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MÖSSBAUER SPECTROSCOPIC STUDIES OF SPIN CROSSOVER COMPOUNDS
P. Gütlich
To cite this version:
P. Gütlich. MÖSSBAUER SPECTROSCOPIC STUDIES OF SPIN CROSSOVER COMPOUNDS.
Journal de Physique Colloques, 1979, 40 (C2), pp.C2-378-C2-385. �10.1051/jphyscol:19792133�. �jpa-
00218505�
JOURNAL DE PHYSIQUE
Colloque C2, supplkment au n o 3 , Tome 40, mars 1979, page
C2-378MOSSBAUER S?ECTROSCOPIC STUDIES OF SPIN CROSSOVER COMPOUNDS
I n s t i t u t fiir Anorganische und AnaZytische Chemie, Johannes Gutenberg-hiversit&t, 0-6500 Mainz, Bundesrepub Zik Deutsch land
R6sum6.- On donne une b r s v e d e s c r i p t i o n i n t r o d u c t i v e du ph6nomGne d e r e n v e r s e m e n t d e s s p i n s d a n s d e s compos6s o r g a n o m 6 t a l l i q u e s . L ' e f f e t Massbauer du "Fe s ' e s t montr6 p a r t i c u l i s r e m e n t e f f i c a c e pour 6 t u d i e r l e s e f f e t s de r e n v e r s e m e n t d e s p i n e t l e s a l t 6 r a t i o n s r e s u l t a n t d ' i n f l u e n c e s c h i m i q u e s t e l l e s que s u b s t i t u t i o n d e l i g a n d s , d i l u t i o n m e t a l l i q u e , n a t u r e du c r i s t a l s o l v a n t ou d e l ' a n i o n e t s u b s t i - t u t i o n i s o t o p i q u e . On donne d e s exemples t y p i q u e s a u x s e c t i o n s 2 , 3 , 4 , e n i n s i s t a n t p a r t i c u l i s r e m e n t s u r l e c o n c e p t de t r a n s i t i o n c o o p 6 r a t i v e d e s p i n i m p l i q u a n t d e s v i b r a t i o n s d e r 6 s e a u e t i n t r a m o l 6 c u - l a i r e . La c o n c l u s i o n e s t une b r s v e r e v u e d e s m o d s l e s t h 6 o r i q u e s d e c r i v a n t l e phenomsne d e r e n v e r s e - ment d e s p i n .
A b s t r a c t . - A b r i e f i n t r o d u c t o r y d e s c r i p t i o n of t h e s p i n c r o s s o v e r phenomenon i n o r g a n o m e t a l l i c com- pounds i s g i v e n . 5 7 ~ e MGssbauer s p e c t r o s c o p y h a s p r o v e n p a r t i c u l a r l y u s e f u l i n t h e s t u d y o f t h e s p i n c r o s s o v e r b e h a v i o u r and i t s a l t e r a t i o n by c h e m i c a l i n f l u e n c e s s u c h a s l i g a n d s u b s t i t u t i o n , m e t a l d i - l u t i o n , n a t u r e o f c r y s t a l s o l v e n t and a n i o n , and i s o t o p i c s u b s t i t u t i o n . S e l e c t e d examples a r e d e s c r i - bed i n s e c t i o n s 2 , 3 and 4 , w i t h p a r t i c u l a r e m p h a s i s on t h e c o n c e p t o f c o o p e r a t i v e s p i n t r a n s i t i o n i n v o l v i n g i n t r a m o l e c u l a r and l a t t i c e v i b r a t i o n s . The l e c t u r e c o n c l u d e s w i t h a b r i e f s u r v e y o f theo- r e t i c a l models which d e s c r i b e t h e s p i n c r o s s o v e r phenomenon.
1 . I n t r o d u c t i o n . - From l i g a n d f i e l d t h e o r y i t i s known t h a t t r a n s i t i o n m e t a l complexes w i t h d \ d 5 , d 6 , d 7 and d 8 e l e c t r o n c o n f i g u r a t i o n s u s u a l l y e x i s t i n e i t h e r h i g h s p i n (HS) o r low s p i n (LS) e l e c t r o n i c ground s t a t e s . The a c t u a l s p i n s t a t e depends o n t h e e n e r g y b a l a n c e due t o v a r i o u s k i n d s o f p e r t u r b a t i o n s e x e r t e d on t h e v a l e n c e e l e c t r o n s o f t h e t r a n s i t i o n m e t a l i o n . The s i m p l e i n e q u a l i t y
A(HS)< < A(LS) ( 1 )
e x p r e s s e s t h e c o n d i t i o n s f o r f i n d i n g a complex t r a n - s i t i o n m e t a l i o n i n e i t h e r HS ground s t a t e (A < P ) o r LS ground s t a t e (A > T ) ; c f . f i g u r e 1 . P i s t h e - mean s p i n p a i r i n g e n e r g y , which c a n be f o r m a l l y
t h o u g h t of b e i n g composed of two p a r t s , v i z . t h e Coulomb e n e r g y and t h e exchange e n e r g y , and which may b e e x p r e s s e d i n t e r m s o f t h e Racah p a r a m e t e r s B and C / 3 , 4 / . F o r o c t a h e d r a l o r p s e u d o o c t a h e d r a l i r o n (11) complex compounds f o r i n s t a n c e , which w i l l b e m o s t l y d e a l t w i t h i n t h e p r e s e n t a r t i c l e , r e c e n t e s t i m a t e s o f
P
h a v e y i e l d e d a v a l u e of 12 800+
400 cm-I 151. A i s t h e l i g a n d f i e l d s t r e n g t h . I t i s a complex q u a n t i t y o r i g i n a t i n g from p u r e l y e l e c t r o - s t a t i c e f f e c t s a s w e l l a s b o n d i n g e f f e c t s , i . e . m e t a l - l i g a n d u and n i n t e r a c t i o n s 121. A c a n be de- t e r m i n e d by e v a l u a t i n g u . v . / v i s . s p e c t r a i n connec- t i o n w i t h model c a l c u l a t i o n s which i n c l u d e , whenever n e c e s s a r y , a x i a l and rhombic f i e l d d i s t o r t i o n s from c u b i c ( o c t a h e d r a l o r t e t r a h e d r a l ) s y m u e t r y , c o n f i g u - r a t i o n i n t e r a c t i o n , s p i n o r b i t c o u p l i n g and covalen- cy e f f e c t s .
