The Isolation of Lithium Rhenide
(Preliminary c o m m u n i c a t i o n )
B y A . v. GROSSE
Contribution from the Research Institute of Temple University, Philadelphia, Pa.
(Z. Naturforschg. 8 b. 533—536 [19531; eingegangen am 23. Juni 1953)
To Dr. Walt er Nod clack
on his 60th
birthday
Das einwertig-negative Rhenid-Ion, Re , ist vor 16 Jahren von L u n d e l l und K n o w 1 e s
in sehr verdünnten, sauren, wässerigen Lösungen beobachtet worden. Diese Lösungen wurden
seither mehrfach von physikalisch-chemischer Seite untersucht, ohne daß jedoch ein Weg zur
Konzentrierung und Isolierung der Rhenide beschrieben wurde.
Anzunehmen war, daß salzähnliche binäre Verbindungen mit Alkalimetallen, wie z. B.
Natriumrhenid — Na+ Re - , existenzfähig sein würden.
Es wurde festgestellt, daß wässerige Kaliumperrhenat-Lösungen in einfacher Weise von
Lithiummetall zu Lithiumrhenid reduziert werden können; das nebenbei in größeren Mengen
gebildete Li0H-H.,0 kann vom leicht löslichen Lithiumrhenid durch fraktionierte Kristallisation
glatt abgetrennt werden. Das feste, salzähnliche Lithiumrhenid ist in Abwesenheit von Sauerstoff bei Zimmertemperatur relativ stabil; somit eröffnet sich jetzt die Möglichkeit zum Studium der Alkalimetallrhenide.
T
Na+
he element rhenium was discovered in 1925 by
I d a T a c k e and W a l t e r N o d d a c k , with
O . B e r g . In the intervening 28 years the very interesting chemistry of this metal has been extensively
studied and many valuable observations recorded 1.
As is well known, this metal is a member of the
subgroup of the halogen family in the Periodic System
and the second higher homologue of manganese or
Mendelejeff's dvimanganese. In our opinion one of
the most striking discoveries in rhenium chemistry
was the observation made sixteen years ago by
Lundell and Knowles 2 at the U.S. Bureau of Standards, that rhenium can exist as a uninegative
rhenide
ion, or Re", in very dilute aqueous solutions.
This is the first time that a metal has been observed
as a negative ion and would imply that salt-like compounds of rhenium and other metals, for example,
the alkalis or alkali earths, should be possible. For
example, sodium rhenide or
1 See I. and W. N o d d a c k ,
"Das Rhenium", Leopold Voss Verlag, Leipzig 1933.
2 G. E. F. L u n d e l l
and H. B. K n o w 1 e s , J. Res.
Nat. Bur. Standards 18, 629 [1937].
3 O. T o m i c e k and F . T o m i c e k , Collect, czechoslov. chem. Commun. 11, 626 [1939].
4 J. J. L i n g a n e, J. Amer. diem. Soc. 64, 1001 [1942].
5 J. J. L i n g a n e , J. Amer. chem. Soc. 64, 2182 [1942].
r> E. K. M a u n and N . D a v i d s o n , J. Amer. diem.
Soc. 72, 3509 [1950].
" C. L. R u 1 f s and P. J. E 1 v i n g , J. Amer. chem. Soc.
73, 3284—3286 [1951].
Re-
would be expected to exist and it would be particularly interesting to study both its similarities
and
differences
from a typical salt, such as:
N a + Cl—
The dilute aqueous solutions of rhenide ion were
studied from a physical-chemical standpoint repeatedly 3 — 7 after their discovery, but no successful attempt to concentrate and isolate pure rhenides from
them is described in the literature. The highest concentration obtained in aqueous solution was 2.6 millimoles rhenide ion per liter in experiments of R u 1 f s
and E 1 v i n g 8 .
W e became interested in this field of rhenium chemistry in connection with our program of preparing
volatile asymetric compounds of various elements for
the study of their microwave spectra in the Department of Physics of Columbia University 9 . If alkali
8 C. L. R u 1 f s and P. J. E 1 v i n g , J. Amer. chem.
Soc. 73, 3287—3292 [1951],
9 See microwave spectrum of MnO.,F, A. J a v a n and
A. v. G r o s s e , Physic. Rev. 87, 227 [1952]; of Re «sOjCl
and Re i " 0 3 C l , A. J a v a n , G. S i 1 v e y , C. H . T o w n e s and A. v. G r o s s e ,
Physic. Rev. 91, 222 [1953];
isolation of Mn0 3 F, A. E n g e 1 b r e c h t and A. v. G r o s s e,
General Program of the 118th Meeting of the American
Association for the Advancement of Science in Philadelphia, Pa., 1951, p. 140; also isolation of Cr0 2 F 2 , A. E n g e l b r e c h t and A. v. G r o s s e , J. Amer. chem. Soc. 74,
5262 [1952].
