Rings, Clusters, and Polymers of the Main Group Elements - American

4 Polyanionic Clusters in Solids and Their Chemical Reactions Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 17, 2015 | http://pubs.acs.org Publication Date: September 30, 1983 | doi: 10.1021/bk-1983-0232.ch004

H. G. VON SCHNERING Max-Planck-Institut für Festkörperforschung Heisenbergstr. 1, D-7000 Stuttgart 80, Federal Republic of Germany

Structural units of binary polycompounds with main group elements consist of one-, two- and three-di mensional polymers, as well as cage-like polycyclic anions. These polyanions can be converted into mole cular compounds by appropriate chemical reactions. The reactions, which are discussed in general, clearly provide an easy means of preparation for new types of compounds. Compounds such as P R , As R and P R have been obtained by these methods. Further homogeneous reactions with P R yield other P R' (R'=Sn me3, Pb me ) compounds which are sur prisingly stable. Reactions of Rb As , for example, in ethylendiamine produce Rb As •3en, which serves as a model of the structure in solution. Redox re actions of Na P or Na P in THF yield P 2- and Na P •6en which can be seen as the first steps of polycondensation of the P units. The formation of allo-Ge and 4H-Ge from Li Ge is an especially interesting reaction in general, in view of the pre paration of new (metastable) modifications of the elements. Some of the binary polycompounds possess interesting physical properties. The congruent sub limation, the crystalline to plastically crystal line phase transitions as well as the raman spectra are discussed in detail. 7

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The e l e c t r o p o s i t i v e e l e m e n t s f o r m s e r i e s o f s o l i d b i n a r y compounds MY w i t h t h e m a i n g r o u p 3B, UB, 5B, 6B e l e m e n t s . I n d e pendent o f t h e r e a l b o n d i n g p r e s e n t i n t h e s o l i d s , t h e s t r u c t u r e s c a n be d e s c r i b e d i n t e r m s o f homoatomie p o l y a n i o n i c c l u s t e r f o r m a t i o n , Y ~ . S u c h p o l y a n i o n i c c l u s t e r s e x i s t as o n e - , t w o - and t h r e e - d i m e n s i o n a l c o n n e c t e d p o l y m e r s , as w e l l as q u a s i i s o l a t e d c a g e - l i k e p o l y c y c l i c u n i t s . Some examples a r e g i v e n i n F i g . 1 and a more g e n e r a l d i s c u s s i o n was p u b l i s h e d r e c e n t l y (_1_). n

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0097-6156/83/0232-0069 $06.00/0 © 1983 American Chemical Society

In Rings, Clusters, and Polymers of the Main Group Elements; Cowley, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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RINGS, C L U S T E R S , A N D P O L Y M E R S

The f o l l o w i n g c o n t r i b u t i o n e s s e n t i a l l y d e a l s w i t h t h e u t i l i z a t i o n o f t h e s e s o l i d b i n a r y compounds as s o u r c e s f o r new p o l y c y c l i c m o l e c u l a r s y s t e m s o r f o r new m e t a s t a b l e m o d i f i c a t i o n s o f t h e e l e m e n t s . New r e s u l t s a r e d i s c u s s e d w h i c h c o n t r i b u t e t o a g e n e r a l u n d e r s t a n d i n g o f t h e s e m a t e r i a l s and t h e i r p r o p e r t i e s . M e t a l Compounds as S o u r c e s

for Polycyclic

Systems

The w i n n i n g o f m o l e c u l a r compounds f r o m h e t e r o g e n e o u s r e a c t i o n s o f s o l i d m a t e r i a l s i s a w e l l known p r o c e s s ( e g . C H from C a C ) . A g e n e r a l scheme i s g i v e n i n F i g u r e 2 f o r compounds o f t h e t y p e M3Y7 and M Y r e s p e c t i v e l y (M = a l k a l i m e t a l ; Y = P, A s , S b ) . I n t h e s t r u c t u r e s o f t h e s e compounds c a g e - l i k e Y7 ~ or Y n anions are p r e s e n t ; thus r e a c t i o n s t o n e u t r a l molecules o f the same s t r u c t u r e s u g g e s t t h e m s e l v e s . The g i v e n scheme h o w e v e r , c a n a l s o be g e n e r a l i z e d f o r a p p l i c a t i o n t o p o l y m e r i c p o l y a n i o n s .