F i g . I : S c h e m a t i c e n e r g y l e v e l d i a g r a m f o r c r y s t a l f i e l d s t a t e s a s f u n c t i o n o f t h e l i g a n d f i e l d s t r e n g t h A. At A=O ( f r e e i o n c a s e ) , t h e c o u p l i n g between t h e v a l e n c e e l e c t r o n s o f t h e t r a n s i t ' n m e t a l i o n y i e l d t h e p u r e R u s s e l l - S a u n d e r s t e r m s $ j + ' L , w i t h the h i g h s p i n (HS) s t a t e b e i n g l o w e s t by v i r t u e of Hund's f i r s t r u l e . At A>O ( l i g a n d s a t t a c h e d t o m e t a l i o n ) , t h e R u s s e l l - S a u n d e r s t e r m s s p l i t i n t o c r y s t a l f i e l d s t a t e s 2Sc1r, t h e number and i r r e d u c i b l e r e p r e s e n t a - t i o n
r
of which are-determined by t h e m o l e c u l a r sym- m e t r y / 1 , 2 / . For A<P mean s p i n p a i r i n g e n e r g y ) , t h e HS c r y s t a l f i e l d s t a t eix
ground s t a t e . At a c r i - t i c a l f i e l d s t r e n g t h A c r i t , P becomes e q u a l t o A and t h e HS and t h e LS (low s p ~ n ) c r y s t a l f i e l d s t a t e s c r o s s . At h>F, t h e LS c r y s t a l f i e l d s t a t e i s more s t a b l e ( c o n t r a r y t o Hund s f i r s t r u l e ) . C r y s t a l f i e l d ground s t a t e s o f i n t e r m e d i a t e s p i n (LS) a r e r a r e l y e n c o u n t e r e d . I n t h e r e g i o n where A becomes comparable t o P (AZP, shadowed a r e a ) , t h e r m a l l y i n d u c e d h i g h s p i n + l o w s p i n t r n s i t i o n i s p o s s i b l e , p r o v i d e d t h e e n e r g y d i f f e r e n c e YA-HJ i s on t h e o r d e r o f kT.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792133
In first-row traniition metal chemistry by far the largest portions of the complex compounds of d4, d5, d6, d7 and d8 electron configurations have only HS ground states or LS ground states. Only a few well-authenticated examples of intermediate spin state are known in the coordination chemistry of iron (III)/6,7/ and iron(I1) 181. A relatively small nun- ber of transition metal complex compounds have been found to exhibit a temperature dependent H S e L S transition, with both spin states coexisting as ground states in equilibrium, whereby the HS ground state prevails at higher temperatures and the LS ground state is the predominant one at lower tempe- ratures; cf. the review articles by Martin and White 191, Sacconi 1101, K h i g Ill/ and Goodwin 1121 and the references given therein. The more or less gra- dual conversion from one spin state to the other as a function of temperature has been followed by the use of various techniques, primarily magnetic suscep- tibility (anomalous behaviour of x(T)) and ~Essbauer spectroscopy (evidence of coexisting spin states with distinctly different ~ossbauer parameters), and in fewer cases by means of vibrational spectroscopy (change in metal-ligand stretching as well as in intra-ligand vibrational frequencies), u.v./vis s p e c troscopy (change of
A),
and x-ray studies (change in metal-ligand bond length, 'HS > 'LS)'Particularly numerous examples of a thermally induced spin transition otherwise termed as magnetic crossover or spin crossover, have been found in six- coordinate iron(I1) complexes 1121. Many of them contain nitrogen coordinating bifunctional ligands (di-imines) such as derivatives of 1,lO-phenanthro- line (phen)
,
forming socalled tris-complexes such as [Fe ( ~ - C H ~ - ~ h e n ) J(~l0r) 2 and related compounds or mixed-ligand complexes such as [Fe (phen),
(NCS) 2 1.
Other examples contain the bifunctional di-imine li- gands 2-picolylamine (2-pic) and 2-(2-pyridy1)- imidazole. There are also iron (XI) spin crossover systems with tridentate (trifunctional) ligands, again with nitrogen as the coordinating atom, such as. the bis-complex
@e(papt~)2] C12.2H20 (paptH = 2-(2-pyridylamino) -4-(2-pyridy1)-thiazole) References to these examples and many others of iron (11) not mentioned here are given in the review ar- ticle by Goodwin / 121. [Fe(isoxazole) J(~10~) 2 is the only example reported on so far for a spin cross- over system with iron(I1) octahedrally coordinated with six monodentate ligands via nitrogen coordina-
tion 1131.
A good number of spin crossover systems have also been found in the complex chemistry of iron(II1).
In fact, the first example of a spin equilibrium was discovered in magnetic studies with a series of iron
(111) N,N-dialkyldithio-carbamates, [F~(S~CNR'R~ I )
by Cambi and coworkers 1141.
