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THE LANTHANIDES: PHYSICO–CHEMICAL
PROPERTIES RELEVANT FOR THEIR
BIOMEDICAL APPLICATIONS
CARMEN MARIA STURZA1*, RICA BOSCENCU2, VERONICA NACEA2
”Arsmedica” 150 Aurel Vlaicu str, Bucharest
University of Medicine and Pharmacy “Carol Davila”
Departament of Inorganic Chemistry, 6 Traian Vuia str, Bucharest
*corresponding author: carmen.sturza@arsmedica.ro
1
2
Abstract
Over the past ten years a resurgence of interest in the lanthanides chemistry has
been noticed, especially in the biomedical applications of these elements. In this review we
try to demonstrate that for the biomedical applications of lanthanides their spectral,
magnetic and coordinative properties are essential.
The lanthanides represent the largest naturally occurring group of elements in the
periodic table. They are highly electropositive and reactive metals. Spectral and magnetic
properties of lanthanide ions are remarkable. Their coordination chemistry is complicated,
especially in solutions having not well defined stereochemistry and uncertain coordination
numbers. However, the complexes of lanthanides and even the lanthanide salts are getting
more and more applications in biology, biochemistry and medicine.
The chelates of lanthanide ions are used as labels in fluorescent immunoassays.
The paramagnetic and luminescent Tb+3- phosphatidylcholine-complexes are unique
evidence for the cation phospholipid interaction. Water-soluble La+3 and Eu+3 complexes
are effective in detecting sugars neutral environment and glycolipids and phospholipids as
well. Furthermore, literature data show that some simple lanthanide salts and complexes
exhibit an antitumoral activity.
All above reveal the actuality of the subject.
Rezumat
În ultimii 10 ani a existat o revenire a interesului pentru chimia lantanidelor, în
special pentru aplicaţiile biomedicale ale acestor elemente. În acest referat încercăm să
demonstrăm că proprietăţile spectroscopice, magnetice şi coordinative ale lantanidelor sunt
esenţiale pentru aplicaţiile lor biomedicale.
Lantanidele reprezintă cea mai largă grupă de elemente naturale din sistemul
periodic. Sunt metale foarte electropozitive şi reactive.
Proprietăţile spectroscopice şi magnetice ale ionilor lantanidelor sunt
remarcabile. Chimia lor coordonativă este complicată, în special în soluţie, de o
stereochimie dificilă şi de numere de coordinare incerte. Cu toate acestea, complecşi ai
lantanidelor şi chiar unele săruri au din ce în ce mai multe aplicaţii în biologie, biochimie şi
medicină. Chelaţi ai lantanidelor sunt utilizaţi ca markeri în imunoteste bazate pe
fluorescenţă. Complecşii paramagnetici şi luminiscenţi Tb+3-fosfatidilcholina oferă dovada
interacţiei cationi metalici- fosfolipide.
Complecşii solubili în apă ai La+3 şi Eu+3 permit identificarea glucidelor în mediu
neutru ca şi a unor glicolipide şi fosfolipide.
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327
Mai mult, date din literatură confirmă activitatea antitumorală a unor săruri
simple şi chiar combinaţii complexe ale lantanidelor.
Toate acestea relevă actualitatea subiectului.


lanthanides
physico-chemical properties
INTRODUCTION
One of the confusions associated with the group of lanthanides is
connected to the terminology. The term “rare earth” was originally used to
describe almost any naturally occurring but unfamiliar oxide and until about
1920 generally included also compounds as ThO2 and ZrO2. By that time the
term began to be applied to the elements themselves rather than to their
oxides, and also was restricted to that group of elements which could only
be separated from each other with great difficulty [1, 2].
The lanthanides comprise the largest naturally-occurring group in
the periodic table. They are belonging to 6-th period.