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rhenides can be isolated it would be of interest to
explore whether a volatile hydrogen rhenide, HRe,
or volatile alkyl and aryl rhenides, such as CH 3 • Re
and C 6 H 5 • Re, would exist.
L u n d e 11 and K n o w 1 e s 2 prepared rhenide ion
solutions by reducing perrhenate ion with amalgamated zinc in dilute sulfuric acid in a Jones reductor
using ice-cold solutions and in the complete absence
of oxygen. These results were confirmed first by
O. T o m i c e k and F . T o m i c e k 3 and later in two
polargraphic investigations by L i n g a n e 4 ' 5 . L i n g a n e 4 also reduced perrhenate ion to rhenide at the
dropping mercury electrode. Numerous studies were
made on the reduction of perrhenate ion to rhenide
ion and the latter's oxidation to various positive
valence states of rhenium 3~8. R u 1 f s and E 1 v i n g s
indicate that coordination complexes are formed with
ethylene diamine and pyridine in hydrohalic acid
media. All of the above investigations, however, do
not materially advance our knowledge of rhenide
compounds from the standpoint of inorganic chemistry beyond the original discovery of L u n d e 11 and
K n o w l e s 2 . Thus S i d g w i c k in his two well
known monumental volumes on " T h e Chemical Elements and Their Compounds" 1 0 does not discuss
rhenides and refers the reader to the literature "for
evidence of Re~ in solution" 2> 5 .
Last year G r i s w o l d , K l e i n b e r g and B r a v o 1 1
reported that they were able to reduce potassium
perrhenate in ethylenediamine-water solutions by
means of potassium metal and obtain a solid mixture
of potassium hydroxide containing rhenide. A method
of separating rhenide from KOH and concentrating
it was not described, however.
In the present investigation our immediate objective
was to see whether rhenides could be concentrated
and whether they were stable enough to be isolated
in the form of a pure compound.
We found that pure aqueous solutions of potassium
perrhenate can be readily reduced with metallic lithium. Lithium is the strongest reducing agent known
in aqueous solution and tops the list of other reducing
agents with a standard electrode potential, E 0 , of the
electrode reaction:
Li —• L i + + e ~
equalling 2.9578 volts at 2 5 ° C 1 - .
The main reason why we chose lithium in preference to the other alkali metals, however, was the
fact that the main reaction product, namely, lithium
hydroxide, crystallizes nicely and easily in the form
of LiOH • 1 HoO and has solubility characteristics
which are advantageous for methods of separation by
fractional crystallization (see experimental part). Furthermore, lithium reacts much less violently with
water than the other alkali metals.
The lithium reduction does not form any of the
colored intermediate valence stages of rhenium in
observable amounts. The solution contains only rhenide ion and unreacted perrhenate ion.
W e found that we can separate the easily soluble
and stable lithium rhenide from even a large excess
of lithium hydroxide by fractional crystallization in
aqueous solution. Unreduced K R e 0 4 is very sparingly
soluble in LiOH solution and there is no difficulty at
all in separating most of it from LiOH and even less
from the easily soluble lithium rhenide.
W e have obtained pure lithium rhenide, probably
as a hydrate, in the form of a practically white crystalline solid (or pale yellow in color in thicker layers).
It is very easily soluble in water. Both the solid and
the concentrated or dilute aqueous alkali solutions are
stable for many weeks at room temperature, if kept
under a nitrogen atmosphere.
These solutions and lithium rhenide crystals are
not particularly sensitive to air or oxygen. Flasks containing these crystals can be opened to the atmosphere, for example, for taking a sample, without
noticeable loss in rhenide content, provided they are
evacuated quiddy, flushed with pure nitrogen and left
under nitrogen. This procedure can be repeated a
number of times. However, if these solutions are permitted to stand in the air for many hours or days the
rhenide is oxidized completely. Thus the oxygen
sensitivity of alkaline rhenide solutions is vastly different from such oxygen-sensitive substances as, for
example, sodium triphenylmethyl; its deep red solution in ether is discolorized in a few seconds when
exposed to air or oxygen.
A lithium rhenide solution readily reduces a hot
acidified potassium permanganate solution. Titration
of rhenide solutions with K M n 0 4 affords a ready
method to determine rhenide quantitatively.
The rhenide ion is oxidized by excess standard perOxford, at the Clarendon Press, England, 1950.
p. 1314.
12 See, for example, H . S . T a y l o r ,
"A Treatise of
11 E. G r i s w o 1 d , J. K 1 e i n b e r g and J. B. B r a v o , Physical Chemistry", Vol. I, 1931. p. 827 (D. Van Nostrand Co., New York. N. Y.).
Science [Washington] 115, 375—376 [1952].
