Fluorides of composition i n t e r m e d i a t e b e t w e e n MF4 a n d MF5 h a v e b e e n recorded for p r o t a c t i n i u m , u r a n i u m a n d p l u t o n i u m ; P a ^ F i , (or PagFg), a b l a c k solid i s o s t r u c t u r a l w i t h U2F9, h a s b e e n m a d e b y h e a t i n g p r o t a c t i n i u m p e n t o x i d e w i t h h y d r o g e n fluoride a n d h y d r o g e n a t 500°
(Stein, 1965) a n d b y t h e r m a l d e c o m p o s i t i o n of a m m o n i u m h e p t a -fluoroprotactinate(V) (NH4)2PaF7 (Brown, 1965). T h e black u r a n i u m c o m p o u n d s U2F9 a n d U4F17 h a v e also b e e n recorded, t h e former o b t a i n e d b y h e a t i n g u r a n i u m tetrafluoride in t h e v a p o u r of t h e h e x a fluoride a t 200° or b y p a r t i a l decomposition of u r a n i u m pentafluoride a t 100-200°, a n d t h e l a t t e r b y h e a t i n g t o g e t h e r t h e stoicheiometric q u a n t i t i e s of u r a n i u m t e t r a - a n d pentafluorides a t 215° or b y h e a t i n g u r a n i u m tetrafluoride in t h e v a p o u r of t h e hexafluoride for t w o d a y s ; t h e r a t e of diffusion of t h e hexafluoride i n t o t h e solid is a p p a r e n t l y t h e controlling s t e p of t h e reaction (Agron et aL, 1958). These u r a n i u m c o m p o u n d s a r e m o r e stable t o hydrolysis t h a n t h e pentafluoride a n d b o t h decompose t o a m i x t u r e of t h e t e t r a - a n d hexafluorides a t h i g h t e m p e r a t u r e s . T h e p r e p a r a t i o n a n d properties of t h e s e c o m p o u n d s , a n d of U5F22, h a v e b e e n described in some detail in a r e c e n t r e p o r t (Nguyen-Nghi, 1961).
A n analogous p l u t o n i u m c o m p o u n d , a brick-red solid t h o u g h t t o be PU4F17, r e m a i n s as a residue in t h e r e a c t i o n of fluorine w i t h p l u t o n i u m tetrafluoride a t 500-600°; its X - r a y p o w d e r p a t t e r n is said t o resemble t h a t of U4F17 (Mandleberg et aL, 1956).
340 κ. w. B A G N A L L
D. Pentachlorides
(i) Preparation and Properties
P r o t a c t i n i u m pentachloride is a pale yellow solid which m e l t s a t 301°
(Grosse, 1934b); t h e P a — C l v i b r a t i o n s in t h e infrared s p e c t r u m a p p e a r a t 362 a n d 322 cm-^, which suggested t h a t t h e c o m p o u n d w a s a t least dimeric (Bagnall a n d B r o w n , 1964). A full s t r u c t u r e analysis h a s n o w shown t h a t p r o t a c t i n i u m is 7-coordinate in t h e c o m p o u n d , t h e s t r u c t u r e consisting of infinite chains of p e n t a g o n a l b i p y r a m i d a l P a C l , groups which share p e n t a g o n edges (Dodge et al., 1967). I t is soluble in d r y m e t h y l cyanide, in t e t r a h y d r o f u r a n , in alcohols w i t h reaction a n d is also slightly soluble in carbon tetrachloride. T h e c o m p o u n d w a s first obtained, a l t h o u g h n o t definitely identified as such, b y h e a t i n g t h e p e n t o x i d e in carbonyl chloride a t 550° (Grosse, 1934b) a n d , later, b y h e a t i n g t h e p e n t o x i d e in carbon t e t r a c h l o r i d e v a p o u r a t 300° (Sellers et al., 1954), w h e n it w a s shown t h a t h y d r o g e n r e d u c t i o n of t h e p r o d u c t yielded t h e tetrachloride. I t h a s also been o b t a i n e d in a b o u t 5 0 % yield b y t h e r m a l decomposition of t h e t h i o n y l chloride a d d u c t , 2PaCl5.SOCl2, a t 150° in a v a c u u m (Bagnall a n d B r o w n , 1964) a n d b y h e a t i n g t h e p e n t o x i d e w i t h a l u m i n i u m chloride (A. G. Maddock, personal c o m m u n i cation). Since severe losses occur w h e n p r o t a c t i n i u m p e n t o x i d e is h e a t e d in a s t r e a m of c a r b o n t e t r a c h l o r i d e v a p o u r , either alone or m i x e d w i t h chlorine, t h e chlorination is best carried o u t either b y h e a t i n g a m i x t u r e of t h e p e n t o x i d e a n d carbon w i t h chlorine a n d carbon tetrachloride a t 500-700° in a sealed t u b e , a reaction w h i c h also yields t h e oxychloride, PagOClg, from which t h e p e n t a c h l o r i d e is s e p a r a t e d b y v a c u u m sublima
t i o n a t 200°, or b y h e a t i n g low fired p r o t a c t i n i u m p e n t o x i d e in a sealed t u b e a t 300-500° w i t h t h i o n y l chloride v a p o u r (Brown a n d J o n e s , 1966a).
