Comparison of covalency in the lanthanide chloride and nitrate complexes based on the adsorption data on zeolite y

Article abstract of DOI:10.1016/j.molstruc.2011.09.052

The changes of the distribution constants K_d of lanthanide chlorides in the system: zeolite Y (solid phase)-sodium chloride (aqueous phase) were investigated. The evident tetrad effect in the change of log K_d values within the lantha

Full text of DOI:10.1016/j.molstruc.2011.09.052

Journal of Molecular Structure 1006 (2011) 469–474
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Journal of Molecular Structure
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Comparison of covalency in the lanthanide chloride and nitrate complexes based
on the adsorption data on zeolite Y
a
a
b
Agnieszka Gładysz-Płaska a, , Marek Majdan , Wiesława Ferenc , Jan Sarzynski
⇑
´
a Faculty of Chemistry, Maria Curie-Skłodowska University, Sq. M.C. Skłodowska 2, 20-031 Lublin, Poland
b Institute of Physics, Maria Curie-Skłodowska University, Sq. M.C. Skłodowska 1, 20-031 Lublin, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The changes of the distribution constants Kd of lanthanide chlorides in the system: zeolite Y (solid
phase)–sodium chloride (aqueous phase) were investigated. The evident tetrad effect in the change of
logKd values within the lanthanide series was observed and the attempt of its explanation through the
comparison of covalence in LnAO bonds existing in: the „AlAO(1/3Ln)ASi„, „AlAO(1/3Ln) species,
Received 24 May 2011
Received in revised form 28 September
2011
Accepted 28 September 2011
Available online 14 October 2011
3þ
i.e. those formed by siloxane and silanols in the zeolite phase and in the aquoions LnðH2OÞ8—9 present
in the aqueous phase was presented. The Ln–zeolite samples were characterized by N2 adsorption and
magnetic measurements. The magnetic moments were determined over the range of 77–303 K. They
obey the Curie–Weiss law. The values of leff calculated for Ln–zeolite Y are much higher than those
obtained for Ln3+ by Hund and Van Vleck.
Keywords:
Lanthanides
Tetrad effect
Adsorption
Zeolites
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
the examples of the tetrad effect in the adsorption systems with
zeolite A [5], zeolite Y [6] and mordenite [7]. In the present paper
One of the most intriguing problems of the modern coordina-
tion chemistry of the lanthanides is so-called tetrad effect, known
also as the double–double effect, related directly to the contribu-
tion of covalency into the metal–ligand bond in the lanthanide
complexes. The name of the effect originates from the division of
the whole lanthanide series into four groups: LaACeAPrANd,
PmASmAEuAGd, GdATbADyAHo, and ErATmAYbALu when the
physicochemical properties of the lanthanides, such as: ionic radii,
unit cell volumes of the compounds, stability constants of the com-
plexes, distribution constants in the liquid–liquid, liquid–solid sys-
tems are analyzed. For the first time the tetrad effect was
discovered independently by Fidelis and Siekierski [1] and by
Peppard et al. [2] based on the extraction data of the lanthanides
in different extraction systems. The four spans were called first,
second, third and fourth tetrads, respectively [3]. The tetrad effect
was explained at first based on the crystal field theory and then
justified more clearly taking into account the change of the Racah’s
parameters of interelectronic repulsion E1 and E3 of the lanthanide
f-electrons during complex formation [4]. Spectroscopic studies of
various lanthanide complexes show that the Racah’s parameters
for 4f electron repulsion decrease with the increasing covalence
in the lanthanide–ligand bonds [4].
we focused our attention on the difference in the shape of the tet-
rad effect when the two complexing ligands are compared: nitrates
and chlorides. The weak complexing ability of Clꢀ and NO3ꢀ ions to-
ward tripositive lanthanide ions is well known [8–16]. No doubt
that the lanthanide chloride and nitrate complexes, i.e. LnCl2+
and LnNO23þ have the character of ionic pairs, outersphere or inner-
sphere in character, i.e. when the central ion in the complex is sep-
arated by water molecules from the ligand or is in the direct
contact with the ligand. The absolute values of the stability con-
stants of these complexes are comparable and it is difficult to find
any essential difference in the contribution of covalency to the me-
tal–ligand bond in them. The method of adsorption using zeolites
was intuitively chosen, knowing that obtaining of a concave or
convex tetrad effect in the lanthanide distribution constants
change within the lanthanide series can reveal the difference be-
tween the studied ligands.