Spin crossover has mostly been observed in the solid state, whereby one can distinguish grossly bet- ween two kinds of systems : (i) those which change spin very abruptly within a temperature range of a few degrees Kelvin around the transition temperature Tc, as is the case in [Fe(phen),(~c~)J 1151 for i m -
tance. (ii) Systems with a gradual spin conversion over a temperature range of a hundred degrees Kelvin or more, as occurs in [ ~ e ( 2 - ~ ~ ~ - ~ h e n ) ~ ] (ClO,), for instance 116,171; the transition temperature in this case is only formally defined as the temperature of 50 per cent conversion. Abrupt spin changes may be connected with a first-order phase change, as has been observed for r~e(~hen) (NCS) 2 ] and [Fe(phen)
,
(NCS~),] by heat capacity measurements / 181. It also appears to be prerequisite to hysteresis effects, which can be understood on the basis of a thermody- namic model 1191 as well as in the framework of an Ising model /20/, and which has indeed experimentally been observed 121,221.
In a few instances spin crossover has also been established in solution using Evans' n.m.r. me- thod for the determination of the magnetic suscepti- bility 1231. Particular attention should be paid to
the measurement of spin state lifetimes in solution for iron(T.1) spin equilibrium systems using the la- ser Raman temperature-jump method 1241, for six- coordinate d5, d6 and d 7 spin-equilibrium metal complexes of iron(IT.1)
,
iron(I1) and cobalt (11) 125, 261 yielding spin state lifetimes of 30<
T 5 120 ns,and to the ultrasonic relaxation experiments of iron (11) spin crossover systems in solutions 1271, which also yielded spin state lifetimes of -30 ns in addi- tion to thermodynamic quantities.
Most of the work performed so far on spin crossover systems concerns thermally induced spin
-
transitions. As the energy difference
I A-PI ,
however, is also pressure dependent due to pressure induced changes in bond lengths and bonding properties, it is anticipated that applied pressure may also have a pronounced influence on the spin behaviour. This has indeed been observed in many experiments by Drickamer and his school 1281.The spin crossover phenomenon in transition metal chemistry has attracted much attention in re-
C2-380
JOURNAL DE PHYSIQUEc e n t y e a r s . The number o f systems e x h i b i t i n g t h i s f e a t u r e i s s t e a d i l y growing, p a r t i c u l a r l y i n t h e complex c h e m i s t r y o f i r o n ( I 1 ) and i r o n ( I I 1 ) . F o r t u - n a t e l y , flijssbauer s p e c t r o s c o p y h a s proven a n e x t r e - mely powerful t o o l f o r t h e s t u d y o f such s y s t e m s . One h a s made ample u s e o f i t i n t h e p a s t f i v e y e a r s i n a t t e m p t s t o l e a r n more a b o u t t h e p h y s i c a l back- ground o f s p i n t r a n s i t i o n p r o c e s s e s . I n t h e following we s h a l r e v i e w t h e most e s s e n t i a l r e s u l t s of t h e s e s t u d i e s
2. Ligand s u b s t i t u t i o n e f f e c t s . - Already t h e e a r l y work of Cambi and h i s coworkers 1141 on i r o n ( I I 1 ) N,N-dialkyldithiocarbamates and t h e c l o s e r i n s p e c -
t i o n of such compounds l a t e r on by v a r i o u s i n v e s t i - g a t o r s /29,30,52/ have c l e a r l y demonstrated t h a t t h e m a g n e t i c b e h a v i o u r i s s t r o n g l y i n f l u e n c e d by chemi- c a l m o d i f i c a t i o n o f t h e l i g a n d s , h e r e by v a r y i n g t h e s u b s t i t u e n t s R of t h e -NR2 group. S i m i l a r e f f e c t s i n i r o n ( I I 1 ) d i t h i o c a r b a m a t e s and many o t h e r s y s t e m s have been observed more r e c e n t l y , and i t h a s become common knowledge t h a t t h e l i g a n d f i e l d s t r e n g t h A may be s i g n i f i c a n t l y a l t e r e d , v i a s t e r i c and e l e c - t r o n i c e f f e c t s , by changing s u b s t i t u e n t s o f a l i g a n d a s i n t h e c a s e of d i a l k y l d i t h i o c a r b a m a t e s , a n d / o r r e p l a c i n g a whole l i g a n d by d i f f e r e n t ones a s , f o r i n s t a n c e , i n [FeII(phen) ( N C S ) 2] and r e l a t e d com- pounds 1311. The l a t t e r example shows, a s a l r e a d y mentioned above, a s h a r p h i g h s p i n e l o w s p i n tran- s i ti o n around 175 K, whereas [Fe1'(phen)
,]
(C10) 2 ,w i t h i r o n ( I 1 ) c o o r d i n a t e d t o t h r e e p h e n a n t h r o l i n e l i g a n d s , i s n e a r l y d i a m a g n e t i c ( p e f f
-
I B.M.) aroundand below room t e m p e r a t u r e .
