- Email: [email protected]
Metal complexation and rotational isomerism of simple carboxylic acids-V molybdate-thiomalic acid complexes in neutral solution studied by 1H NMR
INORG. NUCL. CHEM. LETTERS Vol.|6, pp. 65-70. Pergamon Press Ltd. ]980. Printed in Great Britain
METAL COMPLEXATIONAND ROTATIONAL ISOMERISM OF SIMPLE CARBOXYLICACIDS-V* MOLYBDATE-THIOMALIC ACID COMPLEXES IN NEUTRAL SOLUTION STUDIED BY IH NMR VICTOR Department M.
MADALENA
Department
of Chemistry, CALDEIRA of Chemistry,
M.
S.
GIL
University
and
M.
of Aveiro, Portugal
EMTLIA
University
T.
L.
SARAIVA
of Coimbra,
Portugal
(Received ]2 December ]979) A f t e r a proton NMR study of complexation between the tungstate ion and thiomalic
and
malic
acids in aqueous solution (1,2), we now report a s i m i l a r i n v e s t i g a t i o n of the system molybdate-thiomalic acid, at pH = 7, which, in p a r t i c u l a r , provides d i r e c t information on the numberofcomplexes, t h e i r stoicheiometry, the conformation of the ligand and the i n s t a b i l i t y of the solutions.
Appro-
ximately neutral solutions were studied as they are more amenable to NMR analysis,
presumably due
to the absence of relevant polymerization, and, also, the reduction of
Mo(V) is
Mo(VI) to
mini-
mized. I t has long been known that complexation of
Mo(VI) with thiomalic acid y e l d s a y e l l o w c o l o u r
(maximal absorption at 365 nm) (3) enabling the successful use of t h io malic acid in the analysis of molybdenum (4,5). found
The only structural study we are aware of is that of BUSEV and CHANG FAN (4) who
Mo(Vl) - thiomalic
acid complexation in the r a t i o 1:2 at pH = 3.6. Studies of complexes
with Mo(V) are also know~ (4,6) but they are less relevant in the present case.
RESULTS AND DISCUSSION
|.
Stoich~ome~y of complexes
The IH NMR spectrum of p a r t i a l l y deuterated thiomalic acid (tma), DO2C-CH2-CH(SD)-CO2D, is of the ABX type (7), especially at higher frequencies.
Due to slow exchange at room temperature, sepa-
rate spectra are observed for the various species (bound and free ligand).
FIGURE l (A) shows the
spectrum of a l : l solution (pH* = 7.7) of MoO~- and tma and FIGURE 2 presents a better resolved X part corresponding to the complexes.
A complex a and a complex b are i d e n t i f i e d .
The l a t t e r gives
rise to two ABX spectra s l i g h t l y s h i f t e d from each other in the X and B parts; this shows t h a t b h a s two s l i g h t l y non-equivalent tma molecules~* shows the spectrum of free ligand.
I t is i n t e r e s t i n g to note t h a t
a
1 : 1
The f i n a l assignements of a l l AB lines was aided by
solution comparison
For parts III and IV in this series see Refs. (1,2). The alternative explanations for the multiplicity of b lines, namely further spin coupling or the existence of two distinct b complexes, were ruled out, respectively by comparison of I00 and 270 MHz spectra and from the fact that the two b spectra always have the same intensity (upon change of molar ratio) which is not the case when comparing b and a. **
65
66
Molybdate - Thiomalic Acid Complexes
(A)
ZOHz
O Xb
~Xo ~
Xtm a
(B)
t m a dis.
tma d i s .
FIG. l ° I00 MHz IH NMR sp e ctrum of a i:I solutzon of MoO 42 - /tma (pll* = 7.7) in D20:(A) 15 minutes after addition, (B) 45 days later.
o,6
10Hz
FIG. 2 X spectra for complexes a and b (see Fig. I).