The zeolite Y as the adsorbent was intentionally selected taking
into account its potential application in the nuclear industry as the
adsorbent for the lanthanide radionuclides having in mind the fact,
that the success in the adsorption of lanthanides means also the
positive effect in the adsorption of actinides since the ionic radii
of both element families are very similar. A promising new use
for zeolites is in the reprocessing nuclear waste, transforming it
to the safer form. High-level waste with large amounts of fissile
products, among them lanthanides and actinides, is stored as a salt.
When a zeolite is added, it removes active fission material away,
storing it in a safe way. The zeolite can then be combined with
We continue the idea of the application of adsorption method
for the study of the tetrad effect. In the previous papers we showed
⇑
Corresponding author. Tel.: +48 81 537 57 27; fax: +48 81 533 3348.
E-mail address: agnieszka.gladysz.plaska@onet.pl (A. Gładysz-Płaska).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.09.052

470
A. Gładysz-Płaska et al. / Journal of Molecular Structure 1006 (2011) 469–474
glass and transformed into a solid piece of ceramic, known as
glass-bonded zeolite. In this way nuclear waste can be handled
safely.
Apart from the adsorptive properties of zeolite Y, one should
remember about its importance in the catalysis. Lanthanide modi-
fied zeolites have high catalytic properties and play an important
technology role as catalysts in petroleum cracking [17].
volumetric adsorption analyzer. Prior to the adsorption measure-
ments each sample was degassed at 473 K for at least 6 h until a
pressure of 10ꢀ3 hPa was attained. The specific surface area was
determined according to the standard Brunauer–Emmett–Teller
method, while the pores distribution was determined directly from
the isotherm by applying the Barrett–Joyner–Halenda (BJH) meth-
od [19].
2.5. Magnetic susceptibilities measurements
2. Experimental
Magnetic susceptibilities of the polycrystalline samples of Ln–
zeolite Y were measured by the Gouy method using a sensitive
Cahn RM-2 balance. The measurements were made at a magnetic
field strength of 9.9 khe. The calibrant employed was Hg[Co(SCN)4]
for which the magnetic susceptibility of 1.644 ꢃ 10ꢀ5 cm3 gꢀ1 was
taken [20]. The correction for diamagnetism of the constituent
atoms was calculated by the use of the Pascal’s constants [21].
The temperature-independent paramagnetism of rare earth ions
was assumed to be zero. Magnetic moments were calculated from
the following equations:
2.1. Characteristics of the adsorbent
Zeolite Y (CBV100) in the form of powder (grain and pore size
0.4 nm, unit cell size: 24.65 Å, surface area: 900 m2/g) has been ap-
plied supplied by Zeolyst International. The chemical composition
of the samples has been reported by the supplier as: Na2O 13% (w/
w), SiO2/Al2O3 molar ratio = 5.1. Zeolite Y exhibits the FAU (fauja-
site) structure. It has a 3-dimensional pore structure with pores
running perpendicular to each other in the x, y, and z planes. The
pore diameter is large at 7.4 Å since the aperture is defined by a
12 member oxygen ring, and leads into a larger cavity of diameter
12 Å. The cavity is surrounded by 10 sodalite cages (truncated
octahedra) connected on their hexagonal faces [18].