The s u b s t i t u e n t i n t h e 2 - p o s i t i o n of 1 , l O - p h e n a n t h r o l i n e a p p e a r s t o have a p a r t i c u l a r b e a r i n g on t h e m a g n e t i c b e h a v i o u r of i r o n ( I 1 ) a s a c e n t r a l m e t a l i o n /50,17,32-351. The p o l y c r y s t a l l i n e p e r - c h l o r a t e s o f [Fe11(2-X-phen) 2 + a r e t y p i c a l l l y low s p i n f o r X=H ( u n s u b s t i t u t e d p h e n a n t h r o l i n e ) , b u t pu- r e l y h i g h s p i n f o r X = C 1 ( a l s o f o r X , Y = CH, i n 2,9- bisCH,-phen), and show t h e r m a l l y induced h i g h s p i n +low s p i n t r a n s i t i o n ( 5 ~(Oh)
+
'A (Oh) i n28 l g
t h e o f t e n u s e d n o t a t i o n w i t h i n t h e a p p r o x i m a t i o n of c u b i c symmetry) f o r X=CH,, OCH,, i n which t h e shape of t h e f u n c t i o n o f t h e e f f e c t i v e m a g n e t i c moment o r of t h e a r e a f r a c t i o n o f t h e HS MEssbauer e f f e c t r e - sonances a s a f u n c t i o n o f t e m p e r a t u r e i s e s s e n t i a l l y t h e same, b u t i s s u b s t a n t i a l l y s h i f t e d t o h i g h e r t e m p e r a t u r e s f o r X = OCH, a s compared t o t h e s y s t e m w i t h X = CH,. A t a g i v e n t e m p e r a t u r e , t h e paramagnet-
i c b e h a v i o u r of [Fe ( 2 - ~ - ~ h e n ) (C10k) 2 i n c r e a s e s i n t h e o r d e r of X = H < CH, < OCH, < C1. T h i s o r d e r -
i n g and t h e f a c t t h a t [ ~ e ( 2 , 9 - b i s ~ ~ ~ - p h e n ) , ] ( c l o , ) , i s p u r e l y h i g h s p i n i r r e s p e c t i v e of t e m p e r a t u r e , c l e a r l y d e m o n s t r a t e s t h a t a combination o f b o t h s t e - r i c and e l e c t r o n i c e f f e c t s p l a y s a d e c i s i v e r o l e . CND0/2 m o l e c u l a r o r b i t a l c a l c u l a t i o n s f o r 2-CH3-phen, 2-Cl-phen and t h e p u r e phen l i g a n d have shown how t h e l i g a n d f i e l d s t r e n g t h A can be a l t e r e d i n d i f f e r - e n t ways t h r o u g h e l e c t r o n i c i n t e r a c t i o n s 1351 : I n 2-CH3-phen t h e h i g h e s t occupied IT o r b i t a l i s somarhat d e s t a b i l i z e d (0.2 eV) compared t o phen (due t o a po- s i t i v e i n d u c t i v e e f f e c t by t h e CH g r o u p ) , which i n t u r n r e d u c e s A; i n 2-C1-phen t h e h i g h e s t 0 o r b i t a l i s s t a b i l i z e d by ca. 0.5 eV a s compared t o phen, which i n t u r n weakens t h e ligand-eta1 0 d o n a t i o n and t h u s r e d u c e s A . The s t e r i c h i n d r a n c e e f f e c t by sub- s t i t u e n t s of v a r i a b l e s i z e i s r e a d i l y e x p l a i n e d by t h e s t r o n g dependence of t h e l i g a n d f i e l d s t r e n g t h A on changes i n t h e m e t a l - l i g a n d bond d i s t a n c e
( A
-
I / R ' f o r a p o i n t c h a r g e model of c u b i c symmetry / 2 / ; i n a c t u a l complexes w i t h h i g h d e g r e e s of cova- l e n c y , however, t h e power of t h e R dependence of A h a s been found t o be much s m a l l e r / 5 / ) .S i m i l a r s u b s t i t u e n t e f f e c t s on t h e s p i n e q u i - l i b r i u m have been observed by H o s e l t o n , Wilson and Drago 1.231 w i t h h e x a d e n t a t e l i g a n d s on i r o n ( I 1 ) . I n f a c t , t h e a u t h o r s p o i n t o u t t h a t " m u l t i d e n t a t e li- gands can be of g e n e r a l u t i l i t y i n d e s i g n i n g new e q u i l i b r i u m systems where l i g a n d s u b s t i t u e n t e f f e c t s may be employed t o " f i n e - t u n e " t h e l i g a n d f i e l d s t r e n g t h around t h e c r o s s o v e r region".
3 . Metal d i l u t i o n e f f e c t and t h e concept of coopera- t i v e s p i n t r a n s i t i o n . - One of t h e most s i g n i f i c a n t s t e p s forward towards a b e t t e r u n d e r s t a n d i n g of t h e p h y s i c a l n a t u r e o f t h e s p i n c r o s s o v e r phenomenon i s t h e work o f S o r a i and S e k i 1181. On t h e b a s i s of pre- c i s e h e a t c a p a c i t y measurements on [Fe(phen) 2(NCS) and [Fe(phen) ( N C S ~ ) 2], t h e s e a u t h o r s have s u g g e s t e d t h a t a c o o p e r a t i v e s p i n t r a n s i t i o n t a k e s p l a c e i n t h e s e systems t h r o u g h a s i g n i f i c a n t c o u p l i n g between t h e e l e c t r o n i c s t a t e and t h e phonon s y s t e m , and t h a t t h e s p i n s t a t e c o n v e r s i o n o c c u r s s i m u l t a n e o u s l y i n a group o f m o l e c u l e s of l i k e s p i n , which form a "coope- r a t i v e r e g i o n (domain)". I n t h i s c o n t e x t t h e phonon system i s c o n s i d e r e d t o i n c l u d e i n t e r m o l e c u l a r ( l a t - t i c e ) v i b r a t i o n s a s w e l l a s i n t r a m o l e c u l a r v i b r a t i o n s .