P r o t a c t i n i u m p e n t a c h l o r i d e is a p p r e c i a b l y less volatile t h a n silicon a n d t i t a n i u m tetrachlorides, a p r o p e r t y which h a s b e e n used t o a d v a n t a g e i n t h e s e p a r a t i o n of p r o t a c t i n i u m from p i t c h b l e n d e residues, b y h e a t i n g t h e m , m i x e d w i t h g r a p h i t e , in chlorine a t 800° a n d condensing t h e p r o t a c t i n i u m p e n t a c h l o r i d e in a t r a p m a i n t a i n e d a t 150°, a t e m p e r a t u r e a t w h i c h silicon a n d t i t a n i u m tetrachlorides r e m a i n in t h e v a p o u r p h a s e (Conte et al., 1964).
U r a n i u m p e n t a c h l o r i d e , usually described as d a r k r e d crystals or a b r o w n r e d powder, is dimeric in c a r b o n t e t r a c h l o r i d e (Goren et al.,
1946), explaining t h e observed d i a m a g n e t i s m of t h e p e n t a c h l o r i d e in t h i s solvent (RiidorflF a n d Menzer, 1957). I t is v e r y soluble in t h i o n y l chloride a n d in c a r b o n disulphide.
Since t h e c o m p o u n d is t h e r m a l l y u n s t a b l e e v e n a t 100° a n d dispro
p o r t i o n a t e s readily, its m e l t i n g p o i n t is u n k n o w n a n d i t is difficult t o
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 341
obtain in a pure state. Liquid phase chlorination of uranium trioxide or triuranium octaoxide at 250° with a mixture of carbon tetrachloride and chlorine in a sealed tube appears t o be fairly satisfactory (Michael and Murphy, 1910), but it is probably best prepared by heating uranium tetrachloride in chlorine at about 550° and quenching the vapour (Webb, 1943), although the experimental conditions must be carefully controlled. I t is also formed in most of the chlorination reactions used for the preparation of the tetrachloride, from which it can be separated b y recrystallization from liquid chlorine (Grosse, 1958a). I t can also be recrystallized from carbon tetrachloride (Gans, 1964). The methods used for its preparation have been discussed in some detail b y K a t z and Rabinowitch (1951, pp. 489-491).
Uranium(V) chloride alkoxides, of the form UC1^(0R)5_^, have been made b y reaction of the penta-alkoxide with hydrogen chloride; they are green liquids, soluble in non-polar solvents (Jones et al., 1956).
The magnetic behaviour of uranium pentachloride (and of the thionyl chloride adduct, UCI5.SOCI2) has been reported to be consistent with a
%άλ (Rtidorff and Menzer, 1957) and with a 5/^ (Handler and Hutchinson, 1956) configuration for the ion, the latter being the more likely since the absorption spectrum of the thionyl chloride adduct in carbon tetrachloride fits the 5/^ configuration very satisfactorily (Karracker, 1964). Other observations on the absorption spectra of uranium penta
chloride in various solvents are also available (Sterett and Calkins, 1949; Rohmer et al., 1952; Bagnall et al., 1964c).
