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Octahedral Hexahalide Complexes of the Trivalent Actinides J. L. RYAN Battelle Memorial Institute, Pacific Northwest Laboratory, Richland, Wash.
The trivalent actinides, like the trivalent lanthanides, form only weak chloride complexes in aqueous solution, and although there is evidence of slight formation of anionic complexes in concentrated LiCl from anion exchange data, no anionic chloride complexes have previously been positively identified. Ryan and Jørgensen have recently prepared the trivalent lanthanide hexachloro and hexabromo complexes and studied their absorption spectra. This paper discusses preliminary results of the extension of this work to the trivalent actinides. ^ A c t a h e d r a l h e x a h a l i d e complexes are of c o n s i d e r a b l e interest because the k n o w n h i g h s y m m e t r y a l l o w s m a n y t h e o r e t i c a l arguments to b e a p p l i e d to t h e energy levels.
A s a n e x a m p l e , i n t h e 4f a n d 5f g r o u p
elements t h e electric d i p o l e components of t h e i n t e r n a l / e l e c t r o n t r a n s i tions are f o r b i d d e n b y p a r i t y considerations i n o c t a h e d r a l complexes w i t h a center of i n v e r s i o n , a n d t h e s p e c t r u m i n t h e r e g i o n o f these transitions is d o m i n a t e d b y w e a k v i b r o n i c transitions. T e t r a v a l e n t a c t i n i d e h e x a h a l i d e spectra of this t y p e h a v e b e e n s t u d i e d extensively ( 1, 5, 6, 7, 9,10, 1112). T h e trivalent actinides a n d lanthanides form only weak complexes
i n aqueous
solution.
a b s o r b e d m o d e r a t e l y s t r o n g l y b y a n i o n exchange trated L i C l , the M X
2
+
chloride
A l t h o u g h t h e t r i v a l e n t a c t i n i d e s are resins f r o m
concen-
c o m p l e x appears to b e t h e h i g h e s t c o m p l e x present
to a m e a s u r a b l e extent i n t h e aqueous c h l o r i d e solutions (2). m i d e complexes a p p e a r to b e e v e n w e a k e r (3).
T h e bro-
T h e o n l y other e v i d e n c e
for a n i o n i c t r i v a l e n t a c t i n i d e c h l o r o o r b r o m o complexes has b e e n i n t h e system of U C 1 i n f u s e d C s C l (4). 3
R y a n a n d J0rgensen h a v e r e c e n t l y
prepared the trivalent lanthanide hexachloro a n d hexabromo
complexes
a n d s t u d i e d t h e i r a b s o r p t i o n spectra ( 8 ) . 331 Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
332
LANTHANIDE /ACTINIDE CHEMISTRY
T h i s p a p e r is a p r e l i m i n a r y report of the extension of this w o r k to the t r i v a l e n t actinides. F u r t h e r w o r k is i n progress a n d w i l l b e p r e s e n t e d i n greater d e t a i l later. H e x a c h l o r o a n d h e x a b r o m o complexes of the t r i v a l e n t actinides c a n b e p r e p a r e d b o t h i n t r i p h e n y l p h o s p h o n i u m salts a n d i n s o l u t i o n i n h i g h d i e l e c t r i c , w e a k l y c o m p l e x i n g solvents.