I n o r d e r t o examine t h e s e s u g g e s t i o n s one h a s conducted a s e r i e s of e x p e r i m e n t s on mixed c r y s t a l s of ~ F ~ , z ~ ~ - , ( ~ - ~ ~ c ) , ~ c ~ ~ . E ~ o H , (2-pic = 2 - p i c o l y l a - mine), where t h e h i g h d s p i n low s p i n t r a n s i t i o n h a s been f o l l o w e d a s a f u n c t i o n o f t e m p e r a t u r e and d i l u - t i o n d e g r e e x i n t h e r a n g e 1.0 3 x 2 0.0009 by means
o f "Fe NEssbauer spectroscopy 136,371. T h i s s y s t e m , f o r which t h e t h e r m a l l y promoted s p i n t r a n s i t i o n was f i r s t e s t a b l i s h e d by Renovitch and Baker 1381, i s p a r t i c u l a r l y s u i t e d f o r MEssbauer e f f e c t s t u d i e s , because ( i ) t h e two quadrupole d o u b l e t s a r i s i n g from t h e two c o e x i s t i n g s p i n s t a t e s are w e l l resolved i n t h e whole temperature range o f i n t e r e s t , and ( i i ) t h e i r o n ( 1 1 ) compound forms p e r f e c t s o l i d s o l u t i o n s w i t h t h e corresponding Z n ( I 1 ) complex over t h e whole x range under s t u d y . The h i g h spin*low s p i n t r a n - s i t i o n t a k e s place i n a l l d i l u t e d s y s t e m s , even a t t h e lowest i r o n c o n c e n t r a t i o n o f l e s s than 0.1%. The c o e x i s t e n c e o f t h e two quadrupole d o u b l e t s a r i s i n g from the h i g h s p i n and the low s p i n s t a t e s , respec- t i v e l y , around t h e t r a n s i t i o n temperature T c i n d i c a - t e s a f i r s t order nature ( i n terms o f r e a c t i o n k i n e - t i c s ) f o r t h e s p i n t r a n s i t i o n . W i t h decreasing i r o n c o n c e n t r a t i o n t h e h i g h s p i n s t a t e i s favoured. T h i s i s a l s o r e f l e c t e d i n t h e s p i n conversion curves o f f i g u r e 2 , which e x p r e s s two d i s t i n c t t e n d e n c i e s : ( i ) The t r a n s i t i o n temperature T c i s s h i f t e d t o lower t e m p e r a t u r e s , and ( i i ) the slope i n Tc becomes l e s s s t e e p w i t h decreasing i r o n c o n c e n t r a t i o n .
Fig. 2 : Temperature dependence o f t h e area f r a c t i o n xH o f t h e h i g h s p i n quadrupole d o u b l e t i n "Fe MESS- bauer s p e c t r a o f [ ~ e ~ ~ n ~ - ~ ( Z - p i c )
J c ~ , . E ~ o H
w i t h va- r i a b l e i r o n c o n c e n t r a t i o n . The i n s e r t e d diagram shows t h e dependence o f t h e t r a n s i t i o n temperature T c ( K ) as a f u n c t i o n o f x .Both e f f e c t s a p p a r e n t l y r e s u l t from a gradual weaken- i n g o f t h e c o o p e r a t i v e coupling between t h e e l e c -
t r o n i c s t a t e s and t h e phonon system. T c i s f o r m a l l y d e f i n e d as t h e temperature a t which b o t h s p i n s t a t e s are present i n equal amounts. For most purposes it i s s u f f i c i e n t t o determine T c from t h e area f r a c t i o n
X H o f t h e h i g h s p i n MEssbauer e f f e c t resonances : Tc ( x H = xL = 0 . 5 ) . T c v a l u e s obtained t h i s way de- v i a t e by l e s s t h a n 10% / 3 7 / from t r u e T c v a l u e s de-
termined from a c t u a l r e l a t i v e amounts o f t h e two s p i n s p e c i e s , due t o t h e f a c t t h a t t h e Debye WaLler
f a c t o r s o f t h e h i g h s p i n s p e c i e s a r e , i n g e n e r a l , somewhat smaller than t h o s e o f t h e low s p i n s p e c i e s as a consequence o f t h e s t r o n g e r bonds i n t h e low s p i n compounds. T c ( x ) v a r i e s l i n e a r l y w i t h x i n t h e range 1.0 3 x 2 0 . 2 , but f a l l s o f f somewhat more r a p i d l y f o r x
5
0.2 ( s e e Fig. 2 ) . The r e s u l t s sup- port t h e s u g g e s t i o n by Sorai and S e k i o f t h e s p i n t r a n s i t i o n t a k i n g place through a c o o p e r a t i v e coup- l i n g between t h e e l e c t r o n i c s t a t e and nearby vibra- t i o n a l modes i n v o l v i n g i n t r a m o l e c u l a r s t r e t c h i n g and d e f o r m a t i o n v i b r a t i o n s as w e l l as i n t e r m o l e c u l a r l a t -t i c e modes 1181. T h e i r modulation a s s o c i a t e d w i t h a s p i n change i n a p a r t i c u l a r i r o n atom i s b e l i e v e d t o play a d e c i s i v e r o l e i n t h e mechanism o f communica-
t i n g t h e " s p i n change i n f o r m a t i o n " t o o t h e r molecules w i t h i n a cooperative r e g i o n , which subsequently a l s o
change s p i n . There i s e x p e r i m e n t a l evidence f o r a remarkable r e d u c t i o n o f t h e metal-ligand s t r e t c h i n g and d e f o r m a t i o n v i b r a t i o n a l f r e q u e n c i e s on going from t h e low s p i n t o t h e h i g h s p i n isomer i n s e v e r a l s p i n crossover systems / 3 9 , 1 7 , 3 5 / , supposedly due t o a concomitant decrease i n metal-ligand n back dona- t i o n . I t has been found t h a t a change from low s p i n t o high s p i n i s accompanied by an i n c r e a s e i n bond d i s t a n c e s and changes i n bond angles /40,5 1,53,23, 411. These fundamental changes i n bonding and s t r u c - t u r e are undoubtedly w e l l s u i t e d t o i n i t i a t e f u r t h e r modulation o f v i b r a t i o n a l modes i n t h e v i c i n i t y , which i n t u r n may induce s p i n change processes i n neighboring complex molecules w i t h i n a c o o p e r a t i v e r e g i o n . I n view o f t h i s model i t i s , o f c o u r s e , o f c r u c i a l importance, whether t h e l a t t i c e c o n t a i n s on- l y i r o n complex molecules or a m i x t u r e w i t h
[ ~ n ( 2 - p i c ) , ] C12.EtOH complex m o l e c u l e s , which are diamagnetic because o f t h e d l 0 c o n f i g u r a t i o n o f z i n c
( 1 1 ) i o n s and t h e r e f o r e cannot change s p i n and conse- q u e n t l y a c t as i n t e r v e n i n g molecules i n t h e pathway o f t h e s p i n change i n f o r m a t i o n .
