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Strongly alkaline solutions of hexamminecobalt (III)-salts
INORG.
NUCL.
CHEM.
LETTERS
Vol. 6,
pp.
657-661,
1970.
Pergamon
Press.
Printed
in Great Britain
STRONGLY ALKALINE SOLUTIONS OF HEXAMMINECOBALT (III)-SALTS
Ludwig Heck Institut f0r Anorganische Chemie der Universit~t des Saarlandes 6600
Saarbr~cken, Germany I R e c e i v e d 23 March 1970)
Recently Takemoto and Jones
(i) discussed the problem whether
in strongly basic aqueous media the hexamminecobalt(III)-ion is converted to an amidopentammine-complex or to an ion-pair
[Co(NH3)5NH2 ]2+
[Co(NH3)6]3+x OH-. A conclusive answer
of this question is prerequisite to distinguish between possible mechanisms of base hydrolysis of inert ammine complexes(2). Arguments from the literature and from current work in this laboratory are presented here to support an acid-base-reaction [Co (NH3) 6 ]3+
+
OH-
=
[Co (NH3) 5NH2 ]2+
+
H20
i)
rather than formation of an "outer-sphere" complex [Co (NH3) 6] 3+
+
OH-
=
[Co (NH3) 6] 3+x OH-
2)
Two important points to be examined in detail are i) reactions of platinum(IV)-ammine complexes in basic solution ii) molar absorptivity changes of cobalt(III)-ammine complexes caused by "inner-sphere" and "outer-sphere" ligands.
657
658
ALKALINE SOLUTIONS OF HEXAMMINECOBALT (Ill) - S A L T S
Vol. 6, No. 7
i) Reactions of Platinum(IV)-ammine Complexes in Basic Solution The hexammineplatinum(IV)-ion reacts according to equation i) with a striking change in the ultraviolet region of the absorption spectrum And Faerman
(3). The acidity constants given by Grinberq
(4) and by Joerqensen
(5) (pKa = 7.75) demonstrate
that this reaction is virtually complete in solutions containing minute concentrations of hydroxide ions. In moderately basic solutions the amidopentammineplatinum(IV)-ion
is de-
protonated further
[Pt (NH3) 5NH 2 ]
3+
+
OH-
=
[Pt (NH3) 4 (NH2) 2 ]2+
+
H20
The acidity coefficient pK a has been determined from optical measurements
(6) to be 10.5 ~ 0.i
(25°C; ionic strength I=O).
Figure i) shows the ultraviolet spectra of the two species present in the equilibrium of equation 3). 2500
2000
200(
H~] 3-
[ Pt(NH3)4(N Hz)2] ~+
1500
1500
1000
1000
500
200
250
300
350
m~
FIG.
200
250
300
350
rnp
1
Absorption Spectra of Amido-ammine Complexes of Platinum(IV)
3)
Vol. 6, No. 7
A L K A L I N E SOLUTIONS OF HEXAMMINECOBALT ( l i d - S A L T S
659
The existence of diamidotetrammineplatinum(IV)-ions
in
strongly basic aqueous solutions has been recently confirmed by precipitation of the iodide
[Pt(NH3)4(NH2)2]J 2
(7).
There is even spectroscopic evidence of formation of triamidotriammineplatinum (IV) -ion
[Pt (NH 3) 3 (NH2) 3 ]+ in alkaline
medium of pH higher than 13. This ion is the predominant species in liquid ammonia
(8).
The triply charged chloropentammineplatinum(IV)-ion
is also
reported to react as a divalent acid. Chuqaev prepared the slightly soluble already in 1915 8.1 and 10.4,
[Pt(NH3)4NH2Cl]CI 2
from dilute ammonia
(9). The acidity coefficients found
(4) are
respectively.
From these values the acidity coefficient of hexamminecobalt(III)-ion may be estimated to be 12 + i. Calculations m
based on spectral changes of alkaline solutions 12.8 + 0.2
(i0) gave
(25°C; ionic strength I = O).
m
ii) Absorptivity Chanqes of Cobalt(III)
Ammine Complexes
Changes in the ultraviolet region of the absorption spectra of hexamminecobalt(III) pai~ association
solutions may be attributed to ion-
(Ii) as well as to formation of the con-
jugate base. The ligand-to-metal charge-transfer,
however,
is expected to be far more pronounced for a potential "inner-sphere"
ligand
(NH 2- in case of reaction i) than for
the similar "outer-sphere"
ligand OH- in case of reaction 2.
