The Hexavalent Actinides A. General chemistry

Only four actinide hexahalides are k n o w n , comprising u r a n i u m , n e p t u n i u m a n d p l u t o n i u m hexafluorides a n d u r a n i u m hexachloride, b u t oxyhalides of t h e form MO2X2 are c o m p a r a t i v e l y well k n o w n for all t h r e e e l e m e n t s ; a l t h o u g h t h e americyl ion, Am02^"'', exists b o t h in solution a n d in solid c o m p o u n d s , t h e a t t e m p t e d p r e p a r a t i o n of t h e h e x a ­ fluoride b y r e a c t i o n of t h e oxide ^^^AmgOg w i t h fluorine i n t h e presence of p l a t i n u m , using P t F g as t h e carrier gas, w a s unsuccessful (Tsujimura et al, 1963), in conformity w i t h t h e observed m a r k e d decrease i n t h e t h e r m o d y n a m i c stabilities of t h e hexafluorides w i t h increasing a t o m i c n u m b e r (Table X I I I ) . H o w e v e r , t h e use of t h e longer-lived (7600 yr) A m m i g h t p r o v e m o r e successful since t h e failure w i t h ^^lAm m a y

TABLE X I I I . S o m e p h y s i c a l p r o p e r t i e s o f t h e h e x a h a l i d e s

352 κ . w . BAGNALL

h a v e b e e n d u e t o d e c o m p o s i t i o n r e s u l t i n g f r o m t h e m o r e intense α- r a d i a t i o n f r o m t h e shorter l i v e d ( 4 5 8 y r ) ^^lAm.

T h e v i b r a t i o n a l spectra o f t h e hexafluorides i n d i c a t e t h a t t h e y h a v e r e g u l a r o c t a h e d r a l s y m m e t r y (Oh) ( U F ^ — B i g e l e i s e n et aL, 1 9 4 8 ; C l a a s e n et aL, 1 9 5 6 ; G a u n t , 1 9 5 4 ; 1 9 5 6 ; N p F g a n d P u F g — M a l m et aL, 1 9 5 5 ; W e i n s t o c k a n d C l a a s e n , 1 9 5 9 ) ; u r a n i u m h e x a c h l o r i d e is also o f n e a r o c t a h e d r a l s y m m e t r y ( Z a c h a r i a s e n , 1 9 4 8 d ) . T h e c r y s t a l structures o f some o f t h e m a r e also k n o w n ( T a b l e X I V ) .

TABLE X I V . C r y s t a l l o g r a p h i c d a t a f o r t h e h e x a h a l i d e s

C o l o u r S y m m e t r y S p a c e g r o u p L a t t i c e p a r a m e t e rs, A D e n s i t y

«0 bo Co (gcm-^)

U F e ^ W h i t e O r t h o r h o m b i c Dll-Pnma 9 - 9 0 0 8 - 9 6 2 5 - 2 0 7 5 - 0 6 0

N p F g b B r i g h t O r t h o r h o m b i c Dll-Pnma 9-91 8-97 5-21 5-00

o r a n g e

P u F g d R e d d i s h O r t h o r h o m b i c Dll-Pnma 9 - 9 5 9-02 5-26 — b r o w n

Dll-Pnma

U C l e ^ G r e e n i s h H e x a g o n a l Dla-CSm 10-97 — 6-04 3-56

b l a c k

Dla-CSm

» H o a r d a n d S t r o u p e ( 1 9 5 8 ) . ^ S e a b o r g a n d B r o w n ( 1 9 6 1 ) . c Z a c h a r i a s e n ( 1 9 4 8 d ) . d F l o r i n et al. ( 1 9 5 6 ) .

B. Hexafluorides

T h e h e x a f l u o r i d e s a r e l o w m e l t i n g , v o l a t i l e ( T a b l e X V ) s o l i d s , c o m ­ m o n l y p r e p a r e d b y t h e r e a c t i o n of a l o w e r fluoride w i t h fluorine a b o v e - 2 2 0 ° ( U F e ) , 5 0 0 ° ( N p F g ) o r 7 5 0 ° ( P u F g ) . N e p t u n i u m a n d p l u t o n i u m h e x a f l u o r i d e s a r e less s t a b l e t o h e a t t h a n u r a n i u m h e x a f l u o r i d e a n d t h e i r p r e p a r a t i o n i n t h i s w a y r e q u i r e s a h i g h e r t e m p e r a t u r e , so t h a t i t is n e c e s ­ s a r y t o a l l o w l i q u i d fluorine t o d r i p o n t o t h e h e a t e d l o w e r fluoride, g i v i n g a h i g h l o c a l c o n c e n t r a t i o n of fluorine (NpFg—^Malm et aL, 1 9 5 8 ; P u F g — W e i n s t o c k a n d M a l m , 1 9 5 6 ; F l o r i n eiaZ., 1956). A l t e r n a t i v e l y , t h e n e p t u ­ n i u m c o m p o u n d c a n b e m a d e b y h e a t i n g t h e l o w e r fluoride o n a n i c k e l

TABLE X V . V a p o u r p r e s s u r e d a t a f o r t h e h e x a f l u o r i d e s

R a n g e i ( ° C ) S t a t e l o g i o p{mm)

U F e ^ 0 - 6 4 S o l i d 6 - 3 8 3 6 3 + 0 - 0 0 7 5 3 7 7 t ~ 9 4 2 · 7 6 / ( ί + 1 8 3 - 4 1 6 ) 6 4 - 1 1 6 L i q u i d 6 - 9 9 4 6 4 - 1 1 2 6 - 2 8 8 / ( i + 2 2 1 - 9 6 3 )

1 1 6 - 2 3 0 L i q u i d 7 - 6 9 0 6 9 - 1 6 8 3 - 1 6 5 / ( i + 3 0 2 - 1 4 8 ) K p F e ^ 0 - 5 5 - 1 0 S o l i d 1 8 - 4 8 1 3 0 - 2 8 9 2 - 0 / i - 2 - 6 9 9 0 l o g ί K p F e ^

5 5 - 1 0 - 7 6 - 8 2 L i q u i d 0 - 0 1 0 2 3 - 1 1 9 1 - 1 / ί + 2 - 5 8 2 5 l o g ί PuFei> 0 - 5 1 - 5 9 S o l i d 0 - 3 9 0 2 4 - 2 0 9 5 - 0 / ί + 3 - 4 9 9 0 l o g t PuFei>

5 1 - 5 9 - 7 7 - 1 7 L i q u i d 1 2 - 1 4 5 4 5 - 1 8 0 7 - 5 / ί - 1-5340 l o g ί a O l i v e r et aL (1953). to W e i n s t o c k et al. ( 1 9 5 9 ) .

T H E H A L O G E N C H E M I S T R Y O F T H E A C T I N I D E S 353

filament in fluorine (Seaborg a n d B r o w n , 1961). I n either p r o c e d u r e t h e volatile hexafluoride is carried b y convection t o a condenser or cold surface on w h i c h it is t r a p p e d . A m o r e convenient p r e p a r a t i v e procedure is t o r e a c t n e p t u n i u m or p l u t o n i u m m e t a l or a lower fluoride w i t h p l a t i n u m hexafluoride a t r o o m t e m p e r a t u r e (Malm et al., 1959). H o w ­ ever, w h e n a n h y d r o u s liquid hydrofluoric acid is a d d e d t o caesium hexafluoroneptunate(V), d i s p r o p o r t i o n a t i o n occurs (Asprey a n d P e n n e ­ m a n , 1967), indicating a n easier low t e m p e r a t u r e r o u t e t o n e p t u n i u m hexafluoride; it is possible t h a t t h e p l u t o n i u m ( V ) fluoro complexes m a y b e h a v e in t h e s a m e w a y . T h e f o r m a t i o n of p l u t o n i u m hexafluoride b y fluorination of t h e tetrafluoride a t 200-375° in a flow s y s t e m h a s b e e n investigated as a m e t h o d of recovering p l u t o n i u m from n e u t r o n i r r a d i a t e d u r a n i u m r e a c t o r fuels ( A d a m s et al., 1957; R o b b et al., 1957;

Steindler et al, 1958, 1959); t h e p r o c e d u r e could b e used for t h e l a b o r a t o r y scale p r e p a r a t i o n of t h e c o m p o u n d .