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 17, 2015 | http://pubs.acs.org Publication Date: September 30, 1983 | doi: 10.1021/bk-1983-0232.ch004

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Heterogeneous R e a c t i o n s . I n a heterogeneous r e a c t i o n o f s o l i d M Y , c w i t h s u i t a b l e r e a c t a n t s RX ( e g . C l S i m e ) s o l u t i o n s of R Y , s o l are o b t a i n e d , from which the molecular c r y s t a l l i n e compounds R Y7,c r e s u l t (2_,3_). A d i r e c t p a t h v i a s u b s t i t u e n t e x change t h e n l e a d s f r o m t h e s e m a t e r i a l s t o new compounds, o f t e n w i t h h i g h y i e l d s {h). S e v e r a l e x a m p l e s a r e g i v e n i n F i g u r e 3: A s ( S i m e ) ; P i 1 ( S i m e ) ; P n ( S n m e ) ; a n d above a l l t h e c o m p l e t e s e r i e s P ( M m e ) , w i t h M = S i , Ge, S n , Pb as w e l l as P 7 ( M p h ) w i t h M = S n , Ge (£,6,7). I n t h e above m e n t i o n e d s e r i e s , s u r p r i s i n g l y t h e s t a b i l i t y o f t h e compounds i n a i r i n c r e a s e s . The s i l i c o n compounds decompose i n s t a n t a n e o u s l y i n t o p h o s p h a n e and s i l o x a n e ; t h e l e a d compounds i n c o n t r a s t , h o w e v e r , r e a c t o n l y i n t h e c o u r s e o f some d a y s . The s y s t e m a t i c c h a n g e s i n t h e i . r . s p e c t r a a r e a l s o i n t e r e s t i n g . The M-P and M-C v i b r a t i o n s show t h e e x p e c t e d s h i f t w i t h a t o m i c m a s s , whereas t h e v i b r a t i o n s o f t h e P7-cages remain c o m p l e t e l y u n a f f e c t e d by the s u b s t i t u t i o n (k). A l l a t t e m p t s t o b o n d R P 7 as a l i g a n d t o m e t a l ( 0 ) c o m p l e x e s have f a i l e d . I n c o n t r a s t , t h e p r e p a r a t i o n o f complexes w i t h t h e r e l a t e d c a g e - r n o l e c u l e s P/+(Sime2)3 v a s s u c c e s s f u l (Q) . 3

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Homogeneous R e a c t i o n s . W i t h s u i t a b l e s o l v e n t s t h e b i n a r y m e t a l compounds y i e l d c l e a r , homogeneous s o l u t i o n s . E s p e c i a l l y s u i t a b l e a r e e t h y l e n d i a m i n e ( e n ) , l i q u i d N H , a n d i n some c a s e s THF. The deep r e d s o l u t i o n s i n e n h a v e b e e n known f o r q u i t e a l o n g t i m e (9.), a n d seem t o b e t y p i c a l f o r t h e g r e a t m a j o r i t y o f compounds w i t h p o l y a n i o n s ( c f . t h e s o l u t i o n o f Sng**" ( 1 0 , 1 1 , 1 2 ) ) . We now o b s e r v e , t h a t s o l u t i o n s o f p u r e b i n a r y p o l y p h o s p h i d e s , e g . N a P , i n very c a r e f u l l y p u r i f i e d ethylendiamine ( d i s t i l l e d over L i A l H ^ ; c o n t r o l l e d by c o n d u c t i v i t y measurements; s p e c i f i c cond u c t i v i t y lower by 10~ t h a n t h a t g i v e n i n t h e l i t e r a t u r e ) a r e a b s o l u t e l y c o l o u r l e s s . The w e l l - k n o w n deep r e d c o l o u r a g a i r a p pears i n s t a n t e n e o u s l y w i t h t r a c e s o f oxygen (produced by t h e 3

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V O N SCHNERING

Polyanionic

71

Clusters

Figure 1. Some homoatomic phosphorus units in metal polyphosphides as ex amples for polyanionic clusters.