T h e t r i p h e n y l p h o s p h o n i u m salts
c a n b e p r e p a r e d b y p r e c i p i t a t i o n f r o m n e a r l y a n h y d r o u s e t h a n o l solutions of the a c t i n i d e t r i h a l i d e a n d t r i p h e n y l p h o s p h o n i u m h a l i d e w h i c h are almost saturated w i t h the respective h y d r o g e n h a l i d e . T h e a c t i n i d e t r i h a l i d e s o l u t i o n i n e t h a n o l c a n b e p r e p a r e d b y d i s s o l v i n g the m e t a l , t h e a n h y d r o u s or h y d r a t e d h a l i d e , the o x y h a l i d e , or i n the a p p l i c a b l e instances the sesquioxide i n e t h a n o l c o n t a i n i n g the a p p r o p r i a t e h y d r o g e n h a l i d e . T h e presence of the a n h y d r o u s h y d r o g e n h a l i d e is necessary to decrease t h e c o o r d i n a t i n g p o w e r of i m p u r i t i e s ( chiefly w a t e r ) a n d of the e t h a n o l t h r o u g h f o r m a t i o n of o x o n i u m a n d e t h y l o x o n i u m ions. A t the t i m e of this w r i t i n g the P u C l ~ , P u B r " , a n d A m C l ~ salts h a v e b e e n p r e p a r e d o n a 6
3
6
3
6
3
m a c r o scale, a n d the salt of A m B r " has b e e n p r e p a r e d o n a m i c r o scale. 6
3
T h e P u C l ~ a n d P u B r " salts w e r e p r e p a r e d s t a r t i n g w i t h P u m e t a l a n d 6
3
6
3
a l l o w i n g i t to react w i t h the respective h y d r o g e n h a l i d e i n e t h a n o l c o n t a i n i n g the t r i p h e n y l p h o s p h o n i u m h a l i d e . T h e p a l e grey salts are o x i d i z e d easily b y a i r to y e l l o w P u C l ~ a n d b r i g h t r e d P u B r " , r e s p e c t i v e l y . H e n c e , 6
2
6
2
t h e y m u s t b e h a n d l e d i n a n inert atmosphere, at least u n t i l t h e y are c o m p l e t e l y d r y . T h e c o r r e s p o n d i n g U C 1 ~ salt c o u l d not b e m a d e this 6
w a y because of o x i d a t i o n to U ( I V )
3
e v e n i n the absence of a i r .
The
n e p t u n i u m salts h a v e not b e e n a t t e m p t e d . T h e M X ~ complexes c a n b e p r e p a r e d i n s o l u t i o n i n a c e t o n i t r i l e or 6
preferably
3
i n the h i g h e r d i e l e c t r i c
solvent,
85%
succinonitrile-15%
a c e t o n i t r i l e , s t a r t i n g w i t h the t r i p h e n y l p h o s p h o n i u m salts, the h y d r a t e d or a n h y d r o u s h a l i d e s , or ( p a r t i c u l a r l y i n the case of the b r o m i d e s )
the
o x y h a l i d e s . T h e latter solvent is p r e f e r a b l e i f the t r i p h e n y l p h o s p h o n i u m salts are u s e d as s t a r t i n g m a t e r i a l because of t h e i r h i g h e r s o l u b i l i t y t h a n i n acetonitrile. T h e h e x a c h l o r o complexes are stable i n these solvents i n t h e presence of a m o d e r a t e excess of c h l o r i d e . T h e h e x a b r o m o
complexes
are stable i n the presence of a large excess of b r o m i d e i f the system is q u i t e d r y b u t are better s t a b i l i z e d b y the presence of a n h y d r o u s H B r w h i c h reacts w i t h electron d o n o r groups.
W h e n the M C l e " 3
complexes
are p r e p a r e d i n s o l u t i o n starting w i t h the a n h y d r o u s c h l o r i d e s , a s m a l l a m o u n t of H C l c a n b e u s e d to react w i t h a n y o x y c h l o r i d e present, b u t a large excess of H C l destroys the c o m p l e x b y l o w e r i n g C I " a c t i v i t y t h r o u g h f o r m a t i o n of species s u c h as H C 1 " . I n the b r o m i d e system these species 2
are not of sufficient strength to c o m p e t e w i t h the m e t a l ions, a n d the MBr
6
3
" complexes are stable i n the presence of a l a r g e excess of H B r . T h e
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
24.
R Y A N
Trivalent
Actinides
600
800
400
333
1000 Wavelength,
1200
1400
1600
πιμ
Figure 1. Absorption spectra of plutonium(III). (1) Pu(III) in I M HClO ; (2) the PuCl ~ ion in 85% succinonitrile-15% acetonitrile. This solution was prepared by dissolving PuCl in the solvent saturated with (C H ) NCl and containing a small amount of anhydrous HCl. The PuCl ~ spectrum was cor rected for 2.0ψο Pu(IV) (as PuCl ~) which appears to constitute a slight over correction, and because of this the fine structure in the 670-870 m μ region may not be exactly correct for PuCl ~ Jf
3
6
3
2
5 Il
3
6
2
e
3
6
P u C l ~ and P u B r 6
3
6
3 _
ions i n these solutions are r e a d i l y o x i d i z e d to P u ( I V ) ,
a n d the solutions m u s t b e h a n d l e d i n a n i n e r t atmosphere. T h e a b s o r p t i o n spectra of these complexes
can be obtained i n the
v i s i b l e a n d near i n f r a r e d r e g i o n u s i n g solutions of the t r i p h e n y l p h o s p h o n i u m salts, b u t i f the u l t r a v i o l e t s p e c t r u m is d e s i r e d , the c o m p l e x m u s t be formed i n solution using an aliphatic quaternary a m m o n i u m halide a n d the a c t i n i d e t r i h a l i d e or o x y h a l i d e . T h e a b s o r p t i o n s p e c t r u m of P u ( I I I ) i n the r e g i o n of the f —> f t r a n s i tions
( v i s i b l e a n d near i n f r a r e d )
s l i g h t l y sensitive to c o m p l e x i n g .