The first element of the 6-th period, Cs (Z=55), has the electronic
configuration of Xe for 54 of its electrons and the last electron is placed in
the P shell. Similarly, the next element, Ba (Z=56) has two electrons in the P
shell. The first member of the 5d transition series, 57La, now appears with
the expansion of penultimate shell. However, a new phenomenon appears
with the expansion of the O shell in successive stages from 18 electrons to
32 electrons. The fourteen new elements, between Z= 58 and 72 are called
lanthanons, lanthanides or rare earth. They are very similar to one another.
This is a transition series within another transition series are so
called inner transition series.
Their properties are so similar, that from 1794, when J.Gadolin
isolated “yttria” which he thought it was the oxide of a single new element,
until 1907, when lutetium was discovered, nearly a hundred claims were
made for the discovery of elements belonging to this group.
GENERAL PROPERTIES
The metals are silvery in appearance (except for Eu and Yb, which
are pale yellow) and are rather soft, but become harder across the series.
Most of them exist in more than one crystallographic form.
The electronic configurations of the free atoms are determined only
with difficulty because of the complexity of their atomic spectra, but it is
generally agreed that they are nearly all [Xe] 4fn5d06s2. The exceptions are:
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1. Cerium, for which the sudden contraction and reduction in energy of
the 4f orbitals immediately after La is not yet sufficient to avoid
occupancy of the 5d orbital;
2. Gadolinium, which reflects the stability of the half-filled 4f shell,
4f7;
3. Lutetium, at which point the shell has been filled, 4f14.
However, only in the case of cerium this has any marked effect on
the chemistry in aqueous solution, which is otherwise dominated by the +3
oxidation state, for which the configuration varies regularly from 4f1 (CeIII)
to 4f14 (LuIII). It is notable that a regular variation is found for any property
for which this 4fn configuration is maintained across the series, whereas the
variation in those properties for which this configuration is not maintained
can be highly irregular. This is illustrated dramatically in size variations
between 1,02Å for CeIII and 0,861 Å for LuIII. The radii of LnIII ions
decrease regularly from LaIII (included for completeness) to LuIII. This
“lanthanide contraction” occurs because, although each increase in nuclear
charge is exactly balanced by a simultaneous increase in electronic charge,
the directional characteristics of the 4f orbitals cause the 4fn electrons to
shield themselves and other electron from nuclear charge only imperfectly [3].
Thus, each unit increase in nuclear charge produces a net increase
in attraction for the whole extranuclear electron charge cloud and each ion
shrinks slightly in comparison with its predecessor.
On the other hand, a similar overall reduction is seen in the metal
radii, but irregular.
A contraction resulting from the filling of the 4f electron shell is
not exceptional. Similar contractions occur in each row of the periodic table
and, in the d block for instance, the ionic radii decrease from Sc III to CuIII,
and from YIII to AgIII. The importance of the lanthanide contraction arises
from its consequences:
1. the reduction in size from one LuIII to the next could make their
separation possible, but the smallness and regularity of the reduction makes
the separation difficult;
2. by the time Ho is reached the LuIII radius, it has been
sufficiently reduced to be almost identical to that of YIII; that is why this
much lighter element is invariably associated with the heavier lanthanides;
3. the total lanthanide contraction is of a similar magnitude to the
expansion found in passing from the first to the second transition series, and
which might therefore have been expected to occur also in passing from
second to third. The interpolation of the lanthanides in fact almost exactly
cancels this anticipated increase with the result that in each group of
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329
transition elements the second and third members have very similar sizes
and properties. Redox processes, which of necesity entail a change in the
occupancy of the 4f shell, vary in very irregular manner across the series.
The use of thermodinamic data (4) offers the ionization energies. The
sudden falls of the third ionization energy at Gd and Lu reflect the ease with
which these elements form Gd3+ (f7) and Lu3+ (f14).
CHEMICAL REACTIVITY
The lanthanides are highly electropositive and reactive metals.
With the exception of Yb, their reactivity apparently depends on their size,
so that Eu, which has the largest metal radius, is the most reactive [2, 5].