Μ3Υ7· η Solv (s)

R3Y7 (s) t t R3Y7 ( s o l )

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Figure 2. Different reaction paths A, B, and C of a solid polyanionic cluster shown by the example of M Y (s). 3

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d e c o m p o s i t i o n o f HgO). A n o t h e r o b s e r v a t i o n c o n c e r n s w h i t e p h o s p h o r u s : p u r e Ρ4 d i s s o l v e s i n en g i v i n g a deep r e d c o l o u r ; t h e s o l u t i o n i s d i s c o l o u r e d b y o x y g e n . We s u s p e c t t h a t r a d i c a l s p e c i e s a r e r e s p o n s i b l e f o r t h e i n t e n s e r e d c o l o u r and i n v e s t i g a t i o n s are i n progress. C o n s i d e r a b l e u n c e r t a i n t y s t i l l e x i s t s about t h e s t a t e o f t h e m e t a l compounds i n t h e s o l u t i o n s : " I t ' s d a r k b e h i n d t h e p i c k " ( o l d m i n e r ' s s a y i n g ) ! M. B a u d l e r assumed t h e p r e s e n c e o f L i and P ~ i o n s f o r L i P i n THF s o l u t i o n (J3.). I n t h e s e s o l u t i o n s v a lence f l u c t u a t i o n s of the P ~ cage w e r e o b s e r v e d f o r t h e f i r s t t i m e (^ik). The d e t a i l s w e r e e x p l a i n e d b y G l e i t e r ( 15) "by a t h e o r e t i c a l m o d e l . The v a l e n c e f l u c t u a t i o n s f r e e z e a t 2h0 Κ ( P - N M R ) . We w e r e r e c e n t l y a b l e t o p r o v e t h e f l u c t u a t i o n s f o r N a P i n en (5.) · A f t e r t h e s t r u c t u r e d e t e r m i n a t i o n o f s o l v a t e d compounds, some d o u b t s e x i s t c o n c e r n i n g t h e p a t h o f t h i s v a l e n c e f l u c t u a t i o n . The c a t i o n s i n t h e c r y s t a l l i n e s o l v a t e s R b A s « 3 e n and N a ^ P i ^ - ô e n a r e b o n d e d e x c l u s i v e l y a? i n an u n d i s s o c i a t e d i o n - c o m p l e x t o o n l y one a n i o n and a r e c o o r d i n a t e d o n l y by s o l v a t e m o l e c u l e s ( F i g u r e h) (,l6). The u n u s u a l c o n f i g u r a t i o n c o r r e s p o n d s t o an i s o l a t e d s o l v a t e d M^Y m o l e c u l e and i s q u i t e d i f f e r e n t t h a n t h e c o n f i g u r a t i o n i n s o l v e n O - f r e e L i ~ P (6) and Na^P (17). We a l s o r e g a r d t h i s s t r u c t u r e as a g o o a m o d e l f o r d i s s o l v e d compounds and b e l i e v e t h a t a M e t a l S u p p o r t e d V a l e n c e F l u c t u a t i o n (MSVF) t a k e s p l a c e i n t h e s o l u t i o n . A s i m p l e e l e c t r o s t a t i c e s t i m a t i o n shows t h a t t h e p a r t i c i p a t i o n o f t h e m e t a l atoms n o t i c e a b l y d e c r e a s e s t h e e n e r g y d i f f e r e n c e b e t w e e n g r o u n d - s t a t e and t r a n s i t i o n s t a t e (J_6). +