10
manganate in boiling dilute sulfuric acid, to perrhenic
acid, i.e.
R e — + 4 [O]—• [ReO,] — .
Thus one equivalent of rhenide consumes eight
equivalents of oxygen and forms one equivalent of
perrhenate ion.
A sensitive test for rhenide ion is a hydrochloric
acid solution of bismuth trichloride. A rhenide solution reduces it in a few seconds to a black bismuth
precipitate.
W e can also confirm the thallium test of G r i s wold,
Kleinberg
and B r a v o 1 1 .
nearly saturated aqueous
We
used a
solution of thallous hy-
droxide. T h e L i B e solutions containing also L i O H ,
produced in it, at room temperature, nearly instantly,
a white
precipitate of probably
thallous
rhenide,
T l + R e " . It turned brown in a few seconds and then
b e c a m e black due to reduction. In cold solutions, at
0 ° C or below, the thallous rhenide precipitate was
stabler, but also turned completely blade in a minute
or so
l3.
After the stability of lithium rhenide was demonstrated we made a preliminary experiment of reacting
rhenium metal powder with potassium. On heating,
the two
metals reacted "unter
Feuererscheinung"
(this may have been due to slight oxidation of Dr.
N o d d a c k ' s sample of rhenium!); on decomposing
the reaction product carefully, in the absence of air
and under reduced pressure, with water, a potassium
hydroxide solution was obtained which gave a strong
positive test for rhenide ion
14 .
Experimental Part
1. P r e p a r a t i o n o f L i t h i u m
R h e n i d e S o l u t i o n by R e d u c t i o n of
Potassium Perrhenate Solution with
Metallic Lithium
Chemicals
Used:
a) Rhenium; 10 g of rhenium powder donated to the
writer by Dr. W. N o d d a c k in 1931 from some
of his first supply of rhenium, were used in this
research. Furthermore, chemically pure K R e 0 4 from
the Department of Chemistry, University of Tennessee, Knoxville, Tennessee, was also used.
b) Lithium; lithium wire of the M e t a l l o y C o r p o r a t i o n , Minneapolis, Minnesota was used. 100 cm
wire weighed 7.38 g or 73.8 mg/cm. It was cleaned
and washed under anhydrous ether and used dry.
13 C. L. R u 1 f s and P. J. E 1 v i n g , J. Amer. chem.
Soc. 72, 3304 [1950], attempted to measure the solubility
of TIRe in water without anticipating that it could be
unstable.
c) Pure nitrogen ( 0 2 content = 0.001%) in a cylinder.
R e d u c t i o n of K R e 0 4 S o l u t i o n w i t h L i t h i u m : A typical reduction was carried out as follows:
1.50 g K R e 0 4 ( = 5.18 millimoles) were dissolved in warm
150 cc H 2 0 (solubility of KReO a in H.,0 at 20° C
1.01 g/100 g sol., at 90° C = 9.50 g/100 g sol.) " i n an Erlenmeyer flask. This flask had two glass-ground joints; one
joint was capped and used for the addition of lithium,
while the second led to a gas delivery tube, dipping under
mercury.
7.45 g Li-wire ( = 1.08 g atom, about 102 cm), that is, a
very large excess, were added in pieces 3—4 cm long to
the solution. The lithium reacted rapidly but not violently,
in contrast to the other alkali metals, forming mainly hydrogen and lithium hydroxide. After a while the solution
began to boil. It was kept close to boiling and a little
later, as the concentration of LiOH in the solution increased, the reaction of lithium with the solution slowed
down; one observed various colors on the reacting lithium
surface and after a while white crusts of LiOH began to
appear. After a couple of hours the reaction was complete;
the hot solution was rapidly filtered through a glass frittered plate funnel under nitrogen, from small amounts
of a flocculent brownish precipitate into a second flask
and allowed to cool under nitrogen.
A titration of an aliquot sample of the solution showed
that 10% of the perrhenate was reduced to rhenide ion.
or that the rhenide ion concentration was about 3.5 millimolar (as mentioned previously, no colored intermediate
valence stages of rhenium appeared in this solution). No
attempt was made at this time to determine optimum conditions for maximum rhenide yields. The batdi can be
increased by a factor of IV2 to 2, but to insure ready
control of the reaction it is not recommended to go substantially beyond this point.
2. C o n c e n t r a t i o n a n d I s o l a t i o n
of L i t h i u m R h e n i d e by F r a c t i o n a l
Crystallization
Several reduction batdies can be conveniently combined for the production of larger quantities of lithium
rhenide.