U r a n i u m hexafluoride can also b e p r e p a r e d b y t h e action of b r o m i n e trifluoride on t h e oxides of u r a n i u m (Emeléus et al, 1948; E m e l é u s a n d Woolf, 1950) or u r a n i u m m e t a l (Vogel a n d Vogel, 1951), b y t h e a c t i o n of chlorine trifluoride on u r a n y l fluoride (Ellis a n d F o r r e s t , 1960), a n d b y t h e action of fluorine on u r a n i u m m e t a l or a n y u r a n i u m c o m p o u n d u n d e r suitable conditions. I t is u s u a l l y purified b y distillation (Ellis et al, 1958; Mears et al, 1958), b u t t h e f o r m a t i o n a n d decomposition of t h e alkali m e t a l fluoro complexes can also b e used as a purification m e t h o d p r o v i d e d fluorine is p r e s e n t (Gathers et al, 1958).

U r a n i u m hexafluoride is formed w h e n u r a n i u m tetrafluoride is h e a t e d in d r y o x y g e n a t 800° (Fried a n d D a v i d s o n , 1958),

2 U F , + 0 2- > U F e- f U 0 2 F 2

a useful p r e p a r a t i v e r e a c t i o n for w h i c h free fluorine is n o t r e q u i r e d . Some pentafluoride is also formed, either b y t h e r e a c t i o n

3 U F 4 + O2 -> 2 U F 5 + U O 2 F 2 or, a b o v e 750°, b y t h e reaction

U F 4 + U F e- > 2 U F 5

(Ferris, 1957, 1959). T h e u r a n y l fluoride p r o d u c e d in t h e r e a c t i o n c a n be c o n v e r t e d t o u r a n i u m tetrafluoride, for t h e p r o d u c t i o n of m o r e h e x a ­ fluoride, b y reducing it in h y d r o g e n t o t h e dioxide which is t h e n r e a c t e d w i t h h y d r o g e n fluoride. Some UgOg is also formed in t h e o x i d a t i o n of u r a n i u m tetrafluoride, b y w a y of decomposition of t h e resulting u r a n y l fluoride, w h i c h occurs a b o v e 700° (Ferris a n d G a b b a r d , 1958), t h e r a t e of decomposition i n a n a t m o s p h e r e of d r y helium being first order w i t h

354 κ. w. B A G N A L L

respect t o u r a n y l fluoride (Ferris a n d B a i r d , 1960). P l u t o n i u m h e x a ­ fluoride is also formed b y o x i d a t i o n of t h e tetrafluoride a n d b y t h e reaction of p l u t o n i u m dioxide w i t h a m i x t u r e of h y d r o g e n fluoride a n d oxygen,

2 PuOg + 12 H F -^ O2 2 PuFg + 6H2O

a reaction which p r o b a b l y involves t h e i n t e r m e d i a t e formation of t h e tetrafluoride, or w i t h fluorine

Pu02 + 3 F 2- > P u F e + 02

(Mandleberg et al., 1956). A t 600° p l u t o n i u m trifluoride yields only t h e tetrafluoride a n d dioxide, a r e a c t i o n w h i c h is reversed in a v a c u u m :

4 PuFg + 0 2 ^ 3PUF4 + PUO2 (Fried a n d D a v i d s o n , 1949; D a w s o n et al., 1954b).

F l u o r i n a t i o n of u r a n i u m trioxide or u r a n y l fluoride w i t h s u l p h u r tetrafluoride a b o v e 300° ( O p p e g a r d et al., 1960) is also satisfactory;

u r a n y l fluoride is formed as a n i n t e r m e d i a t e in t h e r e a c t i o n w i t h t h e t r i o x i d e :

U03 + 3 S F 4- ^ U F e + 3 S O F 2

U r a n i u m hexafluoride is r e d u c e d b y s u l p h u r tetrafluoride a b o v e 500°

UFe + S F 4 - > U F 4 + SFe

w h e r e a s p l u t o n i u m hexafluoride is r e d u c e d a t 30° ( J o h n s o n , C. E . et al., 1961), so enabling a s e p a r a t i o n of t h e t w o e l e m e n t s (Steindler, 1962).

U r a n i u m hexafluoride h a s been t h e m o s t s t u d i e d of these c o m p o u n d s b e c a u s e its volatility is of i m p o r t a n c e for t h e s e p a r a t i o n of t h e r a r e r fissile isotope, ^^^U, from t h e non-fissile ^ssxj b y gaseous diffusion m e t h o d s . T h e volatilities of t h e liquid hexafluorides are in t h e e x p e c t e d order UFg > N p F g > PuFg, b u t t h e n e p t u n i u m c o m p o u n d is a n o m a ­ lous in h a v i n g t h e highest v a p o u r pressure of t h e t h r e e in t h e solid s t a t e over t h e ranges so far i n v e s t i g a t e d , a n o b s e r v a t i o n which a p p a r e n t l y c a n n o t b e r e l a t e d t o i t s p a r a m a g n e t i s m . U r a n i u m hexafluoride (Henkel a n d K l e m m , 1935) a n d p l u t o n i u m hexafluoride (Gruen et al., 1956) exhibit weak, a n d a l m o s t t e m p e r a t u r e i n d e p e n d e n t , p a r a m a g n e t i s m . U n p a i r e d electrons are a b s e n t in UFg, b u t t w o n o n - b o n d i n g 5/-electrons are p r e s e n t in t h e p l u t o n i u m c o m p o u n d , a n d t h e s e o c c u p y t h e lowest (ffi) level w i t h p a i r e d spins, t h e / - t e r m being split i n a n o c t a h e d r a l fleld i n t o t h r e e levels, f^^fs a n d f^. I t should be n o t e d t h a t t h e g r o u n d s t a t e in P u F g is n o n - d e g e n e r a t e w h e t h e r t h e spins are p a i r e d or n o t (Grifiith a n d Orgel, 1957). T h e m o l a r susceptibility of N p F g is only 443 χ 10"^

c.g.s. u n i t s a t 300°K a n d 887 χ 10"^ a t 64°K, m a r k e d l y lower t h a n t h e

THE HALOGEN CHEMISTRY OF THE ACTINIDES 355

calculated spin only values of 1240 χ 10-^ a n d 5810 χ 10*^ c.g.s.u. o r those calculated for t h e u n q u e n c h e d orbital a n g u l a r m o m e n t u m cases (Weinstock a n d M a l m , 1957); t h e lowest susceptibility arises w h e n t h e splittings p r o d u c e d b y b o t h t h e s p i n - o r b i t interaction a n d electric field are a p p r o x i m a t e l y e q u a l (Gruen a n d H u t c h i s o n , 1954). P a r a m a g n e t i c resonance a b s o r p t i o n d a t a for n e p t u n i u m hexafluoride c a n b e i n t e r p r e ­ t e d on t h e basis of a 5/^-configuration ( H u t c h i s o n a n d W e i n s t o c k , 1960).

Since t h e volatile hexafluoride could b e utilized for t h e r e c o v e r y of u r a n i u m from i r r a d i a t e d n u c l e a r fuels ( B e r n h a r d t et al., 1959; H y m a n et al, 1956), t h e UFg-ClFa a n d UFg-BrFg p h a s e d i a g r a m s (Fischer a n d Vogel, 1954; Ellis a n d J o h n s o n , 1958) a n d t h e kinetics of t h e r e a c t i o n of UF4 w i t h fluorine ( L a b a t o n a n d J o h n s o n , 1959) a n d chlorine trifluoride ( L a b a t o n , 1959; Nikolaev a n d Shishkov, 1962; N g u y e n - N g h i , 1963), as well as of t h e reactions of s u l p h u r tetrafluoride w i t h u r a n i u m trioxide a n d u r a n y l fluoride ( J o h n s o n a n d Fischer, 1961) a n d of fluorine w i t h u r a n i u m trioxide, t r i u r a n i u m octaoxide (Iwasaki, 1964) a n d u r a n i u m dioxide ( Y a h a t a a n d I w a s a k i , 1964) h a v e been i n v e s t i g a t e d in some detail.