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The s o l u t i o n s o f t h e m e t a l compounds M X c a n he u s e d ( a ) f o r a s t u d y o f r e d o x - r e a c t i o n s u n d e r m i l d c o n d i t i o n s , and (b) t o s e r v e as a d u c t s f o r c a t i o n e x c h a n g e r e a c t i o n s . Thus N a P r e a c t s w i t h p h ^ P C l o r phxfAsCl a c c o m p a n i e d b y q u a n t i t a t i v e p r e c i p i t a t i o n o f N a C l , t o g i v e s o l u t i o n s o f s a l t s w i t h o u t m e t a l c a t i o n s . The l e a s t s o l u b l e m a t e r i a l s w h i c h c r y s t a l l i z e out o f t h e s e s o l u t i o n s are t h e compounds ( p h ^ P ^ P - i e o r (ph/+As ) P i 6 . They c o n t a i n t h e new polyanion P i e ( F i g u r e 5)» w h i c h i s f o r m e d i n t h e s o l u t i o n b y a complex redox r e a c t i o n (18). Na^Piz* i s f o r m e d q u a n t i t a t i v e l y f r o m N a P and P^ ( i n en) i n a w e l l d e f i n e d r e a c t i o n . The c o l o u r l e s s s o l v a t e N a ^ P i ^ ' o e n c a n be o b t a i n e d f r o m en i n c r y s t a l l i n e f o r m h y c o u n t e r - c u r r e n t d i f f u s i o n o f THF ( F i g u r e 5). A g a i n we f i n d a s o l v a t e d m o l e c u l e o r i o n - c o m p l e x Naz+Pi/+ i n t h e c r y s t a l . The p o l y a n i o n P i ^ ~ , whose t r i m e t h y l s i l y l s u b s t i t u t e d d e r i v a t i v e s have a l r e a d y been o b s e r v e d i n s m a l l q u a n t i t i e s i n m a s s - s p e c t r a (J3), c o n s t i t u t e s t h e f i r s t m e t h o d i c a l l y e x e c u t e d s t e p t o an o l i g o m e r i z a t i o n and f i n a l l y t o a comp l e t e polymerization of P ~ c l u s t e r s (16). 3

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R a d i c a l i c Decomposition. A t h i r d path f o r r e a c t i o n s of the b i n a r y m e t a l compounds b e i n g d i s c u s s e d h e r e i s r a d i c a l i c d i s s o c i a t i o n u n d e r t h e i n f l u e n c e o f s u i t a b l e r e a c t a n t s . The t h e r m a l dec o m p o s i t i o n i n t o the elements i s always p o s s i b l e at h i g h temperat u r e s . A t 298 Κ h o w e v e r , i t can o n l y t a k e p l a c e i f a t l e a s t one



VON SCHNERING

Polyanionic Clusters

Figure 3. Structures of PuR and P R as examples for substituted polyanions. The IR spectra of the compounds P (Mme ) with M = Si, Ge, Sn, and Pb show the nearly unchanged fundamental vibrations of the P skeleton (black-filled), the shifts of v(P-M) (black star), and of KM-C ) (white star), and the CH rocking vibrations (not marked). 3

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Figure 4. Structure of Rb As · 3en (a) showing the solvated ion complex (Rb ) As ~ with only weak additional bonds between neighboring complexes, and the proposed Metal-Supported Valence-Fluctuation (MSVF) (b) for the Xj~ cage. 3