The clear colorless solution on cooling and standing,
preferably over night, deposited first the characteristic
white bipyramids of unreacted K R e 0 4 which is only
sparingly soluble in LiOH solution. These crystals can
be recovered at this stage by decantation or filtration
through a glass-frittered funnel or later and in higher
yields as described below. The crystallization of
LiOH • H.,0 and concentration of rhenide is preferably
carried out by means of standard glass ground joint flasks
connected by means of T's, V's and stopcocks to a
vacuum line and to pure nitrogen and mounted on a
14 On repeating the test with lithium and rhenium in a
platinum crucible it was observed that platinum reacts
with molten lithium, in the course of a few minutes (!) at
a temperature slightly above its melting point (186° C!).
We intend to studv this unusual reaction between Li
and Pt.
15 See I. and W. N o d d a c k , c i t . p . 47.
S c h l e n k cross stand 16 . Filtration of the solution can
be conveniently carried out through a glass-frittered plate
(for example, as shown on drawing 37) 16.
The solubility of lithium hydroxide in water is as
follows (in g LiOH/lOOg H , 0 ) :
at 0° C = 12.7 g
30°
= 13.1 g
80 c
100°
-- 15.3 g
17.5 g
The solubility in ethanol is substantially lower.
The rhenide-containing solution was first concentrated by
distillation to V2—V3 of its original volume in an 0 2 -free
atmosphere and then allowed to crystallize so that well
developed crystals of LiOH • H.,0 formed in the mother
liquor. Formation of crusts on the walls is to be avoided.
The crystallization was completed by ice cooling. The
crystals were sucked off rapidly but completely on a
frittered glass filter and washed with small portions of
ice-cold ethyl alcohol, or, if the presence of alcohol is
undesired, with smaller portions of ice-cold water. The
LiOH • H.,0 crystals, if the operations are carried out
properly, are practically free from rhenide. They give
only a yellow color in the bismuth test and only a trace
of brown coloration with thallous hydroxide. If the
crystals should contain noticeable quantities of rhenide
the batch should be recrystallized.
About V2 to 3 of the LiOH can be separated in the
first batch of crystals. The clear filtrate contains practically all of the rhenide, as can be easily ascertained by
the above tests or by permanganate titration.
From the above description it is obvious that the
separation proposed here is in no way similar to Mme
Marie Curie's tedious process of separating radium
chloride from barium chloride. Although the term "fractional crystallization" is used by us, it is meant in the
sense of manual operation only. The lithium rhenide is
not isomorphous with LiOH • H o 0 and is not built into its
crystal lattice. The separation described here is much
more similar to the easy separation of protactinium from
ZrOCl, • 8 H 2 0 crystals 17.
The" batch of LiOH • H 2 0 crystals contain the KRe0 4
crystals referred to previously. By dissolving the alkali
in a requisite amount of water and filtering, 50—60% of
the KRe0 4 originally used in the reduction can be recovered; it obviously can be used "as is" for subsequent
reductions with lithium. The rhenium left as R e O r
in solution can be recovered by any suitable method
(Re2S_, nitron perrhenate, etc.!).
The rhenide filtrate is again concentrated (distilling off
alcohol and H.,0) and the crystallization repeated. On the
third or fourth cycle (with a recrystallization of the head
fractions of LiOH • H.,0 crystals if necessary!) and by
diminishing suitably the scale of the apparatus one
obtains highly concentrated lithium rhenide solutions
which are practically free from lithium hydroxide. By evaporating water slowly from them in a vacuum one obtains
pure lithium rhenide with properties as described on p. 534.
Analytical data to establish the formula of this compound
simply could not be obtained in time to meet the deadline
of Dr. W. N o d d a c k ' s sixtieth birthday celebration.
Our starting solutions were 2.5—7.5 millimolar in
rhenide ion; by the procedure outlined they were concentrated to practically pure lithium rhenide in yields of up
to about 90% of the total rhenide content.
Now that the stability and method of concentration
of lithium rhenide has been demonstrated, it is evident
that other methods of reduction of [ R e O J " to Re" may
be used; after neutralization with lithium hydroxide,
suitable lithium salts can be separated from the lithium
rhenide by fractional crystallization.
It is obvious that this is just the beginning of the study
of the alkali rhenides. The study of their chemical and
physical properties, their reactions, methods of formation
from rhenium and other metals and decompositions at
higher temperatures promises to be of fundamental importance to inorganic chemistry. It is our hope, on Dr. W a l t e r N o d d d a c k ' s sixtieth birthday, that the most
interesting part of rhenium chemistry will just begin to
unfold itself and that the discoverers of rhenium may see
their gift to science fascinate chemists on both sides of
the Atlantic for many years to come.
16 See, for example, E. K r a u s e and A. v. G r o s s e ,
'Die Chemie der metall-organisehen Verbindungen", Verlag von Gebrüder Borntraeger, Berlin (1937), p. 810,
drawing 37.
17 A. v. G r o s s e , Ber. dtsch. chem. Ges. 61, 238 [1928],