(i) Handling and Stability

T h e t h r e e hexafluorides d o n o t r e a c t w i t h q u a r t z or P y r e x in t h e absence of w a t e r or h y d r o g e n fluoride, b o t h of which cause v i r t u a l l y u n l i m i t e d hydrolysis of t h e hexafluorides as a result of r e a c t i o n of h y d r o g e n fluoride w i t h silica a n d its regeneration:

SiOg + 4 H F -> SiF^ + 2 H^O MF, + 2 H2O -> MO2F2 + 4 H F

This is p r e v e n t e d , in t h e case of t h e u r a n i u m c o m p o u n d , b y h a n d l i n g it in t h e presence of a n h y d r o u s sodium or p o t a s s i u m fluoride which t a k e s u p h y d r o g e n fluoride a n d w a t e r , for e x a m p l e as KHF2.2H2O (Grosse, 1958b); t h e use of silica-free a l u m i n i u m p h o s p h a t e glasses is also r e c o m m e n d e d (Grosse, 1958d). Similar m e a s u r e s would p r e s u m a b l y b e effective w i t h t h e o t h e r hexafluorides.

All t h e hexafluorides are decomposed t o lower fluorides b y α-radia­

tion, t h e effect being scarcely noticeable w i t h u r a n i u m hexafluoride m a d e from n a t u r a l u r a n i u m , b u t d e t e c t a b l e w i t h u r a n i u m enriched in t h e s o m e w h a t shorter-lived ^^^U or ^^^V, which h a v e a higher specific

α-activity (Shiflett et al., 1958). T h e decomposition is n o t serious w i t h t h e n e p t u n i u m c o m p o u n d , b u t t h e r a t e of α-radiation decomposition in solid p l u t o n i u m hexafluoride is as high as 2 % per d a y (Steindler, 1963) and t h e c o m p o u n d is best stored in t h e v a p o u r s t a t e i n order t o minimize t h e decomposition, a large p r o p o r t i o n of t h e α-particles t h e n being

356 κ. w. BAGNALL

a b s o r b e d b y t h e walls of t h e container; t h e lower t h e gas pressure a n d t h e smaller t h e d i a m e t e r of t h e container, t h e g r e a t e r t h e p r o b a b i l i t y t h a t t h e α-particles will h i t t h e walls r a t h e r t h a n molecules of t h e hexafluoride. U r a n i u m hexafluoride is relatively stable t o y-radiation u n d e r r e a c t o r conditions (Hull, 1947), whereas t h e p l u t o n i u m com­

p o u n d is quite readily decomposed (Steindler et al., 1964). Similarly, b o t h n e p t u n i u m a n d p l u t o n i u m hexafluorides are susceptible t o p h o t o -decomposition, whereas t h e u r a n i u m c o m p o u n d is stable. T h e r m a l decomposition of p l u t o n i u m hexafluoride t o t h e tetrafluoride is com­

plete in 1 h a t 280°C (Weinstock a n d Malm, 1956) b u t is quite slow a t 200°C; t h e kinetics of t h e decomposition h a v e also b e e n investigated (Fischer et al, 1961; T r e v o r r o w et al, 1961).

(ii) Chemistry

A l t h o u g h a g r e a t deal is k n o w n a b o u t t h e f o r m a t i o n a n d t h e physical properties of t h e hexafluorides, few d a t a are available on t h e i r com­

plexing a n d o t h e r chemical properties. U r a n i u m hexafluoride forms fluoro complexes w i t h t h e alkali m e t a l fluorides (Ruff a n d Heinzel-m a n n , 1911; M a r t i n et al, 1951); t h e sodiuHeinzel-m c o Heinzel-m p o u n d was initially t h o u g h t t o h a v e t h e composition NagUFg (e.g. A d a m s et al, 1958) b u t i^F exchange studies (Sheft et al, 1961) suggested t h a t t h e c o m p o u n d w a s NagUFg; t h i s w a s s u b s e q u e n t l y confirmed b y K a t z (1964). N o fluorine e x c h a n g e was observed b e t w e e n UFg a n d lithium, p o t a s s i u m , silver or zirconium fluorides labelled w i t h ^^F (Sheft et al, 1961). T h e kinetics of t h e formation of t h e sodium complex salt h a v e been investi­

g a t e d (Massoth a n d Hensel, 1958, 1959; P e k a , 1965). This work h a s been e x t e n d e d recently, a n d it h a s b e e n shown t h a t reaction of u r a n i u m hexafluoride w i t h sodium fluoride dispersed in n-perfluoroheptane a t 100° yields t h e w h i t e heptafluoro complex, N a U F , , which decomposes a t 100° in a v a c u u m t o t h e yellow octafluoro complex, NagUFg, which is of b o d y - c e n t r e d t e t r a g o n a l s y m m e t r y . T h e corresponding p o t a s s i u m salts h a v e been o b t a i n e d in t h e s a m e w a y (Malm et al, 1966). O t h e r r e c e n t w o r k on these a n d similar s y s t e m s indicates t h a t b o t h MgUFg a n d

M3UF9

m a y b e formed (M = N a , K , Cs), b u t t h a t t h e r e is n o reaction b e t w e e n u r a n i u m hexafluoride a n d lithium, s t r o n t i u m or a l u m i n i u m fluoride (Peka, 1966). A complex of composition 2 N a F. U F g. H F h a s b e e n recorded, formed from UFg a n d NaHFg ( K a t z , 1963).

O t h e r heptafluoro complexes are well k n o w n ; t h e pale yellow a m m o ­ n i u m salt,

NH4UF7

(cubic, a^ = 13-11 Â) is o b t a i n e d b y condensing u r a n i u m hexafluoride in a suspension of a m m o n i u m fluoride in

C2H2CI4

(Volavsek, 1961) or b y a d d i n g solid a m m o n i u m fluoride t o a solution of u r a n i u m hexafluoride in chlorine trifluoride; t h e caesium salt is

T H E H A L O G E N C H E M I S T R Y O F T H E A C T I N I D E S 357

p r e c i p i t a t e d w h e n a solution of caesium fluoride in chlorine trifluoride is a d d e d t o u r a n i u m hexafluoride (Nikolaev a n d S u k h o v e r k h o v , 1961).

T h e a m m o n i u m salt decomposes t o a m i x t u r e of a- a n d j3-uranium pentafluoride a t 170° in a v a c u u m , a n d t o t h e tetrafluoride a t 450°

(Volavsek, 1963). T h e corresponding h y d r a z i n i u m salt, N2H5UF7, s e p a r a t e s as yellow crystals w h e n a solution of u r a n i u m hexafluoride a n d h y d r a z i n i u m fluoride in a n h y d r o u s hydrofluoric acid is allowed t o s t a n d for 24 h (Frlec et aL, 1964). T h e greenish-yellow n i t r o s o n i u m a n d greenish-white n i t r o n i u m salts,

NOUF7

a n d

NO2UF7,

a r e formed directly from u r a n i u m hexafluoride a n d t h e corresponding fluoride

(Geichman et al., 1963); t h e UF7- ion m a y h a v e a p e n t a g o n a l b i p y r a ­ midal s t r u c t u r e . A d d i t i o n c o m p o u n d s w i t h a few m e t a l t r i - a n d t e t r a ­ fluorides h a v e b e e n r e p o r t e d , for e x a m p l e PbF4.UF6, formed b y reaction of lead difluoride w i t h a n excess of UFg a t 450° (Michallet, 1961;

Michallet et al., 1961). A curious r e d complex w i t h t i t a n i u m t e t r a ­ chloride, UFe.2TiCl4, w h i c h m a y h a v e monoclinic s y m m e t r y (with

= 6-39, = 9-87, = 8-05 Â, ^ = 79°10' a n d d e n s i t y 2-45 g cm-^) is r e p o r t e d t o be formed w h e n t h e c o m p o n e n t s are m i x e d , e v e n a t liquid air t e m p e r a t u r e s (Michallet et al., 1959) in c o n t r a s t t o t h e h a l o g e n r e p l a c e m e n t reaction which occurs b e t w e e n t u n g s t e n hexafluoride a n d t i t a n i u m t e t r a c h l o r i d e (Cohen et al., 1965). More r e c e n t w o r k (O'Donnell et al., 1966) indicates t h a t t h e p r o d u c t s of t h e r e a c t i o n of u r a n i u m hexafluoride w i t h t i t a n i u m t e t r a c h l o r i d e are u r a n i u m hexachloride, u r a n i u m a n d t i t a n i u m tetrafluorides, a n d chlorine.