The shift of a charge-transfer band from the short ultraviolet towards the visible region results in a very marked increase of absorptivity at a given wavelength.
This is
indeed ebserved if a ligand X- moves from "outer-sphere" coordination as in
[Co(NH3)6]3+x X- to the "inner Sphere"
660
A L K A L I N E SOLUTIONS OF HEXAMMINECOBALT (III) - S A L T S
in
as
[Co(NH3)5X]2+.
wavelength
Molar
of 270 nm are
extinction
summarized
Extinction
Coefficients at 270 nm
CO 3
2~
<
2.800
I0
OH-
of large
hundredfold ligands
i0.000
N
1.000
quantitative
increase
passing
If we assume
hydroxo
a molar
[I m o l e -I cm -I]
From measurements paper
we c o m p u t e
cobalt(III)-ion efficient This
to be p u b l i s h e d for the r e a c t i o n
for d i f f e r e n t
relation
for
for the of less
product
than OH-.
in a f o r t h c o m i n g
of h e x a m m i n e -
ion a m o l a r
extinction
as c o n c l u s i v e
evidence
of the a m i d o p e n t a m m i n e c o b a l t ( I I I ) - i o n
alkaline
sphere
[Co(NH3)6]3+x
in d e t a i l
to
co-
[i m o l e -I cm-l].
v a l u e m a y be i n t e r p r e t e d
strongly
a twentyfold
coefficient
reasonable
with hydroxide
15
to the inner
a similar
14
13;14
is found
sphere
extinction
seems
of 150 + 15
the e x i s t e n c e
12 13;
differences
in a b s o r p t i v i t y
from the o u t e r
ligand,
+ 50
450
of the complex.
20
N
?
In spite
Complexes
[Co(NH3)5X] n+ r e f e r e n c e s
< i00
Cl-
Ammine
[i m o l e -I cm -I]
145 ~ 5
Br
i.
of Cobalt(III)
[Co(NH3) 6 ]3+ x X -
X-
ligand
in table
for a
1
TABLE Molar
coefficients
Vol. 6, No. 7
aqueous
solution.
for in
Vot, 6, No. 7
A L K A L I N E SOLUTIONS OF HEXAMMINECOBALT (111) - S A L T S
661
References
i) J. H. Takemoto, 175
M. M.
Jones,
actions,
R. G. Pearson,
p.
124, Wiley,
Mechanisms
N e w York,
3) A. Grinberg,
Z. anorg,
4) A. Grinberg,
G. P. Faerman,
allg.
Joergensen, paper
presented
Hamburg,
April
1968
N. K u k u s h k i n ,
12)
L. Heck,
363
V. V.
138,
Z. anorg,
Scand.
Sibirskaya,
L. V l a d i m i r o v ,
J. Chem.
Linhard,
Unpublished
13) M.
of I n o r g a n i c
333
Re-
(1924)
allg.
Chem.
i__~0, 523
at D e u t s c h e r
Z. N a t u r f o r s c h u n g ,
9) L. Chugaev,
ii) M.
3_~2,
193,
(1956)
Chemiedozententag,
Russ.
J. Inorg.
Chem.
i_~4,
(1969)
8) L. Heck,
i0)
Chem.
1958
Chem.
A c t a Chem.
6) L. Heck,
409,
Nucl.
(1930)
5) Ch. K.
7) Yu.
Inorg.
(1970)
2) F. Basolo,
193
J.
G. Evans,
Educ.
Teil
b,
Comptes 4_~6, 535
Z. E l e k t r o c h e m .
in print redus
160,
840
(1915)
(1969)
5_~0, 224
(1944)
results G. H. Nancollas,
Trans.
Farad.
Soc. 4__2 ,
(1953)
14) M.
Linhard,
M. Weigel,
Z. anog.
15) H.
Siebert,
H. Feuerhake,
Chem.
allg. Ber.