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o f t h e components c a n r e a c t t o g i v e s t a b l e p r o d u c t s . T h i s i s p o s s i b l e i n t h e c a s e o f compounds s u c h as N a P 7 i f Na°=(Na +e~) i s t r a n s f o r m e d i n t o N a , s o l and a s t a b l e r a d i c a l a n i o n ( R * ) ~ . N a P does i n f a c t r e a c t w i t h a s o l u t i o n o f b e n z o p h e n o n e p h C 0 i n THF i n an h e t e r o g e n e o u s r e a c t i o n t o y i e l d t h e w e l l - k n o w n d e e p - b l u e r a d i c a l a n i o n s ( p h CO" ) ~ a n d d i s s o l v e d N a c a t i o n s . The p o l y a n i o n P7 i s d i s c h a r g e d b y t h i s r e a c t i o n a n d f o r m s a l i g h t - r e d amor phous r e s i d u e o f a new, b u t n o t y e t c h a r a c t e r i z e d f o r m o f e l e m e n t a r y p h o s p h o r u s . However, a r e a c t i o n o f L i 7 G e i w i t h p h C O i n THF has b e e n e x p l a i n e d i n d e t a i l (J9») ( F i g u r e 6 ) . Li Ge i s a new p h a s e i n t h e L i / G e s y s t e m , d i s t i n g u i s h e d b y a c o m p l i c a t e d t w o - d i m e n s i o n a l i n f i n i t e p o l y a n i o n i c s t r u c t u r e o f Ge atoms. The compound, w h i c h i s v e r y r e a c t i v e t o w a r d s p r o t i c a g e n t s , i m m e d i a t e l y r e a c t s i n t h e above d e s c r i b e d way t o g i v e t h e r a d i c a l a n i o n a n d a new m e t a s t a b l e m o d i f i c a t i o n o f germanium, a l l o - G e . An e x c l u s i v e l y t o p o t a c t i c a l r e a c t i o n i s involved here, i n which the complex 2 - d i m e n s i o n a l l a y e r s o f t h e o r i g i n a l G e i ~ polyanions, w h i c h a r e no l o n g e r s a t u r a t e d w i t h r e s p e c t t o v a l e n c y , p o l y m e r i z e d i r e c t l y t o allo-Ge. Allo-Ge i s a diamagnetic semiconductor w i t h mechanical pro p e r t i e s s i m i l a r t o g r a p h i t e . Because o f t h e t o p o t a c t i c a l r e a c t i o n path, L i G e i s i n g l e - c r y s t a l s are transformed d i r e c t l y i n t o s i n g l e c r y s t a l s o f a l l o - G e . Apart from t h i s , L i 7 G e i r e a c t s w i t h p r o t i c a g e n t s and a l s o t o p o t a c t i c a l l y t o L i , s o l , H , g a n d a l l o - G e , b u t v e r y s u r p r i s i n g l y , n o t b y t h e f o r m a t i o n o f germanes. H y d r o g e n a p p e a r s i n t h i s r e a c t i o n e x c l u s i v e l y a t t h e edges o f t h e t w o - d i mensional G e i ~ anions, which corresponds e x c e l l e n t l y t o t h e given scheme i n F i g u r e 6 . A l i o - g e r m a n i u m p o s s e s s e s a n o t h e r v e r y i n t e r e s t i n g p r o p e r t y . H e a t i n g i t up t o h20 Κ c a u s e s i t t o change b y an exothermic r e a c t i o n i n t o a f u r t h e r metastable m o d i f i c a t i o n o f germanium, UH-Ge. T h i s i s one o f t h e s t a c k i n g v a r i a n t s o f t e t r a h e d r a l s t r u c t u r e s , w h i c h a r e known f r o m S i C m o d i f i c a t i o n s a n d a r e of considerable t h e o r e t i c a l interest>because here i n a t e t r a h e d r a l s t r u c t u r e o f UB-elements, six-membered r i n g s appear b o t h i n t h e c h a i r - and i n t h e b o a t - c o n f o r m a t i o n . S i m i l a r t o t h e w e l l - k n o w n h i g h p r e s s u r e f o r m 2H-SÎ, t h e a x i a l r a t i o c / a o f t h e h e x a g o n a l u n i t c e l l o f UH-Ge d e v i a t e s f r o m t h e t h e o r e t i c a l v a l u e , an i n d i cation o f the influence of the unfavourable boat-conformation ( e c l i p t i c c o n f i g u r a t i o n o f a p a r t o f t h e Ge-Ge b o n d s ) . A s l o w t r a n s f o r m a t i o n o f kE-Ge t o t h e s t a b l e m o d i f i c a t i o n α-Ge w i t h d i a mond s t r u c t u r e b e g i n s a t a b o u t 770 K. R a p i d h e a t i n g w i l l c a u s e UH-Ge t o m e l t , t h e m e l t i n g p o i n t b e i n g a b o u t 10° b e l o w t h e m e l t i n g p o i n t o f a-Ge (1198 Κ/1208 Κ ) (_19.,20). These r e a c t i o n s y i e l d i n g m e t a s t a b l e new f o r m s o f e l e m e n t s s h o u l d be s t u d i e d f u r t h e r . +

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C r y s t a l l i n e and P l a s t i c Phases M P 3

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and M3P11 (M=alkaline

metal)

To make t h e above m e n t i o n e d r e a c t i o n s s u c c e s s f u l , w e l l d e f i n e d s t a r t i n g m a t e r i a l s a r e r e q u i r e d . By a s e r i e s o f v e r y e x t e n s i v e