That i n t r a m o l e c u l a r and i n t e r m o l e c u l a r v i b r a - t i o n s are a p p a r e n t l y o f utmost importance i n t h e me- chanism o f t h e c o o p e r a t i v e s p i n t r a n s i t i o q h a s re- ceived support from t h e r e s u l t s o f t h e heat c a p a c i t y measurements o f Sorai and S e k i / 1 8 / : The l a r g e s t p o r t i o n ( c a . 7 5 % ) o f t h e measured v a l u e o f ca. 50
~ ~ - l m o l - l f o r t h e t o t a l e n t r o p y change on going fran t h e low s p i n phase t o t h e h i g h s p i n phase was found t o o r i g i n a t e from changes i n t h e v i b r a t i o n a l modes, whereby about 50% o f t h e v i b r a t i o n a l part o f entropy change was a t t r i b u t e d t o changes i n metal-ligand s t r e t c h i n g f r e q u e n c i e s . T h i s i n d i c a t e s t h a t phonon e x c i t a t i o n i s much e a s i e r i n t h e h i g h s p i n s p e c i e s
C2-382 JOURNAL DE PHYSIQUE than in the low spin species. Only about 25% of the
total entropy change can be accounted for by the electronic contribution
ASel= ~ [ l n ( 2 ~ + 1 ) ~ ~ - l n ( 2 ~ + 1 ) ~ ~ ] = Rln5 = 13.4
JK?UO~-'.
Because of the relatively large entropy gain accom- panied with a conversion from low spin to high spin, it is unquestionable that the thermal stability of a spin crossover system is controled by the (Gibbs) free energy G(p,T)=H-TS and not just by the enthalpy.
What counts in the temperature induced transition between two coexisting spin isomers is the free en- ergy difference AG = GH
-
GL = AH-
TAS, where the subscripts H and L stand for high spin and low spin, respectively. AH includes the change in electronic enthalpy, AHe1, as the major contribution, which is the energy difference between the lowest spin orbit level of the high spin crystal field term and that of the low spin crystal field term. AHel has been estimated for [ ~ e ( ~ h e n ) 2 ( ~ ~ ~ ) 2 ] to be ca. 400 cm-I, for instance 1181. Smaller contributions originate from the differences in zero point vibrational ener- gies of both intramolecular and intermolecular modes, AHintravib and AHintervib, respectively. ThusAH = HH
-
EL = AHe1 + AHintravib + AHintervib. (2) With rising temperature TAS plays an increasinglyimportant role; as AS = S -S > 0 , TAS compensates H L
AH =
% -
H > 0 more and more until, at Tc(AG = 0 )L
it outweighs AH completely. The driving force of the spin transition is the total entropy gain AS 'H
-
'L = Asel + "intravib + ASintervib > 0 whereby, as already pointed out above, the major ( 3 ) contributions here originatefrom ASintravib and ASinterv;b 1181. Unpublished new results from 5 7 ~ e Mbssbauer effect studies on solid solutions of
[ ~ e ~ ~ n ~ _ ~ ( p h e n ) ~ ( N C S ) ~ ] , which are presently being carried out by P. Ganguli in our laboratory, agree essentially with the observations made on
[ ~ e ~ ~ n ~ _ ~ ( Z - p i c )
]
c12 . E ~ O H . The rather abrupt spin transition behaviour of the pure iron compound (see figure 3) becomes more and more gradual and Tc is shifted to lower temperatures as the manganese con- centration increases. This again calls for a gradual weakening of the cooperative coupling effect between the electronic state of iron and the lattice by in- tervening [ ~ n ( ~ h e n ) (NCS)~] complex molecules. The drastic increase of the high spin residue at low temperature with decreasing iron concentration, h w ever, is not yet fully understood and is subject to further studies.4. Influences from crystal solvent molecules, anions,
and isotopic substitution.- In view of the suggested model of the cooperative spin transition taking place through an effective coupling between the electronic state and the relevant phonon system within a coope- rative region, it is expected that chemical altera- tions of a system such as changing the nature of n o r coordinating crystal solvent molecules, andlor anions should influence somehow the spin transition beha- viour through concomitant changes in the phonon sys-
tem via changes in the hydrogen bond formation, where relevant, and/or changes in crystal structure.
Fig. 3 : Temperature dependence of the area fraction xH of the high spin quadrupole doublet in 5 7 ~ e M ~ S S -
bauer spectra of [~e,~n, -x(phen) (NCS) 2 7 with va- riable iron concentration.
