Brief Chemistry of Inner-Transition Elements

BRIEF CHEMISTRY OF
INNER-TRANSITION
ELEMENTS
Arnab Patra

In the modern periodic table, the two rows Ce58 to Lu71
and Th90 to Lr103 are set apart from the other elements.
These two rows are collectively known as f-block
elements or f-elements and are divided into Lanthanides
and Actinides. All the f-elements are metallic Lanthanum
and Actinium though have f0 electronic configuration, are
included in the lanthanide and actinide series respectively
property wise.

• In 1794 Johan Gadolin, a Finnish
chemist, isolated Yttria from a mineral
that had recently been discovered at
Ytterby, a village near Stockholm in
Sweden.
• He believed that it was the oxide of
single element, but subsequent work
reveals it to certain the oxide of no fewer
than 10 elements, Yttrium (Y-39),
Terbium (Tb-65), Erbium (Er-68),
Ytterbium (Yb-70), Scandium (Sc-21)
Holmium (Ho-67), Thallium (TI-81),
Gallium (Ga-31), Dysprosium (Dy-66)
and Lutetium (Lu-71). Johan Gadolin

• Shortly after Gadolin discovered another new oxide Ceria was
isolated this was later shown to certain the oxides of Cerium (Ce-
58), Lanthanum (La-57), Neodymium (Nd-60), Praseodymium (Pr-
59), Samarium (Sm-62) and Europium (Eu-63).
• Thorium (Th-90) and Uranium (U-92) are the only naturally
occurring actinide elements. In 1789, it was shown that
Pitchblende (Uraninite), thought previously to be a mixture of Zinc,
Tungsten oxides, also contained the oxide of a new element
Uranium. 39 years later Jons Jacob Berzelius discovered Thoria, a
new oxide from which he subsequently isolated Thorium (Th-90).
• Protactinium (Pa-91) was not discovered until 1913 when
Kazimierz Fazans and others identified Pa-91 as a member of the
radioactive decay series. This isotope of 234Pa is short lived (t ½ =
6.7 hrs.) but the most stable 231-Pa was identified in 1916 by Otto
Hahn has half-life of 32,760 years. None of the other elements
occurred naturally and must be synthesized by nuclear reactions.
Jons Jacob Berzelius
Kazimierz Fazans

POSITION IN THE PERIODIC TABLE
It is strange that Yttrium was initially regarded as being the oxide of just a single
element instead of ten eventually discovered, it must be recognized that there was no
guide available as to how many new elements remained to be found.
Moseley recognized that 14 elements between La-57 and Hf-72. By 1907 all these
Lanthanoids had been identified but the radioactive Promethium (Pm-61) conclusive
evidence for which had to wait until 1947, when it was observed that element-61 in the
radioactive decay product of 235-U.
Niels Bohr realized that the 14 elements between Lanthanum and Hafnium reflected
the fact that the fourth atomic primary quantum shell could accommodate 32 electrons,
14 more than the third. The additional electrons are placed in the 4f orbitals, and the
Lanthanide elements were recognized as forming a new family in the periodic table. For
obvious reasons Lanthanides and Actinides are referred to as the 4f and 5f block
elements, respectively. The 5f block series was not realized until the work GT Seaborg
during the second world war. His suggestion took Thorium, Protactinium and Uranium
out of groups 4, 5 and 6 respectively and into their rightful place at the start of a new
family of elements many of which Seaborg was instrumental in synthesizing.
Moseley
Niels Bohr
Seaborg

ELECTRONIC CONFIGURATION OF LANTHANIDES AND ACTINIDES
Element Symbol Atomic number Configuration
Cerium Ce 58 [Xe]4f15d16s2
Praseodymium Pr 59 [Xe]4f36s2
Neodymium Nd 60 [Xe]4f46s2
Promethium Pm 61 [Xe]4f56s2
Samarium Sm 62 [Xe]4f66s2
Europium Eu 63 [Xe]4f76s2
Gadolinium Gd 64 [Xe]4f75d16s2
Terbium Tb 65 [Xe]4f96s2
Dysprosium Dy 66 [Xe]4f106s2
Holmium Ho 67 [Xe]4f116s2
Erbium Er 68 [Xe]4f126s2
Thulium Tm 69 [Xe]4f136s2
Ytterbium Yb 70 [Xe]4f146s2
Lutetium Lu 71 [Xe]4f145d16s2
Lanthanides: La57 to Lu71 1st element: Lanthanum (La57) [Xe]5d16s2
Element Symbol Atomic number Configuration
Thorium Th 90 [Rn]5f16d17s2
Protactinium Pa 91 [Rn]5f26d17s2
Uranium U 92 [Rn]5f36d17s2
Neptunium Np 93 [Rn]5f46d17s2
Plutonium Pu 94 [Rn]5f67s2
Americium Am 95 [Rn]5f77s2
Curium Cm 96 [Rn]5f76d17s2
Berkelium Bk 97 [Rn]5f97s2
Californium Cf 98 [Rn]5f107s2
Einsteinium Es 99 [Rn]5f117s2
Fermium Fm 100 [Rn]5f127s2
Mendelevium Md 101 [Rn]5f137s2
Nobelium No 102 [Rn]5f147s2
Lawrencium Lr 103 [Rn]5f146d17s2
ACTINIDES: Ac89 to Lr103 1st element: Actinium (Ac89) [Rn]
Current Science journal, 2018, 9, 1644
La57 and Ac89
supposed to
be the 1st
element in 5d
and 6d block
element.
Yb70 and
No102 are not
considered as
the last
elements in 4f
and 5f block.