U r a n i u m hexafluoride is stable t o o x y g e n , n i t r o g e n , c a r b o n dioxide, chlorine, b r o m i n e a n d fluorocarbons; a l t h o u g h stable for some t i m e in c a r b o n tetrachloride, chloroform a n d s-tetrachloroethane a t r o o m t e m p e r a t u r e , reaction occurs on h e a t i n g . T h u s , a b o v e 150° c a r b o n tetrachloride r e a c t s t o give u r a n i u m tetrafluoride, chlorine a n d chloro-fluoromethanes (Nairn et al., 1958). I t readily fluorinates silicon, arsenic, p h o s p h o r u s a n d m o s t organic c o m p o u n d s , t h e last yielding fluorocarbons. A l t h o u g h u r a n i u m hexafluoride is r e d u c e d t o t h e t e t r a ­ fluoride w h e n h e a t e d in h y d r o g e n , t h e r e a c t i o n h a s a h i g h e n e r g y of a c t i v a t i o n a n d is relatively slow even a t 600° (Dawson et al., 1950).

I t is m o r e readily r e d u c e d b y h y d r o g e n chloride (250°) or h y d r o g e n b r o m i d e (80°), w i t h t h e formation of h y d r o g e n fluoride a n d free halo­

gen; reaction w i t h a n h y d r o u s liquid a m m o n i a a t —70° yields

NII4UF5

a l m o s t q u a n t i t a t i v e l y ( J o h n s et al., 1958), w h e r e a s reaction w i t h a m m o n i a gas yields either a m m o n i u m hexafluorouranate(V) or u r a n i u m pentafluoride, as m e n t i o n e d earlier. H y d r o g e n sulphide a n d c a r b o n disulphide reduce u r a n i u m hexafluoride a t 25°, t h e former yield­

ing s u l p h u r tetrafluoride a n d h y d r o g e n fluoride, t h e l a t t e r s u l p h u r

358 κ. w. BAGNALL

tetrafluoride a n d perfluoroalkyl sulphides or, a t high t e m p e r a t u r e s , s u l p h u r hexafluoride a n d c a r b o n tetrafluoride (Trevorrow et aL, 1963).

N i t r o s y l chloride reduces u r a n i u m hexafluoride (and m o l y b d e n u m hexafluoride, b u t n o t t h e t u n g s t e n c o m p o u n d ) (Geichman et aL, 1963), as does n i t r i c oxide, yielding t h e q u i n q u e v a l e n t n i t r o s o n i u m fluoride, NOUFg Î n i t r o g e n dioxide yields t h e corresponding n i t r o n i u m salt,

NO2UF6,

in c o n t r a s t t o m o l y b d e n u m a n d t u n g s t e n hexafluorides, w h i c h d o n o t r e a c t (Geichman et aL, 1961, 1962b, c; Ogle et aL, 1959). U r a n i u m hexafluoride, w h e n p r e s e n t in excess, is r e d u c e d t o lower fluorides b y p h o s p h o r u s trifluoride, m o l y b d e n u m pentafluoride, t u n g s t e n t e t r a ­ fluoride a n d b y m a n y m e t a l a n d n o n - m e t a l chlorides. H o w e v e r , w h e n t h e hexafluoride is t r e a t e d w i t h a n excess of a l u m i n i u m chloride or b o r o n trichloride, u r a n i u m h e x a c h l o r i d e is formed (O'Donnell et aL,

1966).

C. Uranium hexachloride

This is a relatively u n s t a b l e blackish-green solid which begins t o decompose a t 120-150° a n d m e l t s a t a b o u t 178°; it sublimes a t 100°

a n d 10-4 t o r r ( J o h n s o n a n d B u t l e r , 1944). I t is usually m a d e b y dis­

p r o p o r t i o n a t i o n of u r a n i u m p e n t a c h l o r i d e a t 80-180° in a high v a c u u m (10-3_iQ-6 t o r r ) , t h e u n d e c o m p o s e d r e a c t i o n p r o d u c t subliming o u t of t h e r e a c t i o n zone ( J e n k i n s , 1951; Carter, 1956; B r i t i s h p a t e n t 818 321, U . K . A. E. A . , 1959):

2 U C l 5 - > U C l 4 + UC16

a n d , in poorer yield, b y r e a c t i o n of a lower chloride w i t h chlorine a b o v e 350° or b y t h e a c t i o n of c a r b o n t e t r a c h l o r i d e a n d chlorine on u r a n i u m trioxide a t 65-170° u n d e r pressure (Reiber, 1950; V a n D y k e a n d E w e r s , 1955).

UCI5, UOCI3

a n d

UOCI4

a p p e a r t o b e formed as i n t e r m e d i a t e s in t h e l a s t reaction (Lowrie a n d L a r s o n , 1946). T h e m o s t promising r o u t e t o t h e hexachloride a p p e a r s t o b e t h e reaction of t h e hexafluoride w i t h a n excess of a l u m i n i u m chloride or b o r o n trichloride; i t h a s also b e e n shown t h a t t h e hexachloride r e a c t s w i t h t h e hexafluoride t o form t h e tetrafluoride, chlorine being l i b e r a t e d (O'Donnell et aL, 1966).

U r a n i u m hexachloride is soluble in c a r b o n t e t r a c h l o r i d e a n d chloro­

form, slightly soluble in perfluoroheptane a n d insoluble in b e n z e n e ; it is i m m e d i a t e l y h y d r o l y s e d t o u r a n y l chloride b y w a t e r a n d r e a c t s w i t h h y d r o g e n fluoride a t r o o m t e m p e r a t u r e t o form UFg:

2UC16 + 10 H F -> 2 UF5 + 10 HCl + Clg

T h e only recorded complex is a yellow species o b t a i n e d from u r a n i u m hexachloride a n d α,α'-dipyridyl in carbon t e t r a c h l o r i d e (Gans, 1964).

THE HALOGEN CHEMISTRY OF THE ACTINIDES 359

S y m m e t r y a n d s p a c e g r o u p L a t t i c e p a r a m e t e r s (A) C a l c u l a t e d R e f e r ­ d e n s i t y e n c e

Κ Co (gcm-8)

U 0 2 F 2 R h o m b o h e d r a l , R'^m — D^^ 5 - 7 6 4 ( α = 4 2 ° 4 3 ' ) 6-37 1 N p O ^ F ^ R h o m b o h e d r a l , R~^m — Dl^ 5 - 7 9 5 ( α = 4 2 ° 1 6 ' ) 6-41 2 PUO2F2 R h o m b o h e d r a l , R'^m — D^^ 5 - 7 9 7 ( α = 42°) 6 - 5 0 3 UO2CI2 O r t h o r h o m b i c 8-73 8-41 5 - 7 3 5-43 4 1, Z a c h a r i a s e n ( 1 9 4 8 c ) . 2, Z a c h a r i a s e n ( 1 9 4 9 a ) . 3 , A l e n c h i k o v a et al. ( 1 9 5 8 a ) . 4 , B a e n z i g e r a n d R u n d l e ( 1 9 4 4 ) .

(ii) Fluorides

A n h y d r o u s u r a n y l fluoride, a p a l e yellow, hygroscopic solid, is m a d e b y h e a t i n g u r a n i u m tetrafluoride i n oxygen, as m e n t i o n e d earlier, b y t h e action of h y d r o g e n fluoride o n u r a n i u m t r i o x i d e a b o v e 300°C i n t h e presence of o x y g e n

UO3 + 2HF ^ UO2F2 + H2O

( J o h n s o n a n d Clewett, 1946; K u h l m a n , 1948), b y t h e a c t i o n of h y d r o ­ gen fluoride o n u r a n y l a c e t a t e a t 250° (Brooks et al., 1956) or b y t h e action of fluorine o n u r a n i u m oxides a t 350°.

U r a n y l fluoride decomposes, w i t h o u t melting, t o UgOg a t h i g h t e m p e r a t u r e s a s m e n t i o n e d earlier; i t is r e d u c e d t o UO2 b y h y d r o g e n a b o v e 450° a n d some UF4 is formed a t higher t e m p e r a t u r e s , p r e s u m a b l y b y r e a c t i o n of t h e h y d r o g e n fluoride w h i c h is formed w i t h t h e dioxide.

