Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 105. No. 3, June 1993, pp. 155-160.
9 Printed in India.
Periodicity in the formation constant values of lanthanide(lll).
aminopolycarboxylate, resorcinol/orcinol/phloroglucinol mixed ligand complexes
S A N G E E T A V E R M A , S N L I M A Y E a n d M C S A X E N A * Department of Chemistry, Dr H S Gour Vishwavidyalaya, Sagar 470003, India MS received 20 August 1992; revised 28 December 1992
Abstract Formation constants (log
K~AL ) of
mixed ligand complexes[Ln(llI).A.L], where Ln(III) = La 3+ , Ce 3+ , Pr 3+ , Nd 3+, Sm 3+, Eu 3+, Gd 3+, Tb ~+ or Dy3+; A = NTA, HEDTA or EDTA and L = resorcinol Ires), orcinol (orc) or phloroglucinol (phi), have been determined pH-metrically using the Irving-Rossotti approach at 25~ and at an ionic strength, l = 0-2(mole dm- 3 NaCIO4). The log KM~AAL values lie in the sequence: (i) NTA > HEDTA >EDTA and (ii) orc> res > phi with respect to primary (A) and secondary (L) figands, respectively.
Periodicity has been observed in the formation constant values, which lie in the sequence La 3+ <Ce 3+ < P r 3+ < N d 3+ <Sm 3+ <Eu 3+ > G d 3+ <Tb 3+ < D y 3+ with respect to Ln(lll) ions. The evaluated extrastabilization and ,nephelauxetic ratio values lend support to inter-electronic repulsion theory; these values and the magnitude of the tetrad effect lie in the sequence i f > f3 _ f 4 ~ f l o _ f , a. Hydration numbers (HN) of Ln(IIl) ions have been calculated; tetrad effect is present in HN.
Keywords. Periodicity in formation constants; Ln(III).aminopolycarboxylate resorcinol;
orcinol; phloroglucinol; ternary complexes.
1. I n t r o d u c t i o n
Discontinuities are known to occur (Sinha 1983; Verma and Saxena 1988) in the variation profiles of several properties of lanthanides (and actinides). These discontinui- ties
o c c u r p r o m i n e n t l y at the 4 F ( G d ( I I I ) ) stage a n d less m a r k e d l y at the 4 f 3 - 4 f 4 ( N d ( l l I ) - P m ( I I I ) ) a n d 4 f ~~ - 4 f ~ 1 ( H o ( I I I ) - E r ( I l l ) ) s t a g e s d i v i d i n g L n ( I I I ) series into four s e g m e n t s o r tetrads. This t e t r a d i c p h e n o m e n o n has been called the t e t r a d effect ( P e p p a r d et al 1969) o r the d o u b l e - d o u b l e effect ( F i d e l i s a n d Siekierski 1971). T h e p e r i o d i c i t y o b s e r v e d in the p r o p e r t i e s h a s been t e r m e d v a r i o u s l y as m i c r o p e r i o d i c i t y ( T i s h c h e n k o et al 1981), r e g u l a r i t i e s ( F i d e l i s 1970), s y m m e t r y o r s y s t e m a t i c s ( S i n h a 1975, 1978, 1983). A t t e m p t s have b e e n m a d e to p r e d i c t the p r o p e r t i e s o f l a n t h a n i d e s using i n c l i n e d - W s y s t e m a t i c s (Sinha 1975), s e m i - e m p i r i c a l e q u a t i o n s (Jorgensen 1971, 1979; P o l u e k t o v 1982) a n d q u a n t u m c h e m i c a l c a l c u l a t i o n s ( S p i t s y n et al 1984, 1990;L i m a y e a n d S a x e n a 1992). It is believed t h a t c h a n g e s in i n t e r - e l e c t r o n i c r e p u l s i o n s a l t e r the n e p h e l a u x e t i c r a t i o l e a d i n g to e x t r a s t a b i l i z a t i o n ( N u g e n t 1970; J o r g e n s e n 1970, 1971, 1979, 1988) of specific c o n f i g u r a t i o n s i f 3 _ f g , f 7 , f t o _ f t 1 ). T h e possibility
* For correspondence
155
156 Sangeeta Verma, S N Limaye and M C Saxena
of ligand field splitting (Yatsimirskii and K o s t r o m i n a 1964), L - S coupling (Dzhurinskii 1980) and h y d r a t i o n (Williams 1982) c o n t r i b u t i n g to the o c c u r r e n c e of periodicity has also been suggested.