M o l y b d a t e - T h i o m a l i c Acid C o m p l e x e s
of spectra of various molar r a t i o s .
67
Whereas the AB part is almost not s h i f t e d upon complexation,
the X m u l t i p l e t undergoes a l o w - f i e l d s h i f t of 0.53 p.p.m, f o r complex a and 0.66 and 0.63 p.p.m. f o r complex
b.
The formation of the complexes in instantaneous.
However, t h e i r concentrations
decrease
slowly with time (see below). I n t e g r a t i o n of spectra f o r molar r a t i o s from 1:4 to 4:1 (parent equimolar solutions of pH* = 6.4) led to the Job's curves shown in FIGURE 3. parent s o l u t i o n s .
The spectra were recorded a f t e r a d d i t i o n of the
I t is c l e a r that both a and b are I : I complexes.
In view of the
conclusion
above on the b complex this must be a binuclear 2:2 complex.
1:1 -02
//t
,\
\\\
1:51,,// /
/
1:1
4:1/" I /
"\1:1:5
%
~,
\
\j:2k :3
,
\ ,
- 01
I
I
I
.l:&
4:1o/ - -
o!2
oi~
01~
Complex
a b
o18 [THIOMALIC ACID] tot.
[THIOMALICACID]+tot.[MoO~ tot. FIG. 3 Job's curves b a s e d on N M R spectral intensities.
2.
Ligand conformation in the complexes
The conformation of the thiomalic acid moiety in the complexes is revealed by the v i c i n a l HH coupling constants. Table l shows the various spectral parameters obtained from ABX analysis of the spectra of a and b (two tma molecules) as well as those f o r thiomalic acid in the same conditions pH* = 7.7).
(namely,
68
Molybdate - Thiomalie Acid Complexes
TABLE I . JAB Complex
a
(I:I)
Complex b (2:2)
SPECTRAL PARAMETERS JAX
JBX
~B-VA
(-) 15.1 Hz
10.7 Hz
3.8 Hz
0.467 ppm
(-) 15.1
ii.0
4.0
0.421
(-) 15.1
ii.0
4.0
0.436
(-) 15.3
8.9
6.4
0.345
VX-VB 1.265 ppm I. 350
Thiomalic acid
The r e s u l t s
0.790
f o r tma are c o n s i s t e n t w i t h a c o n f o r m a t i o n taken as a weighted average
of
the
t h r e e s t a g g e r e d rotamers CO2H
CO;,H
CO2H
SH
(X)
(c 2)
(c3)
(c~)
(CI) being the most i m p o r t a n t c o n t r i b u t i n g a c c o r d i n g t o LEYDEN and WALTERS ( 7 ) , C3 = 19 %.
CO2H
one b u t (02) and (C3) h a v i n g n o n - n e g l i g i b l e w e i g h t s ( 7 ) ;
the r e l a t i v e
w e i g h t s are a p p r o x i m a t e l y C1 = 51%,
In the case o f the complexes, the JAX v a l u e s are t y p i c a l
o f an e s s e n t i a l l y
rangement of the protons A and X, whereas the JBX values are typical of an essentially
orientation of B and X.
This points to a conformation close to (Cl) in both a and b
C2 = 30% tr~
ar-
gauche complexes:
thiomalic acid is acting as a bidentate ligand through --COO - . . a n d - - S . . - bonds. Possible structures for the complexes are
o
O
Mo
--S
2C
( X1
~ (B) (o)
0
0
Mo
Mo
(A)
( x l ~ S
C02H
(B)~(A) CO2H
02C
(A)
(X}/~ ~ CO2 H (B) (b)
This conclusion, consistent with an assignment of protons A and B that does not change upon complexation, is here preferred to concluding in favour of conformation (C2) which would also show tY~ and gauche coupling constants. Although some support might come from the observed low-field shift of the X proton signal on complexation, a definitive NMR proof would only be found in the vicinal 13C-H couplings.