D. Oxyhalides

(i) General Chemistry

T h e u r a n y l halides a r e quite well k n o w n , b u t t h e corresponding n e p t u n y l , p l u t o n y l a n d americyl c o m p o u n d s h a v e scarcely b e e n in­

vestigated. Since b o t h PuOg^"^ a n d AmO^^ a r e r e d u c e d t o t h e t e r v a l e n t s t a t e b y iodide i n a q u e o u s acid, a n d NpOg^^ is r e d u c e d t o t h e q u a d r i ­ v a l e n t s t a t e u n d e r similar conditions, i t is doubtful w h e t h e r t h e iodides will b e o b t a i n a b l e ; indeed, i t is p r o b a b l e t h a t u r a n y l iodide h a s n e v e r b e e n o b t a i n e d p u r e a n d in a n u n s o l v a t e d s t a t e . Since AmOg^^ is also r e d u c e d t o t h e q u i n q u e v a l e n t s t a t e b y chloride or b r o m i d e i o n i n a q u e o u s solution only t h e fluoride is p r e p a r a b l e from such m e d i a a n d t h e o t h e r halides will h a v e t o b e investigated, if t h e y can b e o b t a i n e d a t all, in n o n a q u e o u s solvents. U r a n y l , n e p t u n y l a n d p l u t o n y l ions give rise t o halocomplex species such a s M^^O^f^^- a n d M^^OgFs" in t h e a p p r o p r i a t e halogen acid a n d salts of t h e s e ions, a n d of t h e analogous americyl chloro complex, h a v e been o b t a i n e d . Some crystallographic d a t a for t h e simple halides a r e given i n T a b l e X V I .

TABLE X V I . C r y s t a l l o g r a p h i c d a t a f o r t h e o x y h a l i d e s , M O 2 X 2

360 κ. w. B A G N A L L

Sulphur reduces it to a mixture of the dioxide and tetrafluoride at 500-600° (Rampy, 1961); only the latter is formed when a mixture of uranyl fluoride and sulphur is heated in hydrogen fluoride at 300-400°.

The anhydrous neptunyl compound, a pink solid, is formed, mixed with sodium fluoride, b y the action of hydrogen fluoride on sodium neptunyl acetate at 300-325° (Fried, 1954); the pure compound is made b y heating neptunium trioxide hydrate in hydrogen fluoride at 300° or b y vacuum drying the precipitate obtained b y adding hydrofluoric acid to a solution of neptunium(VI) (Bagnall et al., 1966c). I t is isomorphous with uranyl fluoride (Table X V I ) . White, gelatinous hydrated plutonyl fluoride is precipitated when methanol and concentrated hydrofluoric acid are added to an aqueous solution containing plutonium(VI) (Anderson, 1949c); the anhydrous salt is obtained when this hydrate i s washed with anhydrous hydrofluoric acid and dried over phosphorus pentoxide (Alenchikova et al., 1958a). The formation of plutonyl fluoride has been observed in the hydrolysis of plutonium hexafluoride, identiflcation being b y X-ray powder photography, the product being isomorphous with uranyl fluoride (Florin et al., 1956; Mandleberg et al., 1956), but crystallographic data were not recorded in these instances.

Americyl fluoride, a brown solid, has been made b y evaporation to small volume of an aqueous solution of americium(VI) in hydrofluoric a c i d , followed b y addition of liquid hydrogen fluoride to the frozen solution (T. K. Keenan, personal communication).

Aqueous solutions of uranyl fluoride are prepared b y dissolving uranium trioxide in hydrofluoric acid and although crystallization is difficult, trihydrates (Brooks et al., 1956) and compounds of the type U O 2 F 2 . 2 H F . 4 H 2 O can be obtained from solution (Buslaev et al., 1963).

Somewhat similar species are formed in the PuOaFg-HF-HaO system (Alenchikova et al., 1961).

Adducts of uranyl fluoride with ammonia, UO2F2.4NH3 obtained with liquid ammonia and U02F2.2(or 3)NH3 with the gas, have been recorded (Unruh, 1909).

Uranyl fluoride is very soluble in water and there is spectrophoto­

metric evidence for the formation of UO2F42- ions in solution (Blake et al., 1951), but this has not been confirmed b y solvent extraction studies (Day and Powers, 1954). Yellow complex salts such as K3UO2F5, in which the UOgFg^- ion is a pentagonal bipyramid (Zachariasen, 1954), N a U 0 2 F 3 , K3(U02)2F7, K 5( U 0 2) 2 F „ ( N H 4) 3 U 0 2 F 5 and Ba3( U 0 2 F 5)2 are readily obtained from aqueous solutions. Thus ( N H 4) 3 U 0 2 F 5 is the solid phase in equilibrium with ammonium fiuoride and uranyl fluoride in water (Ferris, 1960). The hydrated hydrazinium salt, U 0 2 F 2 . ( N 2 H 5 F ) 3. 1- 5 H 2 0 , yields the quadrivalent complex.

T H E H A L O G E N C H E M I S T R Y O F T H E A C T I N I D E S 361

N2H5UF5.H2O,

a t 200° i n a v a c u u m a n d u r a n i u m tetrafluoride a t 400°

(Sahoo a n d S a t a p a t h y , 1964).

KUO3F,

a n orange-red solid w h i c h a p p e a r s t o form a d d u c t s w i t h acetic or oxalic acid, is m a d e b y h e a t i n g u r a n i u m trioxide w i t h a n excess of p o t a s s i u m fluoride a t 850° a n d e x t r a c t i n g t h e u n c h a n g e d p o t a s s i u m fluoride w i t h w a t e r (Mitra, 1963) a n d t h e r e is also evidence for t h e existence of NaUOgF (Ippolitova a n d K o v b a , 1961).

P l u t o n y l fluoro complexes are also k n o w n ; t h e p i n k c o m p o u n d s , all of which are p r o b a b l y h y d r a t e d , are p r e c i p i t a t e d from fluoride solutions of p l u t o n i u m (VI). T h e quinolinium salt a p p e a r s t o b e of t h e form C9II7NHPUO2F3.H2O, b u t t h e p o t a s s i u m , r u b i d i u m , caesium, t e t r a ­ m e t h y l a m m o n i u m a n d p y r i d i n i u m salts h a v e n o t been characterized (Anderson, 1949c).

(iii) Chlorides

A n h y d r o u s u r a n y l chloride is a b r i g h t yellow crystalline solid (Table X V I , p . 259) which becomes orange a t high t e m p e r a t u r e s (Ochs a n d S t r a s s m a n n , 1952); it m e l t s a t 578° a n d is c o n v e r t e d t o oxides, such as UgOg, on ignition in air. I t is conveniently m a d e b y h e a t i n g u r a n i u m t e t r a c h l o r i d e in o x y g e n a t 300-350° ( J o h n s o n et al,, 1958), less satis­

factorily b y h e a t i n g u r a n i u m dioxide in chlorine, a r e a c t i o n originally d u e t o Péligot (1842b), which, a l t h o u g h it does n o t go t o completion, gives higher yields a t 800° u n d e r pressure (Prigent, 1958). A l t h o u g h i t is diflicult t o d e h y d r a t e t h e h y d r a t e s o b t a i n e d b y e v a p o r a t i n g solutions of u r a n i u m trioxide in hydrochloric acid t o d r y n e s s , i t can b e d o n e b y h e a t i n g t h e m in h y d r o g e n chloride a t 300° a n d t h e n in chlorine a n d h y d r o g e n chloride a t 400° (e.g. Ochs a n d S t r a s s m a n n , 1952; B r a d l e y et al,, 1959), b u t n o t b y refluxing t h e h y d r a t e w i t h t h i o n y l chloride, w h i c h leaves t h e m o n o h y d r a t e (Hefley et al,, 1963). T h e h e a t of forma­

t i o n of t h e a n h y d r o u s c o m p o u n d is —301-9 kcal mole"^ ( S h c h u k a r e v et al,, 1958b).

T h e reaction of u r a n i u m trioxide w i t h h y d r o g e n chloride yields t h e m o n o h y d r a t e b u t is e x t r e m e l y slow in t h e absence of m o i s t u r e . T h e m o n o h y d r a t e , w h i c h is monoclinic ( S t a r i t z k y a n d T r u i t t , 1950), c a n be d e h y d r a t e d in h y d r o g e n chloride a t 300° ( K r a u s , 1942b). B o t h m o n o -a n d t r i h y d r -a t e s c-an be isol-ated from -a q u e o u s solution -a n d -a yellow u n s t a b l e complex, UO2CI2.HCI.2II2O, which fumes in air, s e p a r a t e s from solutions of u r a n i u m ( V I ) in hydrochloric acid on cooling t o

— 10° (Aloy, 1901a). N e p t u n y l chloride h a s n o t been recorded, b u t t h e h y d r a t e d p l u t o n y l c o m p o u n d , PUO2CI2.6H2O, h a s b e e n o b t a i n e d b y v a c u u m e v a p o r a t i o n of a q u e o u s hydrochloric acid solutions of plu­

t o n i u m (VI) a t r o o m t e m p e r a t u r e . T h e solid is described as pinkish (Studier, 1954), or greenish yellow (Alenchikova et al,, 1959).