Treatment with water yields hydrous oxides, and the metals
dissolve rapidly in dilute acids, even in the cold, to give aqueous solutions
of LnIII salts. The great bulk of lanthanide chemistry is of the +3 oxidation
state, where, because of the large sizes of the LnIII ions, the bonding is
predominantly ionic and the cations are the typical calss-a acids.
A number of trends connected with the ionic radii are noticeable
across the series; salts become somewhat less ionic as the LnIII radius
decreases; reduced ionic character in the hydroxide implies a reduction in
alkaline properties and, at the end of the series, Yb(OH)3 and Lu(OH)3,
though undoubtedly mainly alkaline, can with difficulty be solved in hot
concentrated NaOH. Paralleling this change, the [Ln(H2O)x]3+ ions are
subject to an increasing tendency to hydrolyse, and hydrolysis can only be
prevented by using increasingly acidic solutions.
However, solubility cannot be simply related to the cation radius.
No consistent trends are apparent in aqueous, or for that matter nonaqueous,
solutions, but an empirical distinction can often be made between the lighter
“cerium” lanthanides and the heavier “yttrium” lanthanides. Thus oxalates,
double sulfates and double nitrates of the former, are rather less soluble and
basic nitrates more soluble than those of the latter. The differences are by no
means sharp, but classical separation procedures depended on them.
Although lanthanide chemistry is dominated by the +3 oxidation
state, and a number of binary compounds which ostensibly involve LnII are
actually better formulated as involving LnIII with an electron in a
delocalized conduction band, genuine oxidation states of +2 and +4 can be
obtained. CeIV and EuII are stable in water and are strongly oxidizing and
strongly reducing respectively. TbIV presumably owes its existence to the
stability of the 4f7 configuration.
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The stabilizing effects of half and completely filled shells can be
similarly invoked to explain the occurrence of the divalent state in Eu II (4f7)
and YbII (4f14).
SPECTRAL PROPERTIES
The electronic configurations of the lanthanides are described by
using the Russell-Saunders coupling scheme. Values of the quantum
numbers S and L corresponding to the lowest energy are obtained in the
conventional manner [3, 5, 6].
Because of lanthanides 4f electrons, ions are largely burried in the
inner core, they are effectively shielded from their chemical environment.
As a result, the spin-orbit coupling is much larger than the crystal field
(2000 cm-1 compared to 100 cm-1) and must be considered first. Note that
this is precisely the reverse of the situation in the d-block elements, where
the d electrons are exposed directly to the influence of neighbouring groups
and the crystal field is therefore much greater than the spin-orbit coupling.
Electronic absorption spectra are produced when electromagnetic
radiation promotes the ions from their ground state to excited states. For the
lanthanides the most common of such transition involve the excited states
which are either components of the ground term or else belong to excited
states, which arise from the same 4fn configuration as the ground term. In
either case, the transitions therefore involve only a redistribution of
electrons within the 4f orbitals (i.e. f→f transitions) and so are orbitally
forbidden, just like d→d transitions. In the case of the latter, the rule is
partially relaxed by a mechanism which depends on the effect of the crystal
field in distorting the symmetry of the metal ion. However, it has already
been pointed out that crystal field effects are very much smaller in the case
of LnIII ions and they cannot therefore produce the same relaxation of the
selection rule. Consequently, the colours of LnIII compounds are usually less
intense.
A further consequence of the relatively small effect of the crystal
field is that the energies of the electronic states are only slightly affected by
the nature of the ligands or by thermal vibrations, and so the absorption
bands are very much sharper than those for d→d transitions. Because of
this, they provide an useful means of characterizing, and quantitatively
estimating, LnIII ions. [7, 8].
Nevertheless, crystal fields cannot be completely ignored. The
intensities of a number of bands (“hypersensitive” bands) show a distinct
dependence on the actual coordinated ligands. Also, in the same way that
crystal fields lift some of the orbital degeneracy (2L + 1) of the terms of dn
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331
ions, so they lift some of the 2J + 1 degeneracy of the states of fn ions,
though in this case only by the order of 100cm-1. This produces a fine
structure in some bands of LnIII spectra.