IONIZATION ENERGIES OF LANTHANIDES
IP1 Ln → Ln+ + e
IP2 Ln+ → Ln+2 + e
IP3 Ln+2 → Ln+3 + e
IP4 Ln+3 → Ln+4 + e
[ IP4 >>> IP1 + IP2 + IP3 ]
IP3 is chemically accessible as
the most common oxidation state
of Lanthanides.
(+4) is not possible as no such
chemical oxidizing agent is there.

The chemistry of lanthanides is determined by the +3 Oxidation State. From the ionization data it reveals that in all cases, the
fourth ionization energy (the energy associated with the process Ln+3 → Ln+4 + e) is greater than the sum of the first three
ionization energies. The extra energy required to remove the fourth electron is so great that in most cases it cannot be
recovered through chemical processes. So, the +4-oxidation state is largely inaccessible. The valence electrons of the neutral
lanthanide elements are distributed in the 4f, 5d and 6s orbitals. As electrons are removed from the neutral atoms, all the
orbitals are stabilized. But the 4f, 5d and 6s levels do not experience the same degree of stabilization with the 4f
being stabilized most and the 6s is least. Once 3 electrons have been removed the additional stabilization of the 4f orbitals
is so large that no electrons remain in the latter two orbitals.
Although the +3-oxidation state is by far the most common, five lanthanides also have a tetravalent chemistry. For
Neodymium and Dysprosium this is confined to solid-state for fluoride complexes, while Praseodymium and Terbium also
form the tetrafluoride and dioxide. The most extensive Ln+4 chemistry is that of Cerium for which a variety of tetravalent
compounds and salts are known. The Ce+4 is chemically accessible due to the high energy of 4f orbitals at the start of the
Lanthanide series.
The most striking feature of the above plot is the very high values for Eu and Yb and the very low values for the elements immediately
following them Gd and Lu.
This may be explained by consideration of the electronic configuration of Ln+2 ions that are being ionized to form the corresponding Ln+3.
Yb+2 has electronic configuration [Xe]4f146s2 and Lu+2, however, has an additional electron in the 5d orbital which is less stable than the 4f and
therefore easier to ionize.
The similar phenomenon happens for the Europium and Gadolinium only the exception is the half-filled f-orbital in place of full filled in the
former case.

ACTINIDES ion-
couples
The trend in the A4+/A3+ couple is one of the decreasing stabilities of A4+ as the series is crossed. Thus +4 is practically the only Oxidation State available for Thorium
and perfectly stable for Protactinium, Uranium and Neptunium. Am(IV) and Cm(IV) are found in solution only as fluoride complexes. The drop in A4+/A3+ at Bk97 is
presumably a reflection of its stable half-filled electronic configuration i.e. [Rn]5f7.
nAn+ → nAn+1 + e
E0
nAn+1/nAn+ ; 𝚫G0 = -nFE0
Actinide elements display a much greater range of
Oxidation State than the lanthanides particularly in the early
part of the series. Some of the lighter actinides i.e., Thorium
to Americium resembles the transition metals in the range of
possible oxidation states, while the later actinides are more
likely the lanthanides in favoring trivalency. Unfortunately, it
is not possible to use ionization energy data to rationalize
the above fact as they are not available for all the nAn+. We
can make progress by considering the standard electrode
potential (E0) for the: A4+ + e → A3+ and, A3+ + e → A2+ ,
Couples.
The trend in A3+/A2+ potential is like that in A4+/A3+ with the +2-oxidation level
becoming increasingly stable as the series is crossed. Note that discontinuity in the
plot appears one element earlier in the A4+/A3+ data, Cm(III) has the half-filled 5f
shell ([Rn]5f7) and hence the result.