is g e n e r a l l y c o n s i d e r e d
to b e
only
T h e a b s o r p t i o n s p e c t r u m of the P u C l ~
i o n i n this r e g i o n is m a r k e d l y different f r o m the spectra of P u ( I I I ) aqueous solutions of v a r i o u s c o m p l e x i n g agents, as seen i n F i g u r e 1.
6
3
in The
m o l a r a b s o r p t i v i t i e s are o n the average a b o u t 18-fold less i n the P u C l ~ 6
3
c o m p l e x t h a n i n the P u ( I I I ) a q u o i o n , a n d the n u m b e r a n d shape of the peaks are c h a n g e d a n d t h e y are s h i f t e d i n energy ( F i g u r e 1 ). A s i m i l a r
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
334
L A N T H A N I D E / A C T I N I D E
C H E M I S T R Y
p r o n o u n c e d decrease i n i n t e n s i t y of t h e A m C l " s p e c t r u m vs. t h a t of t h e 6
A m (III) the f
aquo i o n was observed.
3
T h i s m a r k e d decrease i n i n t e n s i t y of
f transitions of t h e h e x a h a l i d e complexes vs. t h e a q u o i o n ( a n d
other P u ( I I I )
complexes
f o r m e d i n aqueous
solutions s u c h as sulfate,
n i t r a t e , etc.) is q u a n t i t a t i v e e v i d e n c e of o c t a h e d r a l o r near o c t a h e d r a l s y m m e t r y w i t h a center of i n v e r s i o n i n t h e h e x a h a l i d e s a n d of t h e l a c k of s u c h s y m m e t r y i n most
P u (III)
complexes.
T h i s indicates that the
P u ( I I I ) a q u o i o n a n d most o t h e r P u ( I I I ) complexes are n o t o c t a h e d r a l , a n d as f o r t h e t r i v a l e n t l a n t h a n i d e s ( 5 ) , six is n o t a c o m m o n c o o r d i n a t i o n n u m b e r of t h e t r i v a l e n t actinides. F u r t h e r studies of t h e spectra of t h e t r i v a l e n t a c t i n i d e h e x a h a l i d e complexes a r e b e i n g c a r r i e d o u t a n d w i l l b e p r e s e n t e d i n greater d e t a i l later.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Jørgensen, C. K., Acta Chem. Scand. 17, 251 (1963). Marcus, Y., J. Inorg. Nucl. Chem. 28, 209 (1966). Marcus, Y., [private communication]. Morrey, I. R., [unpublished results]. Pappalardo, R., Jørgensen, C. K., Helv. Phys. Acta. 37, 79 (1964). Pollack, S. Α., Satten, R. Α., J. Chem. Phys. 36, 804 (1962). Ryan, J. L., Inorg. Chem. 3, 211 (1964). Ryan, J. L., Jørgensen, C. K., J. Phys. Chem. 70, 2845 (1966). Ryan, J. L., Jørgensen, C. K., Mol. Phys. 7, 17 (1963). Satten, R. Α., J. Chem. Phys. 29, 658 (1958). Satten, R. Α., Schreiber, C. L., Wong, Ε. Y., J. Chem. Phys. 42, 162 (1965). (12) Satten, R. Α., Young, D., Gruen, D. M., J. Chem. Phys. 33, 1140 (1960). RECEIVED October 10, 1966. Work performed under U . S. Atomic Energy Commission Contract AT(45-1)-1830.
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