Such effects on the spin transition behaviour have indeed been observed by various authors. Goodwin et al. 142,431 have observed a solution effect on t k thermally induced incomplete spin transition in the complex [re(papt)
J
and its solvates with chloroform and benzene, where "papt" is the tridentate ligand 2-(2-pyridy1amino)-4-(2-pyridyl)thiazole, which is not capable of hydrogen bond formation to chloroform or benzene. The spin crossover phenomenon in cobalt (11) complexes with two tridentate 2, 2', 2"-terpy- ridine ligands, which are also not capable of hydro- gen bond formation, has been found to be affected by non-coordinating anion and/or crystalline water /44, 45,46,47/. Sams and Tsin 148,491 have found, using the 5 7 ~ e MGssbauer effect and other techniques, that details of the 5 ~(oh) 24
' ~ ~ ( 0 ~ ) spin crossover in tris r2-(2'-pyridyl)-benzimidazole] iron(I1) comple- xes are sensitive to the nature of the anion and,for the perchlorate derivatives, to the number of mole- cules of water of crystallization. Renovitch and Baker 1381 have reported on the differences observed for the spin crossover behaviour of the [Fe(2-pid,]x2 complex compounds with different anions, X = C1, Br, I.R e c e n t l y performed v a r i a b l e - t e m p e r a t u r e 5 7 ~ e Mzssbauer e f f e c t s t u d i e s on v a r i o u s s o l v a t e s of [ ~ e ( 2 - ~ i c ) ~ l ~ . S o l , w i t h S o l = C,H,OH, CH30H, H20 and 2H20 1221 have y i e l d e d s i g n i f i c a n t d i f f e r e n - c e s i n t h e a r e a f r a c t i o n s xH of t h e h i g h s p i n r e s o - nances a s a f u n c t i o n of t e m p e r a t u r e . The e f f e c t s a r e t h r e e - f o l d : ( i ) A l t e r a t i o n of t h e S-shaped c o n v e r s i o n c u r v e x H IT); t h e s p i n t r a n s i t i o n i n t h e s y s t e m w i t h CH30H p r o c e e d s more g r a d u a l l y , a p p a r e n t - l y due t o weaker c o o p e r a t i v e c o u p l i n g , t h a n i n t h e systems w i t h C2H50H o r H , O , r e s p e c t i v e l y . ( i i ) S h i f t o f Tc; t h e s p i n t r a n s i t i o n i n t h e system w i t h 2H20 i s c o m p l e t e l y s u p p r e s s e d , t h e s u b s t a n c e i s diamagne- t i c a t a l l t e m p e r a t u r e s under s t u d y . ( i i i ) Hystere- s i s e f f e c t ; t h e monohydrate shows an enormously 1%- ge h y s t e r e s i s e f f e c t , w h i l e t h e o t h e r s o l v e n t s show no h y s t e r e s i s a t a l l .
A s i n g l e c r y s t a l x-ray s t r u c t u r e a n a l y s i s of [ ~ e ( 2 - ~ i c ) J C L ~ . C ~ H ~ O H 1411 h a s confirmed t h e hydrogen bond f o r m a t i o n i n t h e sequence H(NH2)
...
~ 1 -
. .
.H(NH,) and w i t h t h e C,H50H m o l e c u l e hydrogen bonded v i a OH t o t h e a n i o n ( ~ 1 -. .
.HO-CpH,). The d r a s t i c c r y s t a l s o l v e n t e f f e c t on t h e s p i n crossover seems t o b e p l a u s i b l e on t h i s a c c o u n t . Moreover, p r e l i m i n a r y r e s u l t s from M6ssbauer e f f e c t s t u d i e s on systems w i t h d e u t e r a t e d s o l v e n t m o l e c u l e s , which a r e p r e s e n t l y c a r r i e d o u t by S t e i n h E u s e r and ~ z p p e n i n o u r l a b o r a t o r y , l e n d s u p p o r t t o t h e c o n c l u s i o n t h a t hydrogen bonding i s undoubtedly of c o n s i d e r a b l e importance i n t h e s e systems. Even more s o i f one c o n s i d e r s t h e f a c t t h a t , i n t h e c o u r s e of t h e prepa- r a t i o n of [ ~ e ( 2 - ~ i c ), ] c ~ ~ . R o D
w i t h ROD = C2H,0D, CH,OD one cannot a v o i d d e u t e r a t i o n of t h e -NH2 group t o a s i z e a b l e e x t e n t ( a s e v i d e n c e d by i . r . , n.m.r.and mass s p e c t r o s c o p y ) . Thus a marked change i n t h e v i b r a t i o n a l modes, n o t o n l y i n t h o s e of t h e hydrogen bonds, i s t o be e x p e c t e d . On t h e s e grounds t h e d i s - t i n c t s h i f t of Tc by c a . 1 4 t o h i g h e r t e m p e r a t u r e s observed f o r t h e d e u t e r a t e d system [Fe(2-pic),]~l,.
C2H,0D and [ ~ e ( 2 - p i c ) , ] C1, .CH30D a s compared t o t h e i r n o n - d e u t e r a t e d a n a l o g u e s ( s e e F i g . 4) can b e q d a l i t a t i v e l y u n d e r s t o o d i n t h e f o l l o w i n g way : It s u f f i c e s t o c o n s i d e r t h e e n e r g y d i f f e r e n c e b e t - ween t h e zero-point v i b r o n i c l e v e l s o f t h e h i g h s p i n (H) and t h e low s p i n s t a t e (L). F o r t h e non- , d e u t e r a t e d system i t may b e w r i t t e n a s
15 QJ ?.
AE = E +
-
2 h c (vH-
vL),
( 4 ) where E c o r r e s p o n d s t o t h e e n t h a l p y d i f f e r e n c e AHel( a t T=O) between t h e h i g h s p i n and l o w s p i n e l e c t r o n -
'L 'L
i c ground s t a t e s , V H and
vL
r e f e r t o mean wavenum- b e r s of t h e two s p i n s t a t e s , averaged o v e r t h e 15v a l e n c e f r e q u e n c i e s of t h e s i x - c o o r d i n a t e d i r o n atom.
F i g . 4 : Temperature dependence of t h e a r e a f r a c t i o n xII o f t h e h i g h s p i n q u a d r u p o l e d_oublet i n t h e 5 7 ~ e
?lossbauer s p e c t r a of [ ~ e ( 2 - ~ i c ) ~ J
c12.
S o l , w i t h S o l = C,H,OH ( 0 1 , C2H,0D (01, CH30H(n),
CH,OD (r).'L v may b e approximated by
1
JG
% = -
2rc
where k* and m* e x p r e s s t h e e f f e c t i v e f o r c e c o n s t a n t
and t h e reduced mass, r e s p e c t i v e l y . With
% = f = m *
and e q u a t i o n (5) we o b t a i n ( p r o v i d e d t h e l o c a l sym- m e t r y d o e s n o t change)
A E = E + E K J - ~ ( @ 2
- 4 ) .