1=2
leff ¼ 2:83ð
v
MTÞ
ð2Þ
1=2
leff ¼ 2:83½ðvMðT ꢀ hÞꢁ
ð3Þ
2.2. Determination of the distribution constants Kd of the lanthanides
where leff is the effective magnetic moment, vM the magnetic sus-
ceptibility per molecule, T the absolute temperature, h is the Weiss
constant.
Samples (0.1 g) of the zeolite were equilibrated for 4 h with
100 ml aqueous phase containing 0.0005 mol/dm3 lanthanide ions,
(Sigma–Aldrich, 99% purity) and NaNO3 or NaCl (Sigma–Aldrich,
PA. concentration ranging from 0.001 to 2 mol/dm3) at a tempera-
ture of 296 K. The aqueous phase was filtered (Filtrak 390, Polish
Chemical Reagents) and centrifuged at 10.000 rpm for 10 min. Lan-
thanide nitrates and chlorides used in the experiment were ob-
tained by dissolution of a proper amount of the respective
lanthanide oxide (99.9% purity, Sigma Aldrich) in 0.1 mol/dm3
HNO3 or HCl, respectively (pure for analysis, Polish Chemical Re-
agents) and evaporation of the solution to dryness [5]. The concen-
tration of the lanthanides was determined spectrophotometrically
using the Arsenazo (III) method [6]. The pH values (initial and equi-
librium) of the solutions were controlled using the combined glass
electrode (Sigma Chemical Co.) connected to the pH-meter (CX-
731 type, Elmetron Co.). The distribution constant Kd was calcu-
lated from:
Some of the results are given in Table 2.
3. Results and discussion
3.1. Characteristics of the adsorbent
Fig. 1 shows the photo of the surface of zeolite Y. In the case of
zeolite Y the difference in grain size is small, the single grain sizes
are from 0.3 to 0.7 lm.
Fig. 2 presents the BET surface area, which represents the sum
of the micropore and external surface areas of LnAY zeolite. The
introduction of RE cations promoted an increase on the BET area
value, mainly due to an increase of the external surface and micro-
pore surface area (Table 1). The rise in micropore surface area was
quite similar for all RENa-Y zeolites. In the case of external surface
area a larger increase was observed for La, Ce, Pr, Sm, Eu, Gd, Tb
than for the heavy lanthanides. This may be due to differences in
the hydration of Ln ions, which increases with their ionic radii de-
crease. Strongly hydrated heavy lanthanides block the zeolite pores
more effectively than the light ones. This is the interesting fact
that, the external surface area of the LnAY samples is considerably
higher in comparison with that of NaAY, i.e. from 86 (NaAY) to
150–225 (LnAY) m2/g. This is probably the result of surface hydro-
lysis of the lanthanides. It is difficult to predict what kinds of sur-
face complexes are formed as a result of surface hydrolysis, but it is
rather obvious that the coordination number of the lanthanides
intercalated in the zeolite network is ‘‘completed’’ by H2O mole-
cules and OHꢀ ions.
Kd ¼ ½ðcin ꢀ ceqÞ=ceqꢁ ꢂ V=m
ð1Þ
where cin and ceq refer to the initial and equilibrium concentrations
of the lanthanides in the aqueous phase, V is the aqueous phase vol-
ume expressed in dm3, and m refers to the mass of the zeolite sam-
ple given in grams. The standard error in the determination of the
lanthanide distribution constants Kd, taken from the triple measure-
ments, was 10%.
2.3. Scanning electron microscopy
The zeolite surfaces were investigated by the SEM method (LEO
SEM 1430 VP microscope supplied with an EDX detector; operating
conditions of electron microprobe: 20 kV, 80 lA beam current. The
3.2. Adsorption equilibrium
secondary electron images of the NaAY zeolite sample surface
were registered at the magnification scale 20kꢃ. A vacuum
10ꢀ3 Pa was preserved during measurements.