Molybdate
- Thiomalic
Acid
69
Complexes
and the t e n t a t i v e equations, r e s p e c t i v e l y ,
MoO~- + C4H404S2- =
MoO4(C4H404S)4-
2Moo - + 2 c4H404 s2. + 4H+ = (.o%)2(c4,404s)
+ 2 H20
besides the e q u i l i b r i u m (8): 7 MoO~- + 8 H+ = Mo70~4+4 H20 The second equation above would expain the observed elevation of pH as a r e s u l t of complexation and, also, the c o m p a t i b i l i t y of high formation constants f o r the 2:2 and I : I complexes with the presence of a s i g n i f i c a n t amount of free ligand in I : I
solutions of pH = 7.
I t is intended to extend t h i s study to other pH regions and to involve also 13C NMRandother techniques in a comparative manner.
3.
Redox i ~ t a b i ~ i t y
The yellow colour obtained when tma is added to MoO hours
it
n > I,
turns
whereas
orange green
and colours
red
colours
appear
in
develop n: 1
solutions is not stable. Within a
within
solutions
a
few
days
(n > I ) ;
in
few
solutions l : n w i t h
1 : 1
solutions
red-brown. From the NMR point of view, what is observed is t h a t , f o r any s o l u t i o n , the
become
intensities
of
a l l the o r i g i n a l spectra (due to complexes a and b and, e v e n t u a l l y , to free ligand) decrease while two new ABX spectra appear, s l i g h t h y displaced from each other. Figure I(B) shows the spectrum of a I:I
s o l u t i o n 45 days a f t e r complexation.
The
new spectrum
is
found
to
be
exactly
iden-
t i c a l to that obtained when adding equivalent amountsoftma and H202. I t is therefore a t t r i b u t e d to the o x i d a t i o n product of tma in such conditions:
thiomalic d i s u l f i d e (C02H-CH2-CH(CO2H)S-}2 (9).
Free ligand is the species most r a p i d l y oxidized, followed by complexes b and a. Figure I(A) already reveals the presence of a small amount of the d i s u l f i d e 15 minutes a f t e r addition of tma and Mo(VI) s o l u t i o n s .
Six months l a t e r , the amount of complexes present is
small, almost a l l tma having been oxidized. content is small, e.g. 1:4.
very
This is true even f o r solutions where the molybdenum
This shows that oxidation of tma can not be accomplished by Mo(VI)
÷ Mo(V) reduction; also, reduction of Mo(VI) to lower oxidation states is u n l i k e l y spectra do not r e f l e c t the presence of any paramagnetic ions. thought to be the a l t e r n a t i v e o x i d a t i n g agent.
Oxygen from
the
In f a c t , by running spectra of
and
the
NMR
atmosphere
was
deoxygenated and
normal solutions with and without MoO~-, i t was v e r i f i e d that i t requires both 02 andMoO~-, though the l a t t e r even in very small amounts ( I : I 0 0 ) ,
to r e a l i z e the oxidation of tma w i t h i n the kind of
time i n t e r v a l referred to above. I t can thus be concluded that MoO~- acts as a c a t a l y s t . Since tma d i s u l f i d e is c o l o u r l e s s , the various colours that develop in time must to very small amounts of other species and/or to the p o s s i b i l i t y of c o l l o i d a l s i t u a t i o n s . colour also develops from complexation of Mo(V) with tma (6).
The green colours
are
be due A red probably
the r e s u l t of the yellow complexes a and 6 and the so-called molybdenum blue that appears especially when the MoO~-/tma r a t i o is p a r t i c u l a r l y high.
70
Molybdate - Thiomalic Acid Complexes
EXPERIMENTAL
Analytical grade sodium molybdate and commercially available (racemic) used.
thiomalic
acid were
In order to reduce the OH NMR signal, the former was lyophylized from a solution in
the l a t t e r was dehydrated at 120 C and D20 solutions were used throughout.