CeIII and TbIII are exceptional in providing (in the ultraviolet) bands
of appreciably higher intensity than usual. The reason is that the particular
transitions involved are of the type 4fn→4fn-15d1, and so are not orbitally
forbidden. These 2 ions have 1 electron more than an empty f shell and 1
electron more than a half-full f shell, respectively, and the promotion of this
extra electron is thereby easier than for other ions.
Sm, Dy but more specifically Eu and Tb have excited states which
are only slightly lower in energy than the excited states of typical ligands. If
the electrons on the ligand are excited, the possibility therefore exists that,
instead of falling back to the ground state of the ligand, they might pass first
to the excited state of the LnIII and then fall to the metal ground state,
emitting radiation of characteristic frequency in doing so (fluorescence or,
more generally, luminescence). Excitation by UV light produces
luminescence spectra, which gives information about the donor atoms and
co-ordination symmetry [8].
MAGNETIC PROPERTIES
The lanthanides are transition metals.
The magnetic moments of the transition metals ions are assumed to
be due to electron spin only; as the orbitals occupied by unpaired electrons
are near the peripheries of the ions, it is generally supposed that the orbital
angular momenta are quenched by external fields [3, 5].
From the very high values of the magnetic moments for Tb+3, Dy3+,
3+
Ho and Er+3, it is clear that at least for these ions the orbital contributions
must be taken into account. Since the 4f quantum shells are well shielded
from outside fields by the electrons of the n=5 and n=6 quantum shells, this
is a reasonable conclusion.
It is also clear that there is a minimum value for the magnetic
moment of trivalent ions such as Pb3+, Sm3+ and Eu3+. Agreement between
observed and calculated values is good, except for Sm+3 and Eu3+.
The magnitude of the separation between the adjacent states of a
term indicates the strength of the spin-orbit coupling, and in all cases,
except Sm+3 and Eu3+, it is sufficient to render the first excited state of the
Ln+3 thermally inaccessible, and so the magnetic properties are determined
only by their ground state.
332
Ln
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
FARMACIA, 2008, Vol.LVI, 3
Unpaired
electrons
1(4f1)
2(4f2)
3(4f3)
4(4f4)
5(4f5)
6(4f6)
7(4f7)
6(4f8)
5(4f9)
4(4f10)
3(4f11)
2(4f12)
1(4f13)
0(4f14)
Table I
Magnetic properties and colours of LnIII ions in hydrated salts
Ground
Colour
µe / BM
µe / BM
state
g√J(J + 1) Observed
2
F5/2
Colourless
2.54
2.3-2.5
3
H4
Green
3.58
3.4-3.6
4
I9/2
Lilac
3.62
3.5-3.6
5
I4
Pink
2.68
6
H5/2
Yellow
0.85
1.4-1.7(*)
7
F0
Very pale pink
0
3.3-3.5(*)
8
S7/2
Colourless
7.94
7.9-8.0
7
F6
Very pale pink
9.72
9.5-9.8
6
H15/2
Yellow
10.65
10.4-10.6
5
I8
Yellow
10.60
10.4-10.7
4
I15/2
Rose-pink
9.58
9.4-9.6
3
H6
Pale green
7.56
7.1-7.5
2
F7/2
Colourless
4.54
4.3-4.9
1
S0
Colourless
0
0
* These are the values of µe at room temperature. The values fall when the temperature is
reduced.
The high-spin paramagnetism and long electronic relaxation time
of Gd3+ have made it pre-eminent among contrast agents for magnetic
resonance imaging [9, 10].
In addition, paramagnetic properties of lanthanide complexes are of
great importance.
COORDINATION CAPACITY
Over the past years there has been a great interest in the chemistry
of lanthanide complexes in solution in general and in aqueous solution in
particular.
Coordonation chemistry of lanthanides is quite different from that
of the d-transition metals [12, 7].