The changes of the distribution constants logKd with the ligand
concentrations are given in Fig. 3. It is easy to notice that the dis-
tribution constants in the chloride system are evidently higher
than those in the nitrate one for the same ligand concentrations.
It seems that the stability of the lanthanide chloride complexes
LnCl2+ is lower than that of lanthanide nitrato LnNO23þ complexes.
Therefore the higher concentration of the free uncomplexed
2.4. N2 adsorption measurements
Nitrogen adsorption measurements at 77 K were performed on
the AUTOSORB-1CMS (Quantachrome Instruments, USA, 2005)

A. Gładysz-Płaska et al. / Journal of Molecular Structure 1006 (2011) 469–474
471
Fig. 1. The surface images of the zeolite Y: sodium form (magnification: 20kꢃ).
900
800
700
600
500
400
300
200
100
0
NaY La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
external surface area micropore surface area
Fig. 2. BET surface area of the LnAY zeolite samples.
Table 1
The surface area and pore volume measurements of modified NaALnAY.
Table 2
The magnetic moments of the Ln–zeolite Y samples determined in the 76–303 K
range and calculated for free Ln(III) ions by Hund and Van Vleck at 293 K.
NaALnAY BET
surface
Micropore
surface area surface
External
Pore
volume
Micropore
volume
Sample
leff (lb)
area (m2/ (m2/g)
g)
area (m2/g) (cm3/g)
(cm3/g)
76–303 K
By Hund
By Van Vleck
LaY
CeY
PrY
–
–
–
2.56
3.62
3.68
1.56–1.65
3.40–3.51
7.94
Na
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
606
811
840
859
795
822
842
883
842
813
857
782
791
838
831
520
621
660
665
641
628
648
697
617
641
692
607
641
669
664
86
190
180
194
154
194
194
186
225
172
165
175
150
169
167
0.286
0.405
0.417
0.452
0.343
0.419
0.423
0.474
0.454
0.406
0.451
0.39
0.204
0.212
0.234
0.243
0.222
0.217
0.223
0.262
0.212
0.228
0.262
0.211
0.238
0.247
0.258
8.69–10.24
8.13–9.10
8.27–9.72
4.49–5.70
2.48–3.89
20.88–20.66
29.96–31.16
27.67–28.31
22.58–23.79
28.58–30.30
18.31–19.04
10.02–11.53
–
2.54
3.58
3.62
0.84
0.00
7.94
9.70
10.60
10.60
9.60
7.60
4.50
NdY
SmY
EuY
GdY
TbY
DyY
HoY
ErY
9.70
10.60
10.60
9.60
7.60
4.50
TmY
YbY
LuY
Tm
Yb
Lu
0.403
0.42
0.462
The curves representing the logKd vs. logc dependences have a
plateau for the 0.001–0.1 M concentration range and a sudden
drop for the 0.1–2 M range. For the lower sodium ions concentra-
tions: 0.001–0.1 M the lanthanide tripositive ions locate easily in
the fixed charge sites of the zeolite structure, i.e. on negatively
lanthanide tripositive ions Ln3+ in the chloride system favors the
replacement of sodium ions from the zeolite structure according
to:
1=3Ln3þ þ NaY ) ð1=3LnÞY þ Naþ
ð4Þ

472
A. Gładysz-Płaska et al. / Journal of Molecular Structure 1006 (2011) 469–474
charged siloxane „SiAOAAl„ functional groups and in the vari-
able charge sites formed by silanol groups „SiAOH. For the higher
sodium ion concentrations, i.e. 0.1–2 M, the siloxane functional
groups are loaded with sodium ions and therefore lanthanides sorb
predominantly on the silanol „SiAOH functional groups through
formation of covalent bonds with silanol oxygens; as a result, since
the concentration of „SiAOH groups is lower than that of
„SiAOAAl„, a remarkable decrease of the distribution constants
for 0.1–2 M NaCl, NaNO3 is observed.