D20,
The concentrations
(0.90 ± O.02M) of the molybdate solutions were established by weight and of the acid solutions by potentiometry. The pH was adjusted by dropwise addition of d i l u t e DCI and NaOD solutions. The pH* values quoted are the pH-meter readings. The spectra were recorded at room temperature, on Va rian HA-IO0 and Bruker WH-270 spectrometers.
ACKNOWLEDGEMENTS
This work is a contribution of the Centro de Investigag~o em QuTmica, Coimbra, supported by the I n s t i t u t o Nacional de Investigag~o CientTfica (Portuguese Ministry of Education). The authors thank Dr. J. Feeney for the 270 MHz spectra and Prof. F. Pinto Coelho and Dr. J. Pedrosa de Jesus for helpful discussions.
REFERENCES I.
ANA M. D. PEREIRA and VICTOR M. S. GIL, J. Inorg. Nucl. Chem., 39, 857 (1977).
2.
VICTOR M. S. GIL, M. EMTLIA T. L. SARAIVA, M. MADALENA CALDEIRA and ANA M.D.PEREIRA,J. Inorg.
Nucl. Ohem., in press. 3.
J.A.
CATOGGIO, An. Direcci$n General de Qu~mica, Buenos Aires, 7, 40 (1954).
4.
A.I.
BUSEV and CHANG FAN, J. Anal. Chem. USSR, 16, 177 (1961).
5.
F. BERMEJO MARTINEZ and M. DEL CARMEN MEIJON MOURINO, Inf. Quire. Anal., 17, 115 (1963).
6.
M. LAMACHE, Bull. Soc. Chim. France, 2766 (1974).
7.
D.E.
LEYDEN and D. B. WALTERS, Spect. Acta, 25A, 1869 (1969).
8.
C.F.
BAES and R. E. MESMER, The hydrolysis of cations, Wiley, 1976, p. 253.
9.
B.C.
GILBERT, H. A. H. LAUE, R. O. C. NORMAN and R. C. SEALY, J. Chem. Soc., Perkln II, 892
(1975) and references therein.
METAL COMPLEXATIONAND ROTATIONAL ISOMERISM OF SIMPLE CARBOXYLICACIDS-V* MOLYBDATE-THIOMALIC ACID COMPLEXES IN NEUTRAL SOLUTION STUDIED BY IH NMR VICTOR Department M.
MADALENA
Department
of Chemistry, CALDEIRA of Chemistry,
M.
S.
GIL
University
and
M.
of Aveiro, Portugal
EMTLIA
University
T.
L.
SARAIVA
of Coimbra,
Portugal
(Received ]2 December ]979) A f t e r a proton NMR study of complexation between the tungstate ion and thiomalic
and
malic
acids in aqueous solution (1,2), we now report a s i m i l a r i n v e s t i g a t i o n of the system molybdate-thiomalic acid, at pH = 7, which, in p a r t i c u l a r , provides d i r e c t information on the numberofcomplexes, t h e i r stoicheiometry, the conformation of the ligand and the i n s t a b i l i t y of the solutions.
Appro-
ximately neutral solutions were studied as they are more amenable to NMR analysis,
presumably due
to the absence of relevant polymerization, and, also, the reduction of
Mo(V) is
Mo(VI) to
mini-
mized. I t has long been known that complexation of
Mo(VI) with thiomalic acid y e l d s a y e l l o w c o l o u r
(maximal absorption at 365 nm) (3) enabling the successful use of t h io malic acid in the analysis of molybdenum (4,5). found
The only structural study we are aware of is that of BUSEV and CHANG FAN (4) who
Mo(Vl) - thiomalic
acid complexation in the r a t i o 1:2 at pH = 3.6. Studies of complexes
with Mo(V) are also know~ (4,6) but they are less relevant in the present case.
RESULTS AND DISCUSSION
|.