Coordination number is generally high and stereochemistries, being
determined largely by the requirements of the ligands and lacking the
directional contraints of covalency, is frequently not well defined and the
complexes are labile.
Thus, in spite of widespread opportunities for isomerism, there
appears to be no confirmed example of a lanthanide complex existing in
more than one molecular arrangement. Furthermore, only strongly
complexing (i.e. usually chelating) ligands lead to products which can be
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333
isolated from aqueous solution, and the comparative tenacity of the small
H2O molecule commonly leads to its inclusion, often with consequent
uncertainty to the coordination number involved. This is not to say that
other types of complex cannot be obtained, but complexes with uncharged
monodentate ligands, or ligands with donor atoms other than O, must
usually be prepared in the absence of water.
Lanthanides, both in crystalline complexes as well as in solutions,
show variable coordination number (6….12), in which C.N. 9 is the most
predominant.
Coordination numbers below 6 are found only with very bulky
ligands and even the coordination number of 6 itself is unusual, 7, 8 and 9
being more characteristic.
Coordination numbers of 10 and over require chelating ligands
with small “bites”, such as NO3- or SO42-, and are confined to compounds of
the larger, lighter lanthanides.
The stereochemistries quoted, especially for high coordination
numbers, are idealized and in most cases appreciable distortions are in fact
found.
DISCUSSIONS
The most interesting revelation of the importance of the chemical
lanthanides ions properties (ionic radius, redox properties, coordination
capacity) is offering by the cancer domain (13)
Lanthanides are consuming Reactive Oxygen Species (ROS),
which are mainly oxygen derived free radicals and peroxides, being the
mediators of a number of degenerative diseases. The “antioxidants” are
excellent substrances used as drugs for the treatment of degenerative
diseases.
Tocopherol, ascorbates and a number of other organic compounds
are considered as components of such drugs due to their property shown
towards ROS induced degenerative diseases. Lanthanides are considered of
high potential because of their inherent antioxidant properties [12] Liu et. al.
[14] found LaCl3 effective in inhibiting silica induced lipid peroxidation of
lung macrophagus and smaller of some other lanthanide chlorides inhibited
lipid peroxidation in rat lung.
In fact, it was found that almost all lanthanides compounds,
especially chlorides, are quite effective in inhibiting H2O2 mediated
peroxidation of liposomes [15].
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Table II
Ionic radius of lanthanide (III) ions
Ionic radius
(CN 6)
S.O. III (A0)
Ionic radius
(CN 6)
S.O. III (A0)
Ce Pr Nd
Pm Sm Eu
Gd
1.02 0.99 0.983 0.97 0.958 0.947 0.938
Lower lanthanides
Tb Dy Ho Er Tm Yb Lu
0.923 0.912 0.901 0.89 0.88 0.868 0.861
Higher lanthanides
When tertbutyl hydroperoxide was used to mediate the
peroxidation, lower lanthanides inhibited while the higher lanthanides
promoted peroxidation. Interestingly, Ln(III) lost reactivity of peroxides
when they were bound to membrane.
The lanthanide inhibiting ROS strongly involves their oxyphilicity;
because of the availability of oxygen binding sites on these free radicals,
they are excellent targets for Ln (III) coordination. This causes lanthanide to
play the role of ROS scavenger, therefore presenting good potential for
lanthanides as future drugs.
The involvement of lanthanide in ROS removal is quite different
from the inhibition of ROS by organic compounds like tocopherol,
ascorbate, etc.
Most of the organic antioxidants scavenge free radicals by single
electron exchange with radicals and thus transform themselves into radicals
hence acting as “prooxidants”.
Ln3+ easily interacts with either free radicals or peroxides, but is
not transformed as radicals.
In addition with the antioxidant effect, disorganization in the
cytoskeleton is a phenomenon of common occurrence in tumour cells and
apoptotic sized cells. Microtubules of the cytoskeleton undergo stabilization
and repair and this occurs during the action of some anticancer drugs like
taxol. Depolymerisation of the cytoskeleton is one of the most relevant steps
in the apoptosis process.