(ii) Complexes
A few complexes of the pentachlorides with oxygen or nitrogen donors are now known; the orange-red uranium and pale-yellow protactinium pentachloride-phosphine oxide complexes, MCI5.R3PO, are conveni
ently prepared b y the reaction of protactinium pentachloride with the phosphine oxide in methyl cyanide or, better, methylene dichloride, and b y treating caesium hexachloroprotactinate(V) with the phosphine oxide in methylene dichloride (Brown et al., 1966), a reaction success
fully used for the preparation of the uranium pentachloride complexes (Bagnall et al., 1965c). As with the niobium and tantalum pentachloride analogues, the position of the P = 0 vibrational frequency in these complexes has shifted b y over 200 cm*^ from that of the free ligand, but the protactinium and uranium complexes remain unchanged in the presence of an excess of the ligand (Brown et al., 1966), unlike the niobium and tantalum complexes, which react to give the oxychloride complexes MOCI3.2R3PO {Δν ( P = 0 ) = 25 c m - i ) (Brown et al., 1966;
Copley et al., 1965), a difference which might be due to 77-backbonding
342 κ . w. BAGNALL
from t h e oxygen of t h e h g a n d i n t o readily accessible 5/-orbitals in p r o t a c t i n i u m ( V ) a n d u r a n i u m ( V ) . u r a n i u m p e n t a c h l o r i d e forms a complex w i t h tri-iV^iV-dimethyl p h o s p h o r a m i d e , p r e p a r e d in t h e s a m e w a y as t h e p h o s p h i n e oxide complex, b u t it d i s p r o p o r t i o n a t e s r a p i d l y , as do t h e complexes w i t h a r y l or alkaryl k e t o n e s , so t h a t t h e s e com
p o u n d s h a v e n o t b e e n isolated (Bagnall et aL, 1965c).
T h e a d d u c t s w i t h t h i o n y l chloride, yellow 2PaCl5.SOCl2 (Bagnall a n d B r o w n , 1964) a n d r e d UCI5.SOCI2 (e.g. B r a d l e y et al, 1957), t h e former o b t a i n e d b y dissolving freshly p r e c i p i t a t e d p r o t a c t i n i u m ( V ) h y d r o x i d e in t h i o n y l chloride a n d e v a p o r a t i n g t h e resulting solution in a v a c u u m a n d t h e l a t t e r b y refluxing u r a n i u m trioxide w i t h t h i o n y l chloride u n t i l dissolution is complete, followed b y v a c u u m e v a p o r a t i o n , a r e m o r e correctly f o r m u l a t e d as hexachlorometallates(V), SO(PaCl6)2 a n d SOC^UClg). T h e orange-red u r a n i u m p e n t a c h l o r i d e - p h o s p h o r u s p e n t a chloride complex, UClg.PClg, o b t a i n e d b y h e a t i n g u r a n i u m t r i o x i d e w i t h p h o s p h o r u s p e n t a c h l o r i d e (Cronander, 1873), ionizes as PCI4+UCI6" in p h o s p h o r u s oxytrichloride, a complex of composition 2UCI4.UCI6.6POCI3 being recovered from t h e a n o l y t e ; t h e last is also o b t a i n e d b y a d d i n g small a m o u n t s of w a t e r t o p h o s p h o r u s oxytrichloride solutions of t h e p h o s p h o r u s p e n t a c h l o r i d e complex (Panzer a n d S u t t l e , 1961). A d a r k red complex w i t h trichloroacryloyl chloride, 5UCI5.
CClg^CClCOCl, w h i c h m e l t s a t 96° (sealed t u b e ) is o b t a i n e d as a n i n t e r m e d i a t e in t h e chlorination of u r a n i u m trioxide w i t h hexachloro
p r o p e n e ( P a n z e r a n d S u t t l e , 1960a), b u t n o s t r u c t u r a l d a t a h a v e b e e n r e p o r t e d .