The changes of the distribution constants logKd within the lan-
thanide series are given in Fig. 4. As was mentioned by many inves-
tigators, the shape of the lanthanide tetrad effect is the
consequence of the difference in the covalency of Ln–ligand bonds
in the products and substrates of the complexation reactions [22–
25]. The comparison between the chloride and nitrate systems
show that generally in the case of chlorides the convex tetrad effect
is observed for 1 and 2 M ligand concentrations. Therefore one can
conclude that the covalency in the adsorption products, i.e. in the
complexes formed between the lanthanide ions and silanols or to
a smaller extent the siloxane groups according to:
1=3Ln3þ þ BSiAOH $ BSiAOð1=3LnÞ þ Hþ
ð5Þ
1=3Ln3þ þ BSiAOðNaÞASiB $ BSiAOð1=3LnÞASiB þ Naþ
ð6Þ
is stronger than that in the substrates, i.e. in the LnAO bonds of the
3þ
aquoions LnðH2OÞ8—9. Nitrate ions form anionic complexes with Ln3+
ions [26]. Their participation in the overall concentration of the lan-
thanides increases and at 1–2 M nitrates is probably so high that the
covalency in the adsorption substrates formed according to:
3ꢀn
Ln3þ þ nðNO3Þꢀ $ LnðNO3Þ
ð7Þ
n
is stronger than that in the adsorption products, i.e. in the
„SiAO(1/3 Ln) and in the „SiAO(1/3 Ln)ASi„ species. As a conse-
quence, the concave tetrad effect is observed.
2.0
1.0
2.0
1.0
2.0
1.0
2.0
1.0
(a)
Ce
La
Pr
Nd
0.0
0.0
0.0
0.0
-1.0
-1.0
-1.0
-1.0
-3.0 -2.0 -1.0
0.0
-3.0 -2.0 -1.0 0.0
-3.0 -2.0 -1.0
0.0
-3.0 -2.0 -1.0
0.0
2.0
1.0
2.0
2.0
1.0
2.0
1.0
Tb
Gd
Eu
Sm
1.0
0.0
0.0
0.0
0.0
-1.0
-1.0
-1.0
-1.0
-3.0 -2.0 -1.0
0.0
-3.0 -2.0 -1.0
0.0
Dy
-3.0 -2.0 -1.0
0.0
-3.0 -2.0 -1.0
0.0
Er
2.0
1.0
2.0
1.0
2.0
1.0
2.0
1.0
Ho
Tm
0.0
0.0
0.0
0.0
-1.0
-1.0
-1.0
-1.0
-3.0 -2.0 -1.0
0.0
-3.0 -2.0 -1.0
0.0
Lu
-3.0 -2.0 -1.0
0.0
-3.0 -2.0 -1.0
0.0
Yb
1.5
0.5
1.0
0.0
-0.5
-1.5
-1.0
-3.0 -2.0 -1.0
0.0
La
-3.0 -2.0 -1.0
0.0
log c NaCl
1.0
0.0
1.0
0.0
1.0
0.0
1.0
0.0
(b)
Ce
Nd
Pr
-1.0
-2.0
-1.0
-2.0
-1.0
-2.0
-1.0
-2.0
-3
-3
-3
-3
-2
-2
-2
-2
-1
-1
-1
-1
0
-3
-2
-1
0
-3
-3
-3
-2
-2
-2
-1
-1
-1
0
-3
-2
-1
0
1.0
0.0
1.0
0.0
1.0
0.0
1.0
0.0
Sm
Eu
Tb
Gd
-1.0
-2.0
-1.0
-2.0
-1.0
-2.0
-1.0
-2.0
0
-3
-3
-2
-2
-1
-1
0
0
-3
-3
-2
-2
-1
-1
0
1.0
0.0
1.0
0.0
1.0
0.0
1.0
0.0
Dy
Ho
Er
Tm
-1.0
-2.0
-1.0
-2.0
-1.0
-2.0
-1.0
-2.0
0
0
0
0
1.0
0.0
1.0
0.0
Yb
Lu
-1.0
-2.0
-1.0
-2.0
-3
-2
-1
0
0
log c NaNo3
Fig. 3. The effect of ionic strength: (a) NaCl and (b) NaNO3 on the adsorption of Ln(III) ions on zeolite Y.