Stoich~ome~y of complexes
The IH NMR spectrum of p a r t i a l l y deuterated thiomalic acid (tma), DO2C-CH2-CH(SD)-CO2D, is of the ABX type (7), especially at higher frequencies.
Due to slow exchange at room temperature, sepa-
rate spectra are observed for the various species (bound and free ligand).
FIGURE l (A) shows the
spectrum of a l : l solution (pH* = 7.7) of MoO~- and tma and FIGURE 2 presents a better resolved X part corresponding to the complexes.
A complex a and a complex b are i d e n t i f i e d .
The l a t t e r gives
rise to two ABX spectra s l i g h t l y s h i f t e d from each other in the X and B parts; this shows t h a t b h a s two s l i g h t l y non-equivalent tma molecules~* shows the spectrum of free ligand.
I t is i n t e r e s t i n g to note t h a t
a
1 : 1
The f i n a l assignements of a l l AB lines was aided by
solution comparison
For parts III and IV in this series see Refs. (1,2). The alternative explanations for the multiplicity of b lines, namely further spin coupling or the existence of two distinct b complexes, were ruled out, respectively by comparison of I00 and 270 MHz spectra and from the fact that the two b spectra always have the same intensity (upon change of molar ratio) which is not the case when comparing b and a. **
65
66
Molybdate - Thiomalic Acid Complexes
(A)
ZOHz
O Xb
~Xo ~
Xtm a
(B)
t m a dis.
tma d i s .
FIG. l ° I00 MHz IH NMR sp e ctrum of a i:I solutzon of MoO 42 - /tma (pll* = 7.7) in D20:(A) 15 minutes after addition, (B) 45 days later.
o,6
10Hz
FIG. 2 X spectra for complexes a and b (see Fig. I).
M o l y b d a t e - T h i o m a l i c Acid C o m p l e x e s
of spectra of various molar r a t i o s .
67
Whereas the AB part is almost not s h i f t e d upon complexation,
the X m u l t i p l e t undergoes a l o w - f i e l d s h i f t of 0.53 p.p.m, f o r complex a and 0.66 and 0.63 p.p.m. f o r complex
b.
The formation of the complexes in instantaneous.
However, t h e i r concentrations
decrease
slowly with time (see below). I n t e g r a t i o n of spectra f o r molar r a t i o s from 1:4 to 4:1 (parent equimolar solutions of pH* = 6.4) led to the Job's curves shown in FIGURE 3. parent s o l u t i o n s .
The spectra were recorded a f t e r a d d i t i o n of the
I t is c l e a r that both a and b are I : I complexes.
In view of the
conclusion
above on the b complex this must be a binuclear 2:2 complex.
1:1 -02
//t
,\
\\\
1:51,,// /
/
1:1
4:1/" I /
"\1:1:5
%
~,
\
\j:2k :3
,
\ ,
- 01
I
I
I
.l:&
4:1o/ - -
o!2
oi~
01~
Complex
a b
o18 [THIOMALIC ACID] tot.
[THIOMALICACID]+tot.[MoO~ tot. FIG. 3 Job's curves b a s e d on N M R spectral intensities.
2.
Ligand conformation in the complexes
The conformation of the thiomalic acid moiety in the complexes is revealed by the v i c i n a l HH coupling constants. Table l shows the various spectral parameters obtained from ABX analysis of the spectra of a and b (two tma molecules) as well as those f o r thiomalic acid in the same conditions pH* = 7.7).