TREND IN
METALLIC AND
IONIC RADII
Lanthanide and
Actinide Contraction

The variation in the radii of the metallic Ln and Ln+3 are shown in the diagram. Both the series display a
gradual reduction in radius with increasing atomic number whereas Ln+3 decreases uniformly from Ce+3
to Lu+3. There are marked breaks in the metallic radius trend at Eu and Yb. Metallic actinides and the
An+3, An+4 and An+5 show similar plot. As the lanthanides the ionic radii display a gradual reduction with
increasing atomic number while the trend in metallic radius is more complicated.

EXPLANATION
The reduction in Lanthanide metal and Ln+3 radii with increasing atomic number is often referred to as the Lanthanide contraction, and
the corresponding effect in the 5f series as actinide contraction. This contraction arises from the poor ability of f-electrons to screen the
valence electrons from the nuclear charge although the additional unit of nuclear charge on moving one element to the right is exactly
balanced by the opposite charge of an extra f-electron. The poor screening of the nucleus by the additional f-electron means that the
effective nuclear charge experienced by all the valence electrons increases slightly and the atom or iron contracts.
The Lanthanide contraction has important consequences for the chemistry of 3rd row transition metal (5d series). It might be
anticipated that these elements would be larger than their 2nd row counterparts by an amount like the increase in size by moving from
the 1st to the 2nd transition series. However, the reduction in radius caused by the poor screening ability of the 4f electrons means that
the 3rd row transition metals are of approximately the same size as the second-row congeners.
Indeed, the covalent radius of Au is less than that of Ag (1.25 vs 1.33 Å ) thus the Lanthanide contraction is responsible for the many
similarities in the chemistry of 2nd and 3rd row transition metals. The effect of actinide contraction on the size and Chemistry of
transactinide elements may only be made in very small quantities and decay radioactively with very short half-lives.
But calculations suggest that the actinoid contraction may have an even greater effect than the 4f equivalence. For example, the
element 111 lying under Au is calculated to have the smaller radius than Cu.
All the Lanthanide metals except Eu and Yb may be considered to consist of Ln+3 Ions with three electrons per atom devoted to
metallic bonding. Eu and Yb are best regarded as respectively larger than Ln+3 with only two electrons per atom involved in metallic
bonding. This accounts for the significantly greater metallic radii of Eu and Yb with respect to the other lanthanides.

Magnetic
behavior of
Ln+3 and An+3

The Para magnetism of Ln+3 ions arise from their unpaired 4f electrons which interact little with the surrounding ligand in Ln+3
compounds. The magnetic properties of these compounds are therefore like those of the free Ln+3 ions.
For most Ln+3 ions the magnitude of f-orbital, spin-orbit coupling is sufficiently large so that the excited levels are thermally
inaccessible and hence the magnetic behavior is determined entirely by the ground level.
The effective magnetic moment of this level is given by ⎯
; except Eu(III) and Sm(III)
It may be found out that there is good agreement between calculated and experimental values in case of all lanthanides
except for Eu(III) and Sm(III).
The discrepancy for the latter arises because both the ions have excited levels which are sufficiently close to the ground
level to be thermally accessible. If allowance is made for this by assuming Boltzmann population distribution over the energy
levels, then calculated and experimental values will agree.
The magnetic behavior of actinide compounds are more complicated than their Lanthanide counterparts. The equation
[ ] is laser applicable then for the lanthanides and there is a much temperature dependence of
actinide magnetic susceptibility.

Coordination
Chemistry of
Lanthanides

The following points summaries the properties of lanthanides and actinides which are of direct relevance to
understand their coordination chemistry:
● As hard Lewis’s acids, lanthanide and actinide prefer to co-ordinate hard bases such as F- , H2O etc. The
classical coordination chemistry in an aqueous environment is thus very different from that in say
anhydrous hydrocarbons and this has led to development of what is sometimes termed the neo-classical
coordination chemistry.
● To an incoming ligand, lanthanide ions have the appearance of a noble gas atom, except with a positive
charge (most commonly +3). This is because the 4f orbitals which contain the valence electrons do not
extend out enough to interact to any great degree with live and orbitals. The complexes thus formed are
held Together by electrostatic interaction that is ionic bonding.
● The 5f orbitals of actinides are more accessible however leading to some overlap and covalent character.
But this property decreases with increase in atomic number and the latter actinide ions behave like
lanthanides.
Coordination Chemistry of Lanthanides