Chem. 102,
266,
2951
49 (1969)
(1951)
NUCL.
CHEM.
LETTERS
Vol. 6,
pp.
657-661,
1970.
Pergamon
Press.
Printed
in Great Britain
STRONGLY ALKALINE SOLUTIONS OF HEXAMMINECOBALT (III)-SALTS
Ludwig Heck Institut f0r Anorganische Chemie der Universit~t des Saarlandes 6600
Saarbr~cken, Germany I R e c e i v e d 23 March 1970)
Recently Takemoto and Jones
(i) discussed the problem whether
in strongly basic aqueous media the hexamminecobalt(III)-ion is converted to an amidopentammine-complex or to an ion-pair
[Co(NH3)5NH2 ]2+
[Co(NH3)6]3+x OH-. A conclusive answer
of this question is prerequisite to distinguish between possible mechanisms of base hydrolysis of inert ammine complexes(2). Arguments from the literature and from current work in this laboratory are presented here to support an acid-base-reaction [Co (NH3) 6 ]3+
+
OH-
=
[Co (NH3) 5NH2 ]2+
+
H20
i)
rather than formation of an "outer-sphere" complex [Co (NH3) 6] 3+
+
OH-
=
[Co (NH3) 6] 3+x OH-
2)
Two important points to be examined in detail are i) reactions of platinum(IV)-ammine complexes in basic solution ii) molar absorptivity changes of cobalt(III)-ammine complexes caused by "inner-sphere" and "outer-sphere" ligands.
657
658
ALKALINE SOLUTIONS OF HEXAMMINECOBALT (Ill) - S A L T S
Vol. 6, No. 7
i) Reactions of Platinum(IV)-ammine Complexes in Basic Solution The hexammineplatinum(IV)-ion reacts according to equation i) with a striking change in the ultraviolet region of the absorption spectrum And Faerman
(3). The acidity constants given by Grinberq
(4) and by Joerqensen
(5) (pKa = 7.75) demonstrate
that this reaction is virtually complete in solutions containing minute concentrations of hydroxide ions. In moderately basic solutions the amidopentammineplatinum(IV)-ion
is de-
protonated further
[Pt (NH3) 5NH 2 ]
3+
+
OH-
=
[Pt (NH3) 4 (NH2) 2 ]2+
+
H20
The acidity coefficient pK a has been determined from optical measurements
(6) to be 10.5 ~ 0.i
(25°C; ionic strength I=O).
Figure i) shows the ultraviolet spectra of the two species present in the equilibrium of equation 3). 2500
2000
200(
H~] 3-
[ Pt(NH3)4(N Hz)2] ~+
1500
1500
1000
1000
500
200
250
300
350
m~
FIG.
200
250
300
350
rnp
1
Absorption Spectra of Amido-ammine Complexes of Platinum(IV)
3)
Vol. 6, No. 7
A L K A L I N E SOLUTIONS OF HEXAMMINECOBALT ( l i d - S A L T S
659
The existence of diamidotetrammineplatinum(IV)-ions
in
strongly basic aqueous solutions has been recently confirmed by precipitation of the iodide
[Pt(NH3)4(NH2)2]J 2
(7).
There is even spectroscopic evidence of formation of triamidotriammineplatinum (IV) -ion
[Pt (NH 3) 3 (NH2) 3 ]+ in alkaline
medium of pH higher than 13. This ion is the predominant species in liquid ammonia
(8).
The triply charged chloropentammineplatinum(IV)-ion
is also
reported to react as a divalent acid. Chuqaev prepared the slightly soluble already in 1915 8.1 and 10.4,
[Pt(NH3)4NH2Cl]CI 2
from dilute ammonia
(9). The acidity coefficients found
(4) are
respectively.
From these values the acidity coefficient of hexamminecobalt(III)-ion may be estimated to be 12 + i. Calculations m
based on spectral changes of alkaline solutions 12.8 + 0.2
(i0) gave
(25°C; ionic strength I = O).
m
ii) Absorptivity Chanqes of Cobalt(III)
Ammine Complexes
Changes in the ultraviolet region of the absorption spectra of hexamminecobalt(III) pai~ association
solutions may be attributed to ion-
(Ii) as well as to formation of the con-
jugate base. The ligand-to-metal charge-transfer,
however,
is expected to be far more pronounced for a potential "inner-sphere"
ligand
(NH 2- in case of reaction i) than for
the similar "outer-sphere"
ligand OH- in case of reaction 2.