E. Pentabromides
P r o t a c t i n i u m p e n t a b r o m i d e , a r e d solid, is b e s t p r e p a r e d b y t h e a c t i o n of b r o m i n e on a m i x t u r e of p r o t a c t i n i u m p e n t o x i d e a n d c a r b o n a t 600-700° in a sealed e v a c u a t e d t u b e ; t h e o x y b r o m i d e , PaOBrg, is o b t a i n e d as a b y - p r o d u c t of t h e r e a c t i o n (Brown a n d J o n e s , 1966b).
Earlier p r e p a r a t i v e w o r k , in w h i c h t h e p r o d u c t was n o t definitely identified, includes h e a t i n g t h e p e n t o x i d e w i t h h y d r o g e n b r o m i d e , c a r b o n t e t r a b r o m i d e or a m i x t u r e of t h i o n y l b r o m i d e a n d h y d r o g e n b r o m i d e , a n d t h e p e n t a c h l o r i d e w i t h h y d r o g e n b r o m i d e or t h i o n y l b r o m i d e (Sellers et al, 1954), t h e p r o d u c t of t h e r e a c t i o n of t h e p e n t oxide w i t h a l u m i n i u m b r o m i d e s u b s e q u e n t l y being identified as t h e p e n t a b r o m i d e (A. G. M a d d o c k , personal c o m m u n i c a t i o n ) , a l t h o u g h this p r e p a r a t i v e m e t h o d is n o t v e r y satisfactory because of t h e difficulty of s e p a r a t i n g t h e p r o d u c t from a l u m i n i u m b r o m i d e .
U r a n i u m p e n t a b r o m i d e is o b t a i n e d b y h e a t i n g u r a n i u m trioxide w i t h c a r b o n t e t r a b r o m i d e a t 110-130°, careful t e m p e r a t u r e control being
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 343
necessary in order t o a v o i d t h e f o r m a t i o n of u r a n i u m t e t r a b r o m i d e or t h e o x y b r o m i d e , UOBrg (Prigent, 1954a, 1960); i t is also said t o b e formed b y h e a t i n g UO3 w i t h carbonyl b r o m i d e . U r a n i u m p e n t a b r o m i d e is insoluble in c a r b o n t e t r a b r o m i d e a n d is soluble in, a n d d e c o m p o s e d b y , w a t e r , alcohol a n d acetone.
T h e only recorded complex of a n actinide p e n t a b r o m i d e is t h e m e t h y l c y a n i d e a d d u c t , PaBr5.3CH3CN (Brown a n d J o n e s , 1966b).
F. Pentaiodides
T h e black p r o t a c t i n i u m c o m p o u n d is formed w h e n t h e p e n t o x i d e is h e a t e d w i t h a l u m i n i u m iodide a t 500° (A. G. M a d d o c k , personal com
m u n i c a t i o n ) a n d is p r o b a b l y formed in t h e a n a l o g o u s r e a c t i o n w i t h a m m o n i u m iodide (Sellers et al., 1954), a l t h o u g h t h e p r o d u c t of t h e l a t t e r r e a c t i o n h a s n o t b e e n formally identified. P r o t a c t i n i u m p e n t a iodide is formed in a b o u t 7 0 % yield b y r e a c t i o n of t h e p e n t o x i d e w i t h silicon t e t r a i o d i d e a t 600° in a v a c u u m , b u t it is b e s t p r e p a r e d from t h e elements a t 450° or, m o r e conveniently, b y r e a c t i o n of t h e p e n t a c h l o r i d e or p e n t a b r o m i d e w i t h silicon t e t r a i o d i d e a t 180°. I t sublimes in a v a c u u m a t 450° a n d is soluble in m e t h y l c y a n i d e (Brown et al., 1967).
T h e k n o w n t h e r m a l instability of u r a n i u m t e t r a i o d i d e r e n d e r s it e x t r e m e l y unlikely t h a t u r a n i u m or higher a c t i n i d e p e n t a i o d i d e s will b e isolable.