 Metal Ion having greater coordination number laser is its directional properties.
 Less directional property means more difficult to predict the geometry.
Because of the larger size of Lanthanide ions high coordination number (up to 12) is found. In the
aqueous solution the Ln+3 ions are thought to be surrounded by 8 or 9 O-bound water molecules
for the latter and the earlier metals, respectively.
The lack of covalent bonding extent of directionality means that geometries of Lanthanide
complexes are known with even less precision than the coordination number in solution. However,
many hydrated Lanthanide iron have been explained by X-Ray crystallography in the solution
state.
They generally have tricapped trigonal Prismatic structures. In complexes where geometrical or
optical isomers might be expected, these are extremely difficult to detect or isolate because of
rapid equilibration.
Coordination number and Geometry

Mn+ + H2O → [M(H2O)X]n+
Ann+ + H2O → [An(H2O)X]n+
Ann+ (very good Bronsted acid) → An-O bond gets polarized → O-H bond gets polarized → H+ (acidity)
When a metal cation dissolves in water, it will undergo hydration to some extent, highly charged cations
suggest those formed by the f-elements (particularly actinides) will polarize the -OH bonds in water
strongly and the aquo cations [M(H2O)X] tend to act as Bronsted acid.
[M(H2O)X]n+ → [M(H2O)X-1(OH)](n-1) + H+
The acidity of the solutions of group-III element trications turns decreases rapidly as we move down the
group and that of the trivalent lanthanides increases motility across the series as might be expected from
the reduction in Ionic radius (Lanthanide contraction) and on concomitant increases in charge density of
the ions.
Reaction with water

Bronsted Acidity Series
An+4 > AnO2
+2 > An+3 > AnO2
+
The +4 cations are the most acidic since they have the
highest charge density. For the actinyl ions AnO2
+n , the
question arises as whether the extent of hydrolysis
depends on the net charge of the ion (i.e., +1 or +2) or on
the formal charge of the metal at the center (i.e., +5 or
+6).
In case of AnO2
+2 the O2
-2 ions do not fully quench the
charge on the metal and the effective charge as
experienced by an approaching water molecule has been
calculated to be ~ +3.3. This explains the position of
AnO2
+2 ion between An4+ and An3+ in the above order of
decreasing acidity.

SAMPLE
QUESTIONS
Question: “Addition of NH4SCN to a pale-yellow solution Uranyl nitrate [UO2(NO3)2] intensifies the colour” ---- Explain. [Hint: LMCT]
Question: “Bright yellow solution of Plutonium nitrate [Pu(NO3)4] in strong hydrochloric acid medium changes into dark green.” --- Explain
[Hint: [Pu(NO3)4] in HCl medium forms a polymer like ⎯⎯ Pu ⎯⎯ O ⎯⎯ Pu ⎯⎯ O ⎯⎯ Pu ⎯⎯ and gives MMCT bands]
Question: Highest magnetic moment due to lanthanide ions is ⎯
● 10.65
● 7.94
● 11.4
● 10.61 [Ans (b)]
Question: Explain why magnetic moment of Gd+3 (Z=64) complexes can be obtained by spin only formula but not of Tb+3 (Z=65) complexes.
[UPSC for Geochemist, 2016]
Question: The g-values for Ce+3 (4f1) and Pr+3 (4f2) are respectively ⎯
[CSIR-NET June
2016]
● 3/7 and 2/5
● 5/7 and 4/5
● 6/7 and 3/5
● 6/7 and 4/5 [Ans (d)]

ACKNOWLEDGEMENT
 J. D. Lee
Concise Inorganic Chemistry
Chapman and Hall, London, 1991.
 P. Atkins, T. Overton, J. Rourke, M. Weller and F. Armstrong, Shriver & Atkins
Inorganic Chemistry
4 th Edn, Oxford, 2006.
 F. A. Cotton, G. Wilkinson, C. M. Murillo and M. Bochmann,
Advanced Inorganic Chemistry
6th Edn, John Wiley & Sons, Inc, New York, 1999
 J. E. Huheey, E. A. Keiter, R. L. Keiter and O. K. Medhi Inorgnic
Chemistry: Principles of Structures and Reactivity, 4 th Edn,
Pearson, New Delhi, 2006.
 G. L. Miessler and D. A. Tarr
Inorganic Chemistry
3rd Edn, Pearson, NewDelhi, 2009.
 J.J. Katz, G. T. Seaborg and L. R. Morss (Eds)
The Chemistry of the Actinide Elements, Vols I and II
2nd Edn, Chapman and Hall, London, 1986

Brief Chemistry of Inner-Transition Elements

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