The shift of a charge-transfer band from the short ultraviolet towards the visible region results in a very marked increase of absorptivity at a given wavelength.
This is
indeed ebserved if a ligand X- moves from "outer-sphere" coordination as in
[Co(NH3)6]3+x X- to the "inner Sphere"
660
A L K A L I N E SOLUTIONS OF HEXAMMINECOBALT (III) - S A L T S
in
as
[Co(NH3)5X]2+.
wavelength
Molar
of 270 nm are
extinction
summarized
Extinction
Coefficients at 270 nm
CO 3
2~
<
2.800
I0
OH-
of large
hundredfold ligands
i0.000
N
1.000
quantitative
increase
passing
If we assume
hydroxo
a molar
[I m o l e -I cm -I]
From measurements paper
we c o m p u t e
cobalt(III)-ion efficient This
to be p u b l i s h e d for the r e a c t i o n
for d i f f e r e n t
relation
for
for the of less
product
than OH-.
in a f o r t h c o m i n g
of h e x a m m i n e -
ion a m o l a r
extinction
as c o n c l u s i v e
evidence
of the a m i d o p e n t a m m i n e c o b a l t ( I I I ) - i o n
alkaline
sphere
[Co(NH3)6]3+x
in d e t a i l
to
co-
[i m o l e -I cm-l].
v a l u e m a y be i n t e r p r e t e d
strongly
a twentyfold
coefficient
reasonable
with hydroxide
15
to the inner
a similar
14
13;14
is found
sphere
extinction
seems
of 150 + 15
the e x i s t e n c e
12 13;
differences
in a b s o r p t i v i t y
from the o u t e r
ligand,
+ 50
450
of the complex.
20
N
?
In spite
Complexes
[Co(NH3)5X] n+ r e f e r e n c e s
< i00
Cl-
Ammine
[i m o l e -I cm -I]
145 ~ 5
Br
i.
of Cobalt(III)
[Co(NH3) 6 ]3+ x X -
X-
ligand
in table
for a
1
TABLE Molar
coefficients
Vol. 6, No. 7
aqueous
solution.
for in
Vot, 6, No. 7
A L K A L I N E SOLUTIONS OF HEXAMMINECOBALT (111) - S A L T S
661
References
i) J. H. Takemoto, 175
M. M.
Jones,
actions,
R. G. Pearson,
p.
124, Wiley,
Mechanisms
N e w York,
3) A. Grinberg,
Z. anorg,
4) A. Grinberg,
G. P. Faerman,
allg.
Joergensen, paper
presented
Hamburg,
April
1968
N. K u k u s h k i n ,
12)
L. Heck,
363
V. V.
138,
Z. anorg,
Scand.
Sibirskaya,
L. V l a d i m i r o v ,
J. Chem.
Linhard,
Unpublished
13) M.
of I n o r g a n i c
333
Re-
(1924)
allg.
Chem.
i__~0, 523
at D e u t s c h e r
Z. N a t u r f o r s c h u n g ,
9) L. Chugaev,
ii) M.
3_~2,
193,
(1956)
Chemiedozententag,
Russ.
J. Inorg.
Chem.
i_~4,
(1969)
8) L. Heck,
i0)
Chem.
1958
Chem.
A c t a Chem.
6) L. Heck,
409,
Nucl.
(1930)
5) Ch. K.
7) Yu.
Inorg.
(1970)
2) F. Basolo,
193
J.
G. Evans,
Educ.
Teil
b,
Comptes 4_~6, 535
Z. E l e k t r o c h e m .
in print redus
160,
840
(1915)
(1969)
5_~0, 224
(1944)
results G. H. Nancollas,
Trans.
Farad.
Soc. 4__2 ,
(1953)
14) M.
Linhard,
M. Weigel,
Z. anog.
15) H.
Siebert,
H. Feuerhake,
Chem.
allg. Ber.
Chem. 102,
266,
2951
49 (1969)
(1951)