VON SCHNERING

Polyanionic

Clusters

Figure 5. Structure of the P\e~ anion (left) and structure of the solvated ion complex Na P · 6en (right), in which large black dots are Ν a atoms and shaded areas are P units. 4

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Figure 6. Formation of allo-Ge from Li Ge by a topochemical reaction and the main structural units of Li Gej , allo-Ge, 4H-Ge and a-Ge. 7

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i n v e s t i g a t i o n s we h a v e t h e r e f o r e s t u d i e d t h e m e t a l p h o s p h i d e s M3P7 a n d M3P11 w h i c h a r e d i s t i n g u i s h e d h y ^he a p p e a r a n c e o f t h e v e r y f a s c i n a t i n g cage s y s t e m s P„ " a n d P ^ " ( T a b l e I ) . An i m p o r t a n t r e s u l t i s t h e c l e a r p r o o f "that t h e compounds M^P^ ( M = a l k a l i n e m e t a l ) , q u o t e d i n a l l t e x t b o o k s , d e f i n i t e l y do n o t e x i s t . I n a l l c a s e s t h e p r e v i o u s l y d e s c r i b e d b r i g h t y e l l o w com pounds h a v e t h e c o m p o s i t i o n M P . The c h e m i c a l a n a l y s i s o f p r e v i o u s works (9) agrees c o n s i d e r a b l y b e t t e r w i t h t h e c o m p o s i t i o n 3:7 t h a n w i t h c o m p o s i t i o n 2 : 5 . O b v i o u s l y - a n d t h i s i s q u i t e a g e n e r a l p r o b l e m - t h e r a t i o 2 : 5 v a s c o n s i d e r e d as b e i n g s i m p l e r and more r e a s o n a b l e . The p h o s p h i d e s M3P11 a r e u s u a l l y o r a n g e c o l o u r e d a n d a r e f o r m e d u n d e r v e r y s i m i l a r t h e r m a l c o n d i t i o n s (2Λ). The m a j o r i t y o f t h e M3P7 a n d M3P11 p h o s p h i d e s go t h r o u g h a c r y s t a l l i n e - t o - p l a s t i c f i r s t - o r d e r p h a s e t r a n s i t i o n (VJ_)» ( 2 2 ) .


3

7

The p r e p a r a t i o n o f s i n g l e c r y s t a l s i s d i f f i c u l t , b u t i s s u c c e s s f u l i n some c a s e , s o t h a t we a r e w e l l i n f o r m e d a b o u t t h e s t r u c t u r e s {}_ ^_ yj_ 2Ji_ 22,£3). T h e s t r u c t u r e s o f t h e p l a s t i c p h a s e s are r e l a t e d t o t h e w e l l - k n o w n i n t e r m e t a l l i c phase L i B i , where the centres o f the p o l y c y c l i c P ~ o r P n ~ anions surround t h e p o s i t i o n s o f t h e B i atoms i n L i B i . The o r i e n t a t i o n o f t h e p o l y anions i s d i s o r d e r e d ( d y n a m i c a l l y ? ) . F o r these s t r u c t u r e s t h i s o r i e n t a t i o n leads t o a t y p i c a l electron density d i s t r i b u t i o n o f a seemingly octahedral u n i t . I n contrast t h e o r i e n t a t i o n o f t h e a n i o n s i s f i x e d f o r t h e c r y s t a l l i n e p h a s e s . The symmetry o f t h e u n i t c e l l s as w e l l as t h e d i s t r i b u t i o n o f c a t i o n s and anions i n these M P and M P n type s t r u c t u r e s r e f l e c t t h e d i r e c t r e l a t i o n s h i p t o t h e s t r u c t u r e s o f the p l a s t i c phases. The y e l l o w p h o s p h i d e s M P d i f f e r f r o m a l l o t h e r p h o s p h i d e s by a r e m a r k a b l e p r o p e r t y : t h e y s u b l i m e c o n g r u e n t l y d i s s o c i a t i v e ( c f . N H ^ C l ) . In t h e gas p h a s e o n l y M,g a n d P

Rings, Clusters, and Polymers of the Main Group Elements - American

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