(namely,
68
Molybdate - Thiomalie Acid Complexes
TABLE I . JAB Complex
a
(I:I)
Complex b (2:2)
SPECTRAL PARAMETERS JAX
JBX
~B-VA
(-) 15.1 Hz
10.7 Hz
3.8 Hz
0.467 ppm
(-) 15.1
ii.0
4.0
0.421
(-) 15.1
ii.0
4.0
0.436
(-) 15.3
8.9
6.4
0.345
VX-VB 1.265 ppm I. 350
Thiomalic acid
The r e s u l t s
0.790
f o r tma are c o n s i s t e n t w i t h a c o n f o r m a t i o n taken as a weighted average
of
the
t h r e e s t a g g e r e d rotamers CO2H
CO;,H
CO2H
SH
(X)
(c 2)
(c3)
(c~)
(CI) being the most i m p o r t a n t c o n t r i b u t i n g a c c o r d i n g t o LEYDEN and WALTERS ( 7 ) , C3 = 19 %.
CO2H
one b u t (02) and (C3) h a v i n g n o n - n e g l i g i b l e w e i g h t s ( 7 ) ;
the r e l a t i v e
w e i g h t s are a p p r o x i m a t e l y C1 = 51%,
In the case o f the complexes, the JAX v a l u e s are t y p i c a l
o f an e s s e n t i a l l y
rangement of the protons A and X, whereas the JBX values are typical of an essentially
orientation of B and X.
This points to a conformation close to (Cl) in both a and b
C2 = 30% tr~
ar-
gauche complexes:
thiomalic acid is acting as a bidentate ligand through --COO - . . a n d - - S . . - bonds. Possible structures for the complexes are
o
O
Mo
--S
2C
( X1
~ (B) (o)
0
0
Mo
Mo
(A)
( x l ~ S
C02H
(B)~(A) CO2H
02C
(A)
(X}/~ ~ CO2 H (B) (b)
This conclusion, consistent with an assignment of protons A and B that does not change upon complexation, is here preferred to concluding in favour of conformation (C2) which would also show tY~ and gauche coupling constants. Although some support might come from the observed low-field shift of the X proton signal on complexation, a definitive NMR proof would only be found in the vicinal 13C-H couplings.
Molybdate
- Thiomalic
Acid
69
Complexes
and the t e n t a t i v e equations, r e s p e c t i v e l y ,
MoO~- + C4H404S2- =
MoO4(C4H404S)4-
2Moo - + 2 c4H404 s2. + 4H+ = (.o%)2(c4,404s)
+ 2 H20
besides the e q u i l i b r i u m (8): 7 MoO~- + 8 H+ = Mo70~4+4 H20 The second equation above would expain the observed elevation of pH as a r e s u l t of complexation and, also, the c o m p a t i b i l i t y of high formation constants f o r the 2:2 and I : I complexes with the presence of a s i g n i f i c a n t amount of free ligand in I : I
solutions of pH = 7.
I t is intended to extend t h i s study to other pH regions and to involve also 13C NMRandother techniques in a comparative manner.
3.
Redox i ~ t a b i ~ i t y
The yellow colour obtained when tma is added to MoO hours
it
n > I,
turns
whereas
orange green
and colours
red
colours
appear
in
develop n: 1
solutions is not stable. Within a
within
solutions
a
few
days
(n > I ) ;
in
few
solutions l : n w i t h
1 : 1
solutions
red-brown. From the NMR point of view, what is observed is t h a t , f o r any s o l u t i o n , the
become
intensities
of
a l l the o r i g i n a l spectra (due to complexes a and b and, e v e n t u a l l y , to free ligand) decrease while two new ABX spectra appear, s l i g h t h y displaced from each other. Figure I(B) shows the spectrum of a I:I
s o l u t i o n 45 days a f t e r complexation.
The
new spectrum
is
found
to
be
exactly
iden-
t i c a l to that obtained when adding equivalent amountsoftma and H202. I t is therefore a t t r i b u t e d to the o x i d a t i o n product of tma in such conditions:
thiomalic d i s u l f i d e (C02H-CH2-CH(CO2H)S-}2 (9).
Free ligand is the species most r a p i d l y oxidized, followed by complexes b and a. Figure I(A) already reveals the presence of a small amount of the d i s u l f i d e 15 minutes a f t e r addition of tma and Mo(VI) s o l u t i o n s .
