"

x

+ 70. + 2.

4

A ( x l 0 c m " ) = + 76. + 1.

4

+ 2 + 65. + 2

+ 65.

+ 208O. + 30

2.035 + .002

1.999 + .002

= 1.9760 + .001

2.038 + .002

+ 2.

+ 67.

1.996 + .002

+ 2.

+ 67.

+ 2 1 6 0 + 50.

2.04 + .01

1.9740 + .0006

+ 128Ο + 15.

u

+ 70. + 2.

+ 1320. + 15.

1.999 + .002

3

2.04 + .01

CsMgI

+ 70+2.

+ 1.

+ 4.

3

1.996 + .002

CsMgBr

+ 74. + 1.

= + 76.

+ 961.

= 1.9750 + .0006

1.9730 + .0006

3

Parameters

D ( x l 0 c m " *) = + 8 5 8 . + 7.

g

B(xl0 cm" )

4

A(xl0 cm"

4

D(xi0 cm"

iι

g,,

CsMgCl

Spin Hamiltonian

Table 5

11.

SEKUTOWSKi E T A L .

Linear

Chain

Complexes

161

6.4% Cr^CsMgCI

3

5.0 ί

10.000

15.000

25.000

20.000

Figure 16. Electronic absorption spectrum at 77°K of CsCrCh and CsCrCls doped into CsMgCl (16). Spectra with Ε polarized parallel and perpendicular to the crystallographic c axis are denoted by \ \ and l . c values are in liter mole cm .

29% Cr^CsMgCl

s

5.000

5.000

10,000

10.000

6.000

20.000

25.000

15.000

20,000

25.000

1

(cm ) 1

M°=l S =2 a

ΙΙΙΦ

m-

Cr'

Cr'

ΔΜ;=+Ι

ΔΜ =0

ΔΜ =0

8

8

AS=0

ν - · - exciton • exciton

Figure

17.

Simultaneous

T

ΔΜΪ=*1

ΔΜ!=

pair

AS=0 hi/ w r * - e x c i t o n • magnon

excitation

model

(44, 45)


1

162

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

of m a t e r i a l s f o r t h e i n v e s t i g a t i o n o f c o o p e r a t i v e electronic effects. Acknowledgment. The s u p p o r t o f t h e N a t i o n a l S c i e n c e F o u n d a t i o n under G r a n t s NSF-GH-33634 and GP-31016X a r e g r a t e f u l l y acknowledged. The e x p e r i m e n t a l m a g n e t i c s u s c e p t i b i l i t y measurements were made w i t h t h e g r e a t l y appreciated a s s i s t a n c e o f P r o f e s s o r David Hendrickson of t h e U n i v e r s i t y o f I l l i n o i s . Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Salzmann, J. J., H e l v . Chim. A c t a ( 1 9 6 8 ) , 5 1 , 5 2 6 . Vonk, C. G., J. Cryst. M o l . Struct. ( 1 9 7 3 ) , 3, 201. G r u b e r , S. J., Harris, C. Μ., and Sinn, Ε., J. Chem. Phys. (1968), 49, 2183. G i n s b e r g , A. P., Martin, R. L., and Sherwood, R. C., I n o r g . Chem. ( 1 9 6 8 ) , 7 , 932. C o u t t s , R. S. P., W a i l e s , P. C., and Martin, R. L., J . O r g a n o m e t a l l i c Chem. (1973), 47, 375.

M a r t i n , R. L., in "New Pathways in I n o r g a n i c C h e m i s t r y , " Pages 175-231, E. A. U. E b s w o r t h , A. G. Maddock and A. G. S h a r p e , Cambridge P r e s s , London ( 1 9 6 8 ) . D a h l , L. F. and P e t e r s e n , J. L., private communication. P e t e r s e n , J. L. and D a h l , L. F., J. Amer. Chem. Soc. ( 1 9 7 4 ) , 9 6 , 2248. M a r t i n , R. L. and W i n t e r , G., J. Chem. Soc. (1965),

1965,

4709.

S t u c k y , G. D., A c t a C r y s t . ( 1 9 6 8 ) , B24, 330. Y e l o n , W. B. and Cox, D. Ε., Phys. Rev. B. ( 1 9 7 3 ) , 7 , 2024, and included r e f e r e n c e s . Goodgame, D. M. L. and Weeks, M. J., J. Chem. Soc. ( 1 9 6 4 ) , 1 9 6 4 , 5 1 9 4 . Asmussen, R. W. and Soling, Η., Z. A n o r g . A l l g e m . Chem. (1965), 203, 3. Asmussen, R. W. and B o s t r u p , Ε., A c t a Chem. Scand. (1957), 11, 745.

Li, Ting-i, S t u c k y , G. D. and McPherson, G., A c t a C r y s t . (1973), B29, 1330. McPherson, G. L., K i s t e n m a c h e r , T. and S t u c k y , G. D., J. Chem. Phys. ( 1 9 7 0 ) , 5 2 , 815. McPherson, G. L. and S t u c k y , G. D., J. Chem. Phys. (1972),

57,

3780.

Li, Ting-i, S t u c k y , G. D. and McPherson, G. L., A c t a C r y s t . ( 1 9 7 3 ) , B 2 9 , 1330.


11.

SEKUTOWSKI

19. 20. 21.

22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.

E T AL.

Linear

Chain

Complexes

163

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Electronic and Magnetic Properties of Linear Chain Complexes

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