The present w o r k attempts to investigate the occurrence of periodicity in the f o r m a t i o n c o n s t a n t values, i.e. free energy of c o m p l e x a t i o n of the mixed ligand complexes o f the type [ L n ( I I I ) . A . L ] , where L n ( I I I ) = La 3 +, Ce 3 +, P r 3 +, N d 3 +, Sm 3 +, Eu 3+, G d 3+, T b 3+ or D y 3 + ; A = n i t r i l o t r i a c e t a t e (NTA), 2 - h y d r o x y - e t h y l e t h y - lenediaminetriacetate ( H E D T A ) or ethylenediaminetetraacetate (EDTA), and L = resorcinol (res), orcinol (orc) or p h l o r o g l u c i n o l (phi). A t t e m p t s have also been m a d e to examine the systematics in the h y d r a t i o n n u m b e r s o f L n ( I I I ) ions.
2. Experimental
Chemicals of s t a n d a r d purity were used. Ln(III) nitrates o f 99.99% purity were supplied by the I n d i a n Rare Earths Limited. O t h e r chemicals were of Merck G R a n d S i g m a / F l u k a make. T h e solutions were p r e p a r e d in d o u b l e distilled water. T h e L n ( I I I ) solutions were standardized by c o m p l e x o m e t r i c titrations (West 1969). The f o r m a t i o n constants log K~L and log KuA L were determined p H - m e t r i c a l l y using the MA I r v i n g - R o s s o t t i a p p r o a c h (Irving a n d Rossotti 1953, 1954; C h i d a m b a r a m a n d B h a t t a c h a r y a 1970) at 25~ and at an ionic strength, I = 0-2 (mole d m - 3 , N a C I O 4 ) . An Elico digital (model LI-120) p H - m e t e r with an a c c u r a c y of + 0.01 p H unit a n d a 1-0ml m i c r o b u r e t t e reading up to 0-01 ml were used. T h e final metal a n d ligand concentrations were maintained at [ L n ( I I I ) ] = [ A ] = [ L ] = 1.0 x 1 0 - 3 mole d m - 3. A 0.2 mole d m - 3 carbonate-free s o d i u m h y d r o x i d e solution was used for p H titrations, which were always repeated to ascertain reproducibility. T h e f o r m a t i o n c o n s t a n t values were refined statistically by the linear plot m e t h o d (Rossotti a n d Rossotti 1955; Pecsok et al 1976). Refined values of the f o r m a t i o n c o n s t a n t s are presented in table 1.
Table 1. Formation constants of binary ML(IogK~L ) and ternary MAL(IogKMAL) MA
complexes of Ln(III) ions with NTA, HEDTA, EDTA (A) and res, orc, phi (L).
Formation
constant A L La 3+ Ce 3+ Pr a+ Nd a+ Sm 3+ Eu 3+ G d 3+ Tb 3+ Dy 3+
- - res 7.18 7.21 7.46 7.58 7.75 7.85 7'73 7"90 7-98 log KMML - - orc 7.08 7.24 7.54 7-64 7 . 8 9 8.00 7-76 8"19 8.29 - - phi 6.61 6-83 6.91 7.12 7 . 2 2 7 . 3 7 7"17 7-49 7'59 NTA res 5-94 6"03 6-13 6-33 6 . 4 0 6'55 6 - 4 0 6"65 6.73 log KMA MA L HEDTA res 5-91 5-98 6-03 6-12 6"28 6'40 6"23 6"48 6-58 EDTA res 5-40 5"50 5-60 5-80 5 - 9 0 6.04 5.93 6-21 6.29 NTA orc 6-20 6 . 3 4 6-444 6-54 6'72 6 . 7 9 6 - 6 4 6"92 7.00
log KMA L MA HEDTA orc 6-08 6'17 6-22 6.37 6"49 6"61 6-57 6"67 6.79 EDTA orc 5-70 5.85 5-95 6-02 6 . 0 9 6 - 2 4 6"13 6'42 6.50 NTA phi 5-57 5'72 5-85 6-02 6"19 6 . 3 2 6 - 2 2 6 . 5 3 6.74 log KMA M^ L HEDTA phi 5-52 5.71 5-85 5"92 6"08 6 . 2 5 6 - 1 5 6'22 6.32 EDTA phi 4-90 5"02 5-17 5"28 5"42 5-68 5-40 5'80 5-98
Standard deviation: + 0.005 to 0"03 Temp.: 25~ Ionic strength, 1 = 0.2 (tool din-3, NaC104)
Formation constants of Ln(lIl)
1573. Results and discussion
The formation constant values evidently show a dependence on ligand characteristics.
