General Characteristics of d-block Elements
1. Melting and Boiling Point
Transition metals are hard having high melting
point (melting point of Fe = 1535°C) and boiling point (boiling point of Fe = 2800°C).
The hardness, high melting point and high boiling point are attributed to their
small atomic size producing strong interatomic attraction and presence of greater
number of valence electrons available for interaction. (The strength of bonding
between the atoms of metals depends upon the interaction between the electrons
in their outermost shell). Greater the number of valence electrons available
for interaction, the stronger will be the resulting bonding and higher will be melting
point and boiling point Transition metals have 3 or more electrons available
for bonding. Hence the interatomic bonding is very strong. That is why their melting
point and boiling point are high.
Zinc (m.p. = 419°C and b.p. = 907°C), Cd
(m.p. = 320.9°C and b.p. = 765°C) and Hg (m.p. = - 38.4°C and b.p. = 357°C) are
notable exceptions and have exceptionally and relatively low values of melting
point and boiling points.
The reason of their low melting point and
boiling point is due to their filled (n – 1) d-orbitals with no unpaired
electrons [ns2 (n–1)d10]. Due to completely filled
d-orbital, d-orbitals, d-electrons are not available for interatomic
bonding. Hence their melting point and boiling
point are comparatively low.
There
is a regular increase in melting and boiling points across each transition
series along each period (except in Mn, Ni, Cu which have usually weaker IBF)
due to decrease in atomic radii and increase of number of valence electrons.
[In first transition series, vanadium has the highest melting point while
scandium has the highest boiling point]. The melting point and boiling point
increase along a each transition series and reach a maximum in the middle of
the series (near group VB and VIB) and then decreases beyond VIB. In moving
across a transition series, the number of unpaired d-electrons rises to maximum
of five in group VIB beyond which electron pairing begins.
The melting
and boiling point of d-block elements increase on descending a group due to
increasing atomic mass.
2. Interstitial
Compound Formation
Transition elements form compounds of
indefinite structures and properties and variable composition which are called Interstitial
compounds which are non-stoichiometric compounds formed by the adsorption (penetration)
of small, non-metallic atoms like H, B, C, N into the interstices of lattice of
transition metals.
These compounds are non-stoichiometric as atoms
are not present in simple ratios and have variable composition. So they are not
true compounds because they are not in accordance with laws of chemical
combination. For the same reason, they cannot be expressed by simple chemical
formula. e.g. TiH1.73, VH0.56, LaH2.76, CeH2.69,
CuH22.3.
This non-stoichiometry (i.e. non-stoichiometric
compound formation) is due to their variable oxidation states and due to their
defects in their lattices. In their solid crystal lattice, there are holes (i.e.
inter-atomic spaces or interstices or void spaces) where small atoms like C, H,
B, N etc are adsorbed to form their hydrides, carbides, borides and nitrides respectively
giving a solid solution.
The
non-stoichiometry of compounds of transition metals with O, S, Se and Te is
very prominent. e.g. in ferrous oxide (FeO) ratio of Fe and O is not 1:1 but
the composition varies between Fe0.94O and Fe0.84O
showing non-stoichiometric nature of compound. Vanadium selenide is in another
non-stoichiometric compound whose composition varies between VS0.98 and
VS2.
3. Variable Oxidation States
Transition elements exhibit a variety of
oxidation states (except Zn, Cd, Sc, Y etc.) from 0 to +8 (found in ruthenium
and osmium e.g RuO4, OsO4 etc) in their compounds. The lower oxidation states
are generally obtained by the loss of ns electrons while higher oxidation
states are achieved by sharing ns and all or some of the (n – 1 )d-electrons.
In their lower oxidation states, they form ionic compounds but in their higher
oxidation states they form covalent compounds. Their maximum oxidation states
are largely confined to +2 to +3 but it may exceed to +3 which are only rare.
(e.g. Fe shows +4, +5, +6 and Ru and Os show +8 oxidation states). The
different pattern in oxidation states of Fe, Ru and Os is attributed mainly to
increased size facilitating the loss of more than one 3d electrons).
The exhibition of variable oxidation states
is attributed mainly due to small energy difference between ns and (n –
1)d-orbitals and partly due to partially filled (n – 1)d-orbital. Thus both ns
and (n – 1)d-electrons participate in compound formation giving rise to their
variable oxidation states. Along with ns electrons, one, two or more (n – 1)
d-electrons may be removed or shared. For example; titanium (Z = 22) shows an
oxidation state of +2 if both 4s electrons are used for bonding and show
additional oxidation states of +3 and +4 when one and two 3d-electrons are also
removed respectively in addition to 4s electrons. Similarly, in Vanadium (Z =
23) if only 4s electrons are removed, it will show oxidation state +2. If in
addition, 1, 2 or 3, 3-d-electrons are also removed, it will show additional
oxidation states of +3, +4 and +5 respectively.
Maximum oxidation state is equal to the sum
of the number of ns electrons and number of unpaired (n – 1)d-electrons. For
example:
 | |||
The maximum oxidation state in first
transition series increases from Sc (+3) to Mn (+7) then decreases from Fe (+6)
to Zn (+2) as d-electrons pair up.
Maximum oxidation state is not stable.
Therefore, most common oxidation states are +2 and +3. More commonly, only one
electron may be removed from 3d. Thus cobalt does not show +5 oxidation state.
The stability of lower oxidation state (+2)
relative to higher oxidation state (+3 or even higher) increases from left to
right across the series with increasing atomic number (nuclear charge). It
reflects the