A. Gładysz-Płaska et al. / Journal of Molecular Structure 1006 (2011) 469–474
zeolite Y - NaCl
473
[27]. Therefore one would expect the reverse order of covalency.
However when we compare the absolute electronegativities [28],
they are 7.54 and 8.30 (eV) for O and Cl respectively and in this
case the more covalent bond for oxygen ligand is expected. The
most important property is however ionic radius of the investi-
gated ions. For oxygen ligand (O2ꢀ) and for chloride ligand (Clꢀ)
they are 140 and 181 pm [27], which in fact is the strong argument
for the unquestionable more pronounced covalency of LnAO bond,
since the overlapping of the p orbitals from ligand and vacant 6s,
5d orbitals from lanthanide central ion is better for oxygen ligand.
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
3.3. Magnetic properties of the adsorption products
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Legend
In order to estimate the nature of metal ligand bond in the zeo-
lite structure the magnetic susceptibility of the Ln–zeolite Y sam-
ples was determined. The magnetic susceptibility of the analyzed
Ln–zeolites was measured in the range 77–303 K. The compounds
follow the Curie–Weiss law since their magnetic susceptibility val-
ues decrease with the increasing temperature (Fig. 6.). The values
of the Weiss constant (h) for the compounds were found to have
a positive sign which may be caused by ferromagnetic spin interac-
tions [29]. The magnetic moment values experimentally deter-
mined for the analyzed Ln–zeolites change from 2.48 lb (76 K) to
0.001 M NaCl
0.01 M NaCl
0.1 M NaCl
1.0 M NaCl
2.0 M NaCl
Zeolite Y – NaNO3
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
31.16 lb (303 K) (Table 2 and Fig. 7). The zeolites containing La(III)
and Lu(III) ions are diamagnetic in nature. The leff values for the
analyzed zeolites are considerably higher than those calculated
for free Ln(III) ions [30]. This difference may result from a spin –
coupling and the ferromagnetic alignment of the spins in the
molecular centers of compounds connected with the superex-
change coupling process between Ln(III) ions and diamagnetic an-
ions or may be caused by the difference of geometry of
paramagnetic center surroundings [31]. The superexchange mech-
anism is probably connected with the p electron transfers from the
bridging oxygen anions present in the zeolite Y framework to 6s
and 5d orbitals of Ln(III) ions. The participation of f orbitals in this
mechanism is presumably limited only to the change of their en-
ergy. The overlapping of f-orbitals with p oxygen anion orbitals is
minor, however the change of their energy under the influence of
6s and 5d electrons results from the unpaired electrons arrange-
ment causing the increase of the magnetic moment values [32–34].
In the case of Ln(III) ions there are partially occupied orbitals on
the metal atom and the intra-atomic coupling with the transferred
electron gives a ferromagnetic effect [35]. The magnetic moment
values experimentally determined are much greater than those
calculated for the free Ln(III) ions (with the exception of Eu). It re-
sults from the superexchange coupling process that may occur be-
tween Ln(III) ions and anions being closely related in the zeolite
network, i.e. negatively charged siloxane oxygens having non-
bonding hybridized sp3 orbitals with lone electron pairs [36]. The
transfer of electron from the diamagnetic anion orbital into the
not full – filled s, p, d, f orbitals on the Ln(III) ion, responsible for
the covalency in the Ln–ligand bonds [37] may take place causing
the increase of zeolite leff values. The magnetic data confirm also
the electrostatic metal ligand bonding in the zeolite structure with
the small contribution from covalency. In the case of Eu(III), the
Hund’s calculation of magnetic moment is much too low in
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Legend
1.0 M NaNO3
2.0 M NaNO3
0.001 M NaNO3
0.01 M NaNO3
0.1 M NaNO3
Fig. 4. The changes of the distribution constants (Kd) of the lanthanides in the
system: NaCl–zeolite Y and NaNO3–zeolite Y, (M = mol/dm3).