362 κ . w . BAGNALL

U r a n y l c h l o r i d e i o n i z e s t o

UO2CI+

a n d

UO2CI3-

r e s p e c t i v e l y i n d i l u t e a n d c o n c e n t r a t e d c h l o r i d e s o l u t i o n s i n 3 0 - 6 0 % e t h a n o l ( H e f l e y a n d A m i s , 1960). T h e r e i s e v i d e n c e f o r t h e f o r m a t i o n o f h y d r o l y t i c s p e c i e s , s u c h a s U02(0H)C1, i n d i l u t e s o l u t i o n s o f u r a n y l c h l o r i d e ( P o z h a r s k r i et al., 1963) a n d e l e c t r o p h o r e s i s , e l e c t r o l y s i s a n d i n f r a r e d s t u d i e s h a v e l e d t o t h e s u g g e s t i o n t h a t o n e o f t h e s p e c i e s p r e s e n t i n s u c h s o l u t i o n s m i g h t b e r e p r e s e n t e d a s [ ( U 0 3 H) 2 ]

[UCl6(OH)2]

( D u v a l , 1962). T h e f o r m e r c o u l d b e r e g a r d e d a s t h e p a r e n t a c i d o f t h e o x o c h l o r o c o m p l e x s a l t s

AIUO3CI

d e s c r i b e d l a t e r . S o l i d u r a n y l , n e p t u n y l a n d p l u t o n y l t e t r a c h l o r o c o m p l e x e s h a v e b e e n i s o l a t e d f r o m a q u e o u s s o l u t i o n . T h e y e l l o w t o y e l l o w - g r e e n s a l t s

M2UO2CI4

( M =^

NH4,

K , R b , d i h y d r a t e s a n d C s , Ν Μ θ 4 ,

NEt4

a n d NMcgH, a n h y d r o u s ) c r y s t a l l i z e f r o m a q u e o u s s o l u t i o n s o f t h e a p p r o p r i a t e c h l o r i d e s a n d h y d r o c h l o r i c a c i d (NH4, Κ — B e r z e l i u s , 1824; P é l i g o t , 1842a; a l l — R i m b a c h , 1904). T h e g o l d e n y e l l o w a n h y d r o u s s o d i u m a n d p o t a s s i u m s a l t s a r e o b t a i n e d b y p a s s i n g u r a n y l c h l o r i d e v a p o u r o v e r t h e h e a t e d a l k a l i m e t a l c h l o r i d e ( A l o y ,

1901a) a n d t h e p o t a s s i u m s a l t h a s a l s o b e e n m a d e b y f u s i n g s t o i c h e i o ­ m e t r i c a m o u n t s o f t h e c o m p o n e n t s a t 280° o r b y t h e a c t i o n o f h y d r o g e n c h l o r i d e o n p o t a s s i u m u r a n a t e ,

K2UO4,

a t 250°. I t m e l t s a t a b o u t 290°

a n d f o r m s t h e d i h y d r a t e i n m o i s t a i r (Lucas, 1964). T h e y e l l o w - g r e e n a n h y d r o u s q u a t e r n a r y a m m o n i u m , p y r i d i n i u m a n d q u i n o l i n i u m ( L o e b e l , 1907), 1 , 1 0- p h e n a n t h r o l i n i u m ( M a r k o v a n d T s a p k i n , 1961) a n d 2 , 2' - d i p y r i d y l i u m ( M a r k o v a n d T s a p k i n , 1959) s a l t s h a v e b e e n p r e p a r e d f r o m a q u e o u s s o l u t i o n ; t h e p y r i d i n i u m s a l t h a s a l s o b e e n m a d e b y p a s s i n g h y d r o g e n c h l o r i d e i n t o a n e t h a n o l i c s o l u t i o n o f u r a n y l c h l o r i d e h y d r a t e a n d p y r i d i n e ( B r a d l e y et al., 1959). T h e 2 , 2' - d i p y r i d y l i u m a n d o - p h e n a n t h r o l i n i u m s a l t s a r e a l s o f o r m e d b y a i r o x i d a t i o n o f u r a n i u m t e t r a c h l o r i d e i n D M P i n t h e p r e s e n c e o f 2 , 2' - d i p y r i d y l o r o - p h e n a n t h r o ­ l i n e ( G a n s a n d S m i t h , 1964b). S i m i l a r l y , t h e d i p h o s p h o n i u m s a l t s , ( R 3 P H ) 2 U 0 2 C l 4 , h a v e b e e n m a d e b y a i r o r h y d r o g e n p e r o x i d e o x i d a t i o n o f t h e h e x a c h l o r o u r a n a t e s( I V ) i n b o f l i n g e t h a n o l ( G a n s a n d S m i t h ,

1964a). M i x e d h a l i d e s a l t s , s u c h a s K 2 U 0 2 C l 2 B r 2 , h a v e b e e n r e c o r d e d , m a d e b y f u s i n g u r a n y l c h l o r i d e w i t h p o t a s s i u m b r o m i d e a t 300°, b y t h e a c t i o n o f h y d r o g e n c h l o r i d e o n t h e s a l t K 2 U 0 3 B r 2 a t 2 9 0° C , o r t h e a m m o n i u m s a l t a t 150°, a n d b y d e h y d r a t i o n o f t h e d i h y d r a t e w h i c h c r y s t a l l i z e s f r o m a s o l u t i o n o f u r a n i u m t r i o x i d e i n 2 0 % h y d r o c h l o r i c a c i d c o n t a i n i n g p o t a s s i u m b r o m i d e ( P r i g e n t a n d L u c a s , 1960; L u c a s , 1964). T h e a n h y d r o u s c o m p o u n d m e l t s a t a b o u t 290° a n d f o r m s t h e d i h y d r a t e i n m o i s t a i r . T h e d a r k y e l l o w c a e s i u m ( B a g n a l l a n d L a i d l e r , 1966) a n d t e t r a e t h y l a m m o n i u m t e t r a c h l o r o n e p t u n a t e( V I ) a n d t h e y e l l o w t e t r a e t h y l - , t e t r a p r o p y l - a n d t r i e t h y l a m m o n i u m p l u t o n a t e( V I ) c r y s t a l l i z e f r o m c o n c e n t r a t e d h y d r o c h l o r i c a c i d s o l u t i o n s o f t h e

THE HALOGEN CHEMISTRY OF THE ACTINIDES 363

a c t i n i d e ( V I ) a n d t h e appropriate cation chloride ( R y a n , 1963). T h e t e t r a m e t h y l - a n d t e t r a e t h y l a m m o n i u m u r a n i u m a n d p l u t o n i u m salts have also been made b y evaporating 4 M hydrochloric acid solutions of the a c t i n i d e ( V I ) w i t h t h e t e t r a - a l k y l a m m o n i u m chloride; all are o f tetragonal s y m m e t r y excepting t h e t e t r a e t h y l a m m o n i u m u r a n i u m com­

pound, w h i c h is o f monoclinic s y m m e t r y ( S t a r i t z k y a n d Singer, 1952).

A l t h o u g h a m e r i c i u m ( V I ) is r a p i d l y reduced b y chloride i o n , t h e d a r k red tetrachlorocomplex salt, C s 2 A m 0 2 C l 4 , is obtained b y t r e a t i n g t h e a m e r i c i u m ( V ) complex, C s 3 A m 0 2 C l 4 , w i t h concentrated hydrochloric acid, i n w h i c h neither salt is appreciably soluble. This reaction appears to involve a spontaneous oxidation w h i c h has been ascribed t o t h e high lattice stabilization energy of t h e a m e r i c i u m ( V I ) compound. I t exists i n t w o crystal modifications (Bagnall et al., 1967c), one o f w h i c h is iso-structural w i t h CS2UO2CI4, w h i c h is of monoclinic s y m m e t r y ( H a l l et al., 1966).