The log K~L values lie in the order ore > res > phi, which is the sequence of ligand basicity. The log KMMA L values increase in the sequence EDTA < H E D T A < NTA with respect to aminopolycarboxylates. This order appears to be a consequence of electrostatic effect accompanying ternary complexation as MA + L = MAL (charges omitted) although the steric and statistical effects are also expected to contribute to the observed order. The calculated values of the stability quantifying parameter _ MA _ log K~L) are all negative, increasing numerically in the sequence A log K ( - log KMA L
NTA < H E D T A < EDTA; e.g. A l o g K with res lies in the range NTA: - 1 . 2 4 to - 1.25, HEDTA: - 1.27 to - 1.40, EDTA: - 1.69 to - 1.78. Such larger negative values of A log K with greater negative charge on A"- species have been observed earlier too (Bhattacharya 1981; Singh and Saxena 1990, 1991) with charged ligands.
A perusal of log KMML a n d log KMA L MA values, as recorded in table 1, reveals the stability sequence of La 3 + < Ce 3 + < Pr 3 + < Nd 3 + < Sm 3 + < Eu 3 + > G d 3 + < T b a + < Dy 3 + with respect to Ln(III) ions. A clearly visible depression at Gd a + indicates the presence oftetrad effect in the formation constant values. The periodicity in log K (i.e. formation constant in general) values has been examined by: (i) log K vs
4fq(q
is the number of 4f electrons in Ln(Ill)ions) plots, which show (figure not included) in the present case, as usual, a single prominent break at the 4f 7 (Gd(III)) stage; (ii) straight line approximation method (Siekierski 1981) in accordance with which the values of deviations A t = P e . p , - P i . , , where P p,, =experimental value of a property and Pi.t = interpolated value) have been found to be, in general, negative (A < 0) for q = 1 , 2 , 5 , 6 , 8 , 9 , 1 2 , 1 3 and positive ( A > 0 ) for q = 3 , 4 , 7 , 1 0 , 1 1 . This shows the existence of minor discontinuities at the f 3 _ f 4 and r i o _ f l l stages also besides a major break at f . The periodicity in log K values is clearly demonstrated; and (iii) differential plot method (Verma and Saxena 1987, 1988) in compliance with which plots (figure l) of A logK/Ar
vs 4f q (where A log K and A r represent the differences in the values of log K and ionic radius, r, of Ln(III).ion, respectively, between two successive Ln(III) ions), i.e. rate of change in l o g K with ionic radius vs 4f ~, show60
- - - A : L n ( I I I ) r e s
5 0 - - B : L n ( I I I ) . E D T A . r e s oo
6 0
2~ 3o
~ 2 0 /~
0
- 1 0 I I I I I , I , I
3 5 7 9
N u m b e r o f 4 f - e l e c t r o n s
Figure !. Representative differential plots showing the variation profiles of A log K,'Ar with 4)" q.
158
Sanoeeta Verma, S N L i m a y e and M C S a x e n awell-marked depression at the three stages: 1/4th-filled shell: f 3 _ f 4 , 1/2-filled shell:
f 7 and 3/4th-filled shell: f~o _ f l ~ . The superiority of this method lies in the fact that it not only shows the presence of periodicity, but also indicates that the magnitude of tetrad effect lies in the sequence f 7 > f 3 _ f 4 ~ f ~ o _ f l 1. The same conclusions have been arrived at with the help of the straight line approximation method by calculating the magnitude of tetrad effect at different 4f ~ configurations. These results offer experimental evidence in favour of the inter-electronic repulsion theory of Jorgensen (1990) and Nugent (1970).
Extrastabilisation of various 4fq configurations may be evaluated using a semi-empirical equation (Spitsyn
et al1984, 1990), the reduced form of which may be written as,
AG~L(o r AGMA L) MS _
-- EB_
fiE, t,(1)
MA stands for free energy change accompanying complexation, where AG~t" or AGMA L
EB = energy of the baricentre of 4f r configuration and fiEs, = extrastabilization energy.
The parameters E B and fiE,t may be correlated by the expressions,
E 8 = C + q V + [ q ( q - 1)/2]. E ~ (2)
6E~t = K I "fiE 1 + K 3 f i E 3.