The d i f f e r e n c e between t h e d e u t e r a t e d ( d ) and non- d e u t e r a t e d (h) system w i l l be
6(AE) = AE ( d l
-
AE ( h )b e c a u s e t h e e f f e c t i v e f o r c e c o n s t a n t w i l l n o t change upon d e u t e r a t i o n . As, however, kL > k: and
*(d> > m*(h), A(&) w i l l b e p o s i t i v e , i . e .
AE(d) > AE(h), which i m p l i e s a s h i f t of Tc t o h i g h e r t e m p e r a t u r e s .
5. T h e o r e t i c a l models.- To e x p l a i n t h e anomalous Curie-Weiss b e h a v i o u r of m a g n e t i c s u s c e p t i b i l i t y i n complex compounds e x h i b i t i n g s p i n c r o s s o v e r one gene- r a l l y u s e s t h e c o n v e n t i o n a l v a n Vleck f o r m a l i s m of magnetism. I n t h e e a r l y a t t e m p t s one h a s found s a t i s f a c t o r y agreement between t h e o r y and experiment by t a k i n g t h e Boltzmann d i s t r i b u t i o n o v e r a l l s p i n - o r b i t l e v e l s of t h e h i g h s p i n and t h e low s p i n c r y s t a l f i e l d s t a t e s w i t h i n c l u s i o n o f d i s t o r t i o n from c u b i c symmetry, s p i n - o r b i t c o u p l i n g , and c o n f i g u r a t i o n i n t e r a c t i o n 154,551. T h i s model was r e f i n e d by consi- d e r i n g t h e c r y s t a l - f i e l d s p l i t t i n g e n e r g y and t h e e n e r g y s e p a r a t i o n between t h e d i f f e r e n t s p i n s t a t e s , r e s p e c t i v e l y , a s a f u n c t i o n of t e m p e r a t u r e / 5 6 , 5 7 / , and by i n c l u s i o n of a n o r b i t a l r e d u c t i o n f a c t o r a n d a f r a c t i o n ci of permanently- p a r a m a g n e t i c "im?urity"/57/.
C2-384
JOURNAL DE PHYSIQUE S o r a i and S e k i 1181 have p o i n t e d o u t t h a tt a k i n g t h e energy s e p a r a t i o n between t h e d i f f e r e n t s p i n s t a t e s t o be t e m p e r a t u r e dependent must be con- s i d e r e d d o u b t f u l , u n l e s s e x p e r i m e n t a l p r o v e , from v a r i a b l e - t e m p e r a t u r e i . r . and u . v . / v i s . s p e c t r a f o r i n s t a n c e , s u p p o r t i n g t h i s assumption i s a v a i l a b l e . I n any c a s e , t h e work of S o r a i and Seki /18/ h a s ma- d e c l e a r t h a t t h e t h e r m a l s t a b i l i t y of a s p i n c r o s s - o v e r system a t f i n i t e t e m p e r a t u r e s i s c o n t r o l l e d by t h e f r e e energy d i f f e r e n c e AG = AH
-
TAS r a t h e r t h a n j u s t by t h e e n t h a l p y (energy) d i f f e r e n c e . Thus any macroscopic (thermodynamic) model f o r s p i n t r a n s i t i o n s h o u l d be based on t h e Gibbs f r e e e n e r g y G(p,T) == H
-
TS ( o r t h e Helmholtz f r e e energy F(V,T) =U-TS) /19,18/.Following t h e s u g g e s t i o n s of S o r a i and Seki a phenomenological thermodynamic model h a s been em- ployed s u c c e s s f u l l y t o d e s c r i b e t h e h i g h s p i n
*
low s p i n t r a n s i t i o n i n t h e s o l i d s o l u t i o n s of
[pexZn (2-pic) JCl2.EtOH a s evidenced by ~ G s s b a u e r 1 -x
s p e c t r o s c o p y 137,581. The model a l l o w s t o e s t i m a t e a s a f u n c t i o n of t h e i r o n c o n c e n t r a t i o n : t h e t r a n - s i t i o n t e m p e r a t u r e Tc(x), t h e number n(x) of i r o n i o n s of l i k e s p i n i n a c o o p e r a t i v e domain, t h e t o t a l e f f e c t i v e e n t h a l p y and e n t r o p y changes a s w e l l a s t h e p a r t i a l c o n t r i b u t i o n s t o AHeff and ASeff a r i s i n g from i n t r a m o l e c u l a r changes ( e l e c t r o n i c and v i b r a - t i o n a l ) a s w e l l a s from i n t e r m o l e c u l a r ( l a t t i c e v i - b r a t i o n a 1 ) c h a n g e s . The model i s a p p l i c a b l e t o o t h e r s p i n c r o s s o v e r systems.
The m e t a l d i l u t i o n e f f e c t i n t h e system [ ~ e ~ ~ n (2-pic) J C I ~ . E ~ O H can a l s o be d e s c r i b e d by
I -x
a s t a t i s t i c a l thermodynamic model 1591, which e x p l i - c i t l y s t a t e s t h e importance of c o n s i d e r i n g t h e chan- ges i n v i b r a t i o n a l modes going a l o n g w i t h a s p i n t r a n s i t i o n .
A r e c e n t l y p u b l i s h e d model by Zimmermann and ~ G n i g 1201, based on an I s i n g t y p e t r e a t m e n t , a l s o i n c l u d e s e x p l i c i t l y t h e e f f e c t of l a t t i c e vibra- t i o n s ; thermal h y s t e r e s i s e f f e c t s may a l s o b e under- s t o o d on t h e grounds of t h i s model.
o c h e r models have been developed which have t h e i r m e r i t s and t h u s s h o u l d n o t be overlooked i n a t t e m p t s t o u n d e r s t a n d t h e p h y s i c s of s p i n c r o s s o v e r /60,61,62/.
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