Lanthanide compounds have been shown to influence the stability
of microtubules [16, 17], mixed lanthanide compounds [16] increasing the
amount of orderliness of microtubules in PAMC82 cells.
Other scientists [17] found different lanthanides showing different
behaviours and this was ascribed to the similarity of Ln (III) predominantly
with Ca2+ or the similarity of Ln (III) with Mg2+.
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FARMACIA, 2008, Vol.LVI, 3
Is true that Ln3+ is very similar to Ca2+ in aspects like softness,
covalence tendency and redox properties. Ln+3 and Ca2+ ions are strong
Lewis acids, have a low covalence tendency and negligible redox properties.
Property
Electronic
configurations of ions
Coordination number
Coordination geometry
Donor atom preference *
Ionic radius (A0)
Type of bonding
Hydration number
Water exchange rate
constant (s-1)
Diffusion coefficient
Crystal field stabilization
Ca (II)
[Ar]
Table III
Properties of Ca2+ and Ln3+
Ln (III)
[Xe]
6-12
6 or 7 favoured
Highly flexible
0>N>S
1.00-1.18 (C.N. 6-9)
Essentially electrostatic
6
5.108
6-12
8 or 9 favoured
Highly flexible
0>N>S
0.86-1.22 (C.N. 6-9)
Essentially electrostatic
8 or 9
5.107
1.34
None
1.30
Negligible
*Ca+2 and Ln3+ are class a acceptors or “hard” acids; form most stable complexes with
ligands containing N, O or F donor atoms.
Ca2+ ion was found to destabilize microtubules, contrary to Mg2+.
The lower lanthanides behaved more like Ca2+, because of their
larger size and hence the propensity of exhibiting relatively higher
coordination number.
Heavier lanthanides like Tb, Dy, Ho, being smaller in size, behave
more like Mg2+, having a greater potential for strengthening the
microtubules [13].
Also, Ln3+, when administered in high doses, behaves like Mg2+
supporting the association of tubulin. However, when administered in high
doses, the Ln3+ interferes with the assembly by distorting the protein
conformation, altering crosslinking and consequently destabilizing the
polymers.
The complexes of lanthanides are getting more and more
applications in cancer therapy, especially those derived from poly
(aminocarboxilic) acids. The formation constants of these chelates are of the
order of 1020 to 1025, which enables them to remain intact while diffusing
into extracellular spaces with rapid clearance through kidneys. Due to the
high thermodynamic stability and extreme kinetic inertness of these poly
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(aminocarboxylates) of Ln (III), the intact excretion ennances, thereby
lowering considerably the body retention of chelated Ln (III) complexes
[18, 19].
The binding sites of the biological substrate which are preferred by
Ln (III) for complexation involve donor sites O≥F>Cl>N>S. The inherent
strong oxyphilicity of the lanthanide causes the interaction, preferred
binding sites being: COOH, OH (phenolic), OH (hydroxylic), O (carbonyl),
N (amino, imido, imino), S (sulphydril). Nitrogen sulphur donor sites of the
biomolecules also enter into complexation, when Ln (III) undergoes
chelation. The lability of the lanthanide complexes, their strong
oxyphilicity, the very fast exchange reaction, the nondirectionality of the
ligand bond and varying coordination number, all contribute towards the
lanthanide interaction with biomolecules.
The ionic size of Ln (III) varies from one lanthanide to another; in
addition, the ionic size of a particular lanthanide ion varies significantly
with the coordination number.
Smaller size of chelating biomolecular ligand can even suit larger
lanthanides, with lowered coordination number and can form stable chelates
with larger biomolecules. This can explain the different coordinating power
(and also their biological behaviour) of different lanthanides under different
physiological conditions.
It is clear that over the past years (20-26) there has been a great
interest in the chemistry of lanthanides, in aqueous solution in particular.
The properties of the lanthanides ions in aqueous solution are of a great
importance, not only scientifically but also biologically and medically.
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