For the 0.001–0.1 M ligand concentrations range, chlorides
show the concave tetrad effect, whereas nitrates convex one for I
and IV tetrads. The shape of the effect in III tetrad (GdAHo) is dif-
ficult to account for due to a probable change of the coordination
3þ
number of the lanthanides in their aquoions Ln ðH2OÞ8—9 in this re-
gion of the lanthanide series [26] and consequent increase of the
electron-donor ability of O atoms of water molecules. The situation
when outersphere ionic pairs are formed (0.001–0.1 M range), i.e.
lanthanide ions are separated from ligands by sheaths of water
molecules, is not the same for nitrates and chlorides. The nitrate li-
gand in the second coordination sphere of the lanthanide ion prob-
ably reduces the electron-donor properties of oxygen from water
molecules in the first coordination sphere, i.e. oxygen originating
from water in the outersphere nitrato complex is a weaker electron
donor than that oxygen originating from the siloxane groups
„SiAOAAl„ or to a smaller extent from the silanol groups
„SiAOH and the convex tetrad effect is found in the case of ni-
trate. The chloride ligand in the second coordination sphere of
the lanthanide ion attracts weakly the water molecules in the first
coordination sphere, since it is probably unable to form such a
strong hydrogen bond with water oxygens as with the nitrate ion
(Fig. 5). Therefore oxygen originating from water in the outer-
sphere chloride complex is a stronger electron donor than that
originating from the siloxane groups „SiAOAAl„ or to a smaller
extent the silanol groups „SiAOH. As a result, the concave tetrad
effect occurs.
It is difficult to understand the reason for the higher covalency
in LnAO bond when compared it with LnACl bond, since according
to Pauling the electronegativities of O and Cl are: 3.44 and 3.16
Fig. 5. The complexation of Ln3+ ion by oxygen donors originating from siloxane,
silanol and water.

474
A. Gładysz-Płaska et al. / Journal of Molecular Structure 1006 (2011) 469–474
5
5
5
5
4
0
2. The increase of the magnetic moments of the lanthanide ions
3x10
2x10
2x10
1x10
5x10
0x10
2x10
Legend
incorporated into the zeolite Y network is probably the result
of the electron donation from the zeolite framework to the lan-
thanide vacant orbitals.
Ce
Pr
Nd
Sm
Eu
Yb
3. It is rather unquestionable that the covalency in the lanthanide
nitrato complexes is remarkably stronger than that in the chlo-
ride ones.
4. The elimination of the lanthanides from the aqueous environ-
ment by the adsorption method should be conducted from
the chloride media since the distribution constants of the lan-
thanides to zeolite Y from 0.001 to 0.1 M NaCl are evidently
higher than those from the nitrate media.
100
150
200
250
300
6
6
5
0
Legend
Gd
Tb
Dy
Ho
Er
References
1x10
5x10
0x10
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T (K)
Fig. 6. The magnetic susceptibility of LnAY zeolites.
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30
25
20
15
10
5
Legend
76 K
163 K
303 K
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Naukowe UAM, Poznan, 2000.
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0
La
Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
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Fig. 7. The magnetic moment values of the Ln–zeolite Y samples at different
temperatures (K).
´
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comparison with that obtained experimentally. The multiplet
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by Van Vleck [30].
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4. Conclusions
1. The adsorption of the lanthanides on the zeolite Y is the sensi-
tive method for the evaluation of subtle differences between
the covalency in the LnAO bond existing in the adsorption sub-
strates and products, manifesting in the shape of the tetrad
effect.
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