Six months l a t e r , the amount of complexes present is
small, almost a l l tma having been oxidized. content is small, e.g. 1:4.
very
This is true even f o r solutions where the molybdenum
This shows that oxidation of tma can not be accomplished by Mo(VI)
÷ Mo(V) reduction; also, reduction of Mo(VI) to lower oxidation states is u n l i k e l y spectra do not r e f l e c t the presence of any paramagnetic ions. thought to be the a l t e r n a t i v e o x i d a t i n g agent.
Oxygen from
the
In f a c t , by running spectra of
and
the
NMR
atmosphere
was
deoxygenated and
normal solutions with and without MoO~-, i t was v e r i f i e d that i t requires both 02 andMoO~-, though the l a t t e r even in very small amounts ( I : I 0 0 ) ,
to r e a l i z e the oxidation of tma w i t h i n the kind of
time i n t e r v a l referred to above. I t can thus be concluded that MoO~- acts as a c a t a l y s t . Since tma d i s u l f i d e is c o l o u r l e s s , the various colours that develop in time must to very small amounts of other species and/or to the p o s s i b i l i t y of c o l l o i d a l s i t u a t i o n s . colour also develops from complexation of Mo(V) with tma (6).
The green colours
are
be due A red probably
the r e s u l t of the yellow complexes a and 6 and the so-called molybdenum blue that appears especially when the MoO~-/tma r a t i o is p a r t i c u l a r l y high.
70
Molybdate - Thiomalic Acid Complexes
EXPERIMENTAL
Analytical grade sodium molybdate and commercially available (racemic) used.
thiomalic
acid were
In order to reduce the OH NMR signal, the former was lyophylized from a solution in
the l a t t e r was dehydrated at 120 C and D20 solutions were used throughout.
D20,
The concentrations
(0.90 ± O.02M) of the molybdate solutions were established by weight and of the acid solutions by potentiometry. The pH was adjusted by dropwise addition of d i l u t e DCI and NaOD solutions. The pH* values quoted are the pH-meter readings. The spectra were recorded at room temperature, on Va rian HA-IO0 and Bruker WH-270 spectrometers.
ACKNOWLEDGEMENTS
This work is a contribution of the Centro de Investigag~o em QuTmica, Coimbra, supported by the I n s t i t u t o Nacional de Investigag~o CientTfica (Portuguese Ministry of Education). The authors thank Dr. J. Feeney for the 270 MHz spectra and Prof. F. Pinto Coelho and Dr. J. Pedrosa de Jesus for helpful discussions.
REFERENCES I.
ANA M. D. PEREIRA and VICTOR M. S. GIL, J. Inorg. Nucl. Chem., 39, 857 (1977).
2.
VICTOR M. S. GIL, M. EMTLIA T. L. SARAIVA, M. MADALENA CALDEIRA and ANA M.D.PEREIRA,J. Inorg.
Nucl. Ohem., in press. 3.
J.A.
CATOGGIO, An. Direcci$n General de Qu~mica, Buenos Aires, 7, 40 (1954).
4.
A.I.
BUSEV and CHANG FAN, J. Anal. Chem. USSR, 16, 177 (1961).
5.
F. BERMEJO MARTINEZ and M. DEL CARMEN MEIJON MOURINO, Inf. Quire. Anal., 17, 115 (1963).
6.
M. LAMACHE, Bull. Soc. Chim. France, 2766 (1974).
7.
D.E.
LEYDEN and D. B. WALTERS, Spect. Acta, 25A, 1869 (1969).
8.
C.F.
BAES and R. E. MESMER, The hydrolysis of cations, Wiley, 1976, p. 253.
9.
B.C.
GILBERT, H. A. H. LAUE, R. O. C. NORMAN and R. C. SEALY, J. Chem. Soc., Perkln II, 892
(1975) and references therein.