(3)
In (2) and (3), C and V are constants, q, the number of f-electrons in Ln(III) ion, E ~ E ~ and E 3, the inter-electronic repulsion Racah parameters. K~ and Ka, the S- and L-dependent coefficients given by,
K~ = 1 8 / 1 3 [ S ( 1 / 2 -
S)],
K 3 = L / 1 8 0 [ L ( 2 2 2 -
48L) - 234],
(4) (5)
where S and L stand for the spin- and total orbital angular momentum-quantum number, respectively.
The quantities fiE t , fiE ~ and fiE 3 have been calculated using the above equations with a computer program (G5215AP with PL-I) (Limaye
et al1990, 1991). These and the evaluated values of nephelauxetic ratio
(fiEa/fiE 1)(abbreviated as NFE) for the three sets of 4f q configurations, where discontinuities occur due to periodicity, have been recorded in table 2. The observed trends in the values of extrastabilization and nephelauxetic ratio lead to the following conclusions: (i) The 6E,t values lie in the sequence f 7 > f 3 _ f 4 ~ f 1o _ f ~ 1 showing that these configurations are preferentially extrastabilised in the above sequence giving rise to tetrad effect, (ii) the fiEf values reveal a certain dependence on the basicity of secondary ligands, (iii) the observed range, in which the present
6E3/fiE tvalues lie, agrees well with the range
-- 0"194 <
fiEa/fiE l< 0-214
predicted by theoretical quantum chemical calculations (Spitsyn
et al1984, 1990;
Ionova 1990) and (iv) the plots of fiEst vs 4f ~ (figures not included) exhibit an inverted organ pipe shape for
K t f i E tand inverted tulip for
K 3 f i E 3in agreement with inter-electronic repulsion theory.
In order to examine the role of hydration on the occurrence of periodicity, the
values of the first, second, third and total hydration numbers (HN) of Ln(III) ions
have been calculated using a recently published method (Sood 1987) assuming that
Formation constants of Ln(lll)
159
Table 2. Extrastabilization(6E,)andnephelauxeticeffect(NFE)in[Ln(lll).L],[Ln(lll).A.L]
complexes (A = NTA, HEDTA, EDTA; L = res, orc, phi).
Formation constant
Extrastabilization energy 6E (cal.deg- a mol- i)
NFE
A L f 9 , f s o f , , f ~ f7 (6E3/fE t) fiE ~ 6E 3
log K M
ML
log KMA c MA
log KMA L MA
log KMA c MA
-- res 60 98 229 0"0855 0-0157 0.00136
-- orc 281 398 818 0.1401 0-0563 0-07897
- - phi 175 258 581 0' 1112 0-0400 0-00445
NTA res 130 192 437 0-1067 0-0302 0-0032
HEDTA res 171 233 436 0.1735 0-030 0-0052
EDTA res 84 130 314 0.0895 0-0216 0.00193
NTA ore 185 258 519 0.147 0-0357 0.0053
HEDTA orc 22 41 137 ff012 0.0095 0.0001
EDTA orc 190 264 511 0' 1609 0-0352 0 . 0 0 5 6
NTA phi 130 208 527 0.0736 0-0364 0-00268
HEDTA phi 130 173 306 0.1980 0"0211 0.00419
EDTA phi 235 335 698 0"133 0.0480 0-00643
25
2O
z
/'1
Figure 2.
/'W/t"
I I , I , I , I , I
4 G I! 10 12 14
Number of 4 f - e l e c t r o n s
Differential plots showing variations in A(HN)/Ar with 4ft.
the hydration sphere of Ln(IlI) ions consists of three concentric zones (Frank and Evans 1945; Gurney 1953). The total HN varies from about 21.91 to 24.22 across Ln(III) series. Application of differential plot method reveals the presence of tetrad effect in HN values (figure 2).
The possibility of contribution from ligand field stabilization and consideration of L - S states are also engaging our attention.
Acknowledgement
One of the authors (SV) is grateful to the University Grants Commission, New Delhi,
for the award of a research fellowship. SNL thanks the Ministry of Human Resource
160 S a n g e e t a V e r m a , S N L i m a y e a n d M C S a x e n a
D e v e l o p m e n t , N e w D e l h i f o r t h e a w a r d o f a U S S R G o v e r n m e n t p o s t - d o c t o r a l f e l l o w s h i p . F i n a n c i a l s u p p o r t f r o m t h e U u n i v e r s i t y G r a n t s C o m m i s s i o n is g r a t e f u l l y a c k n o w l e d g e d .
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