Salts such as K2UO3CI2 have been made b y heating t h e chlorodi-b r o m o u r a n a t e ( V I ) i n oxygen a t 250° (Prigent a n d Lucas, 1960, 1961), b y heating potassium u r a n a t e , K2UO4, i n hydrogen chloride a t a b o u t 200° (Lucas, 1964); t h e potassium salt is also made b y reaction of t h e stoicheiometric quantities of u r a n y l chloride m o n o h y d r a t e w i t h potas­

sium h y d r o x i d e , a n d t h e a m m o n i u m salt b y t r e a t i n g t h e m o n o h y d r a t e w i t h gaseous a m m o n i a , reactions w h i c h suggest t h a t u r a n y l chloride m o n o h y d r a t e could be considered as a n acid, H2UO3CI2. T h e potassium a n d a m m o n i u m salts are converted t o t h e corresponding tetrachloro-u r a n a t e ( V I ) b y hydrogen chloride a t 150° (Prigent a n d Ltetrachloro-ucas, 1961).

A second series o f oxochloro complexes is obtained b y heating t h e alkali m e t a l halide w i t h u r a n i u m trioxide i n a v a c u u m a t u p to 600° (Allpress a n d W a d s l e y , 1964); these are o f monoclinic s y m m e t r y a n d appear t o be non-stoicheiometric, t h e caesium salt being close t o CS0.9UO3CI0.9.

D e r i v a t i v e s of t h e d i u r a n y l i o n , U 2 0 5 ^ + , a n d t r i u r a n y l i o n , U 3 0 8 ^ + , such as K2TJ2O5CI4 a n d K2U3O8CI4, have been made b y heating t h e d i -a n d triur-an-ates, K2U2O7 -a n d K2U3O10J i n hydrogen chloride, respec­

t i v e l y a t 240° a n d 2 1 0 - 2 5 0 ° . T h e y f o r m respectively t e t r a - a n d hexa­

hydrates i n moist air (Lucas, 1964).

M a n y adducts o f u r a n y l chloride w i t h oxygen a n d nitrogen donors, usually o f t h e t y p e UO2CI2.2L, have been reported ( K a t z a n d R a b i n o ­ w i t c h 1 9 5 1 , p. 5 8 4 ) ; a l l o f t h e m are orange t o yellow or greenish-yellow crystalline solids. T h e orange d i a m m i n e is precipitated f r o m ethereal solutions o f u r a n y l chloride b y gaseous a m m o n i a a n d w h e n solid u r a n y l chloride is exposed t o t h e gas (Peters, 1909, 1912) b u t no definite product has been isolated f r o m t h e reaction of u r a n y l chloride w i t h anhydrous l i q u i d a m m o n i a (Rosenheim a n d Jacobsohn, 1906). A

364 κ. w. B A G N A L L

t r i a m m i n e , as well as complexes w i t h alkyl a m i n e s (UO2CI2.2-3L) a n d h y d r a z i n e (UO2CI2.4L) h a v e also been o b t a i n e d from t h e c o m p o n e n t s . T h e a m m i n e s are t h e m o s t t h e r m a l l y stable of these c o m p o u n d s ; u r a n y l chloride, b u t n o t t h e trioxide, is r e d u c e d t o t h e dioxide a t r o o m t e m p e r a t u r e b y a q u e o u s lOM h y d r a z i n e (Kalnins a n d Gibson, 1959).

A d d u c t s w i t h 1,10-phenanthroline (UO2CI2.L a n d UO2CI2.2L) precipi­

t a t e from ethanolic solution of t h e c o m p o n e n t s , w h e r e a s t h e 2,2'-di­

p y r i d y l c o m p l e x (UO2CI2.L.2H2O) can be o b t a i n e d from ethanolic or a q u e o u s solution (Markov a n d T s a p k i n , 1959). T h e complex w i t h t r i -b u t y l p h o s p h a t e , UO2CI2.2L, is p r e s e n t i n ligand solutions of u r a n y l chloride ( K o m a r o v a n d P u s h l e n k o v , 1961a) a n d t h e a b s o r p t i o n s p e c t r a of such solutions also i n d i c a t e t h e existence of t h i s complex a n d of t h e complex U O 2 C I 2. 3 T B P (Vdovenko et al, 1963). T h e trialkyl a n d t r i a r y l p h o s p h i n e oxide complexes (UO2CI2.2L) are m a d e b y o x i d a t i o n of t h e corresponding u r a n i u m t e t r a c h l o r i d e bis-(phosphine oxides) w i t h 100 v o l u m e h y d r o g e n p e r o x i d e (Gans a n d S m i t h , 1964a). T h e supposed tri-p h e n y l tri-p h o s tri-p h i n e a d d u c t (Majumdar et al, 1964) is t h e tri-p h o s tri-p h i n e oxide complex ( F i t z s i m m o n s et al, 1966).

Complexes w i t h pyridineiVoxides, UO2CI2.3L (L = 4 m e t h y l p y r i -dine-iV-oxide) a n d UO2CI2.2L (L = 4 - m e t h o x y - a n d 4-nitropyridine-iV^-oxide) p r e c i p i t a t e from a c e t o n e solutions of u r a n y l chloride a n d t h e ligand, whereas 4-chloropyridine-iV-oxide p r e c i p i t a t e s t h e complex UO2CI2.4L from h o t ethanolic solution ( B a l a k r i s h n a n et al, 1966). T h e e t h a n o l a d d u c t , UO2CI2.2L, is formed in t h e azeotropic d e h y d r a t i o n of h y d r a t e d u r a n y l chloride i n b e n z e n e - e t h a n o l m i x t u r e , from w h i c h it is s e p a r a t e d b y e v a p o r a t i o n of t h e solvent (Bradley et al, 1959) a n d t h a t w i t h acetic a n h y d r i d e , UO2CI2.L, is formed b y t h e reaction of u r a n i u m trioxide w i t h acetyl chloride (Chrétien a n d Oechsel, 1938). T h e acet­

a m i d e complex, UO2CI2.2L.H2O, s e p a r a t e s as green crystals w h e n t h e ligand is a d d e d t o a m e t h a n o l solution of u r a n y l chloride a n d u n d e r similar conditions u r e a forms t h e complex UO2CI2.4L (Markov a n d T s a p k i n a , 1962), b u t from a q u e o u s solution u r e a forms t h e c o m p l e x UO2CI2.2L.H2O or, if a l a r g e excess of u r e a is used, UO2CI2.3L.H2O (Markov a n d T s a p k i n a , 1959). A v a r i e t y of complexes is formed w i t h iValkyl s u b s t i t u t e d ureas, UO2CI2.2L w i t h 1,3dimethylurea a n d t e t r a -m e t h y l u r e a , UO2CI2.3L w i t h e t h y l u r e a a n d 1,3-diethylurea, a n d UO2CI2.4L or UO2CI2.5L w i t h 1,3-dimethylurea (Deptula, 1965). T h e i\ri\r-dimethylformamide complex, UO2CI2.3L is m a d e b y dissolving u r a n y l chloride in t h e ligand a n d e v a p o r a t i n g t h e solution in a v a c u u m (Lamisse et al, 1964), w h e r e a s iViV-dimethyl a c e t a m i d e forms t h e com­

plex UO2CI2.2L in a c e t o n e solution (Bagnall et al, 1966d). NNN'N'-t e NNN'N'-t r a m e NNN'N'-t h y l m a l o n a m i d e (TMMA) a n d - g l u NNN'N'-t a r a m i d e (TMGA) p r e c i p i NNN'N'-t a NNN'N'-t e

T H E H A L O G E N C H E M I S T R Y O F T H E A C T I N I D E S 365

the complexes UOgCla. 1 · 5 Ε f r o m acetone solution a n d t h e corresponding derivatives of α,αdimethylmalonamide ( H M M A ) a n d 3 , 3 d i m e t h y l -glutaramide ( H M G A ) precipitate t h e complexes UOaClg.L. UO2CI2.

1 · 5 Τ Μ 0 Α is appreciably dissociated t o UOgClg.TMGA a n d free ligand i n boiling m e t h y l cyanide a n d U O 2 C I 2. H M G A is monomeric i n t h a t solvent, w i t h t h e u r a n i u m presumably 6-coordinate, possibly i n a n octahedral environment. T h e other dicarboxylic acid a m i d e complexes are insoluble i n polar solvents a n d appear t o be polymeric (Bagnall et aLy 1966b). I n f r a r e d spectra of t h e amide a n d urea complexes indicate t h a t coordination is b y w a y o f t h e carbonyl oxygen.

(iv) Bromides

T h e only a c t i n i d e ( V I ) bromide k n o w n is t h e u r a n y l compound. T h e anhydrous salt is a blood-red hygroscopic solid w h i c h becomes yellow as i t hydrates; i t is unstable, decomposing slowly even a t r o o m t e m p e r a ­ ture w i t h t h e evolution o f bromine. I t is m a d e b y h e a t i n g t h e t e t r a ­ bromide i n oxygen a t 150-160°, t h e product being a b o u t 9 6 % pure (Powell, 1944; P o w e l l a n d N o t t o r f , 1944; Spedding et al, 1958), b y heating UOBrg i n oxygen a t 150° ( 9 8 % pure) or b y heating u r a n i u m dioxide w i t h bromine i n a sealed t u b e ( 9 5 % pure) (Prigent, 1954a, 1960). Some u r a n y l bromide is f o r m e d w h e n a m i x t u r e of u r a n i u m dioxide a n d carbon is heated i n bromine v a p o u r ; e x t r a c t i o n of t h e pro­

duct w i t h ether yields UOgBrg.EtgO ( U n r u h , 1909). Aqueous solutions are obtained b y dissolving u r a n i u m trioxide or u r a n y l acetate i n aque­

ous hydrobromic acid (Sendtner, 1879) or w h e n a n aqueous suspension of u r a n i u m dioxide is heated w i t h bromine o n a w a t e r b a t h (Richards a n d Merigold, 1902). E v a p o r a t i o n o f t h e solution yields t h e yellow-orange, deliquescent t r i h y d r a t e , formerly t h o u g h t t o be a h e p t a h y d r a t e (Sendtner, 1879), also f o r m e d w h e n t h e anhydrous compound is allowed t o h y d r a t e (Shchukarev et al, 1959). T h e t r i h y d r a t e loses one molecule of w a t e r a t 60° a n d decomposes a t higher temperatures. I t forms solvate hydrates w h e n recrystallized f r o m ether or isopropanol. T h e yellow basic salt, U 0 2( O H ) B r. 2 H 2 0 , crystallizes f r o m acid deficient solutions;

this is stable i n dilute aqueous solution, b u t concentrated solutions deposit h y d r a t e d u r a n i u m trioxide (Peterson, 1961).

T h e yellow t o yellow b r o w n h y d r a t e d complex halides, M 2 U 0 2 B r 4 . 2 H 2 O ( M = NH4, K ) , are less stable t h a n t h e chloride analogues. T h e y are made b y dissolving t h e corresponding uranates i n hydrobromic acid a n d concentrating t h e resulting solution (Sendtner, 1879). T h e potassium salt can also be prepared f r o m a 1:1 m i x t u r e o f t h e com­

ponents i n w a t e r ; w i t h t h e stoicheiometric quantities some potassium bromide is included i n t h e crystals o f t h e product (Lucas, 1964); t h e

366 κ, w. B A G N A L L

p y r i d i n i u m salt separates o n cooling w h e n pyridine is added t o a solu­

t i o n o f u r a n i u m trioxide i n boiling alcoholic hydrobromic acid (Loebel, 1907). T h e anhydrous caesium salt, CsgUOgBr^, has been obtained f r o m hydrobromic acid solution ( E l l e r t et al., 1965) a n d has been shown to be of monoclinic s y m m e t r y ( M i k h a i l o v et al., 1965). T h e dihydrates can be d e h y d r a t e d w i t h o u t decomposition i n nitrogen a t 120°; t h e anhydrous potassium compound melts a t about 290° a n d reforms t h e d i h y d r a t e i n moist air. T h e anhydrous potassium compound yields KaUOgBra w h e n heated i n oxygen a t 250°; this is also f o r m e d w h e n u r a n y l bromide monohydrate, obtained w h e n t h e t r i h y d r a t e is allowed to stand over phosphorus pentoxide i n a v a c u u m , is t r e a t e d w i t h t h e stoicheiometric q u a n t i t y of potassium h y d r o x i d e ; t h e corresponding a m m o n i u m salt is formed b y treating t h e m o n o h y d r a t e w i t h a m m o n i a gas a t room temperature. B o t h salts react w i t h hydrogen chloride a t 150° t o give t h e m i x e d halo complex, MaUOgClaBra (Prigent a n d Lucas, 1961). KUOgBr, made i n t h e same w a y as t h e chloride, has also been investigated (Allpress a n d W a d s l e y , 1964).

Adducts w i t h 2, 3 or 4 molecules o f a m m o n i a are formed i n ethereal or ethanolic solution ( U n r u h , 1909) a n d a black adduct w i t h acetic anhydride, U 0 2 B r 2. 2 L , is obtained w h e n u r a n y l acetate is refluxed w i t h acetyl bromide ( P a u l et al., 1961). Complexes w i t h 2 molecules of t r i b u t y l phosphate ( K o m a r o v a n d Pushlenkov, 1961a) or t r i b u t y l phosphine oxide ( K o m a r o v a n d Pushlenkov, 1961b) have been prepared i n t h e ligand solution a n d t h e complex w i t h 3 molecules of iViV-dimethyl-formamide prepared f r o m t h e components is of monoclinic s y m m e t r y ( K a u f m a n et al., 1963). H o w e v e r , iViV^-dimethylacetamide forms t h e complex U 0 2 B r 2. 2 L (Bagnall et al., 1966d). Phosphine oxide complexes, U02Br2.2R3PO ( R = M e , E t , P h ) , are obtained f r o m methanol or acetone solution (Gans, 1964). T h e reported triphenylphosphine adduct ( M a j u m ­ dar et al., 1964) is t h e phosphine oxide complex (Fitzsimmons et al.,

1966).

(v) Iodides

I t is doubtful whether pure anhydrous u r a n y l iodide has ever been obtained; its adducts are m u c h less stable a t r o o m t e m p e r a t u r e even t h a n those o f t h e bromide. R e a c t i o n o f ethereal u r a n y l n i t r a t e t r i ­ h y d r a t e w i t h b a r i u m iodide ( A l o y , 1901b) or o f u r a n y l chloride w i t h sodium iodide i n ether (Lamisse a n d R o h m e r , 1963) a n d evaporation o f the filtrate yields a n orange-red etherate, stable a t 0° b u t decomposing a t r o o m t e m p e r a t u r e . I t is v e r y hygroscopic, v e r y soluble i n w a t e r a n d soluble i n m e t h a n o l , ethanol, ether, acetone, pyridine a n d m e t h y l acetate. A d d i t i o n of iV^iV-dimethyl formamide to its ethereal solution

T H E H A L O G E N C H E M I S T R Y O F T H E A C T I N I D E S 367

yields t h e a d d u c t UO2I2.4L (Lamisse et al,, 1964). A supposed complex w i t h t r i p h e n y l p h o s p h i n e , UO2I2.2L ( M a j u m d a r et al,, 1964) is t h e phosphine oxide a d d u c t ( F i t z s i m m o n s et al,, 1 9 6 6 ; D a y a n d V e n a n z i ,

1966b). Aqueous solutions o f u r a n y l iodide h a v e been m a d e b y r e d u c t i o n of u r a n y l iodate w i t h aqueous sulphur dioxide ( R i c h a r d s a n d M e r i g o l d ,

1902) a n d b y double decomposition o f u r a n y l sulphate a n d b a r i u m or calcium iodide ( T r u t t w i n n , 1925; L y n d s , 1962). V a c u u m e v a p o r a t i o n o f t h e resulting solution a t r o o m t e m p e r a t u r e gives a d a r k - b r o w n p r o d u c t w h i c h m a y c o n t a i n some u r a n y l iodide h y d r a t e . A m m i n e s h a v e been r e p o r t e d ( U n r u h , 1909) a n d t h e o n l y recorded iodo complex, d a r k r e d ( P h 3 B u P ) 2 U 0 2 l 4 , has recently been p r e p a r e d f r o m m e t h y l cyanide solution ( D a y a n d V e n a n z i , 1966b).

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In document The Halogen Chemistry of the Actinides K. W. B A G N A L L* Atomic Energy Research Establishment, Chemistry Division, Harwell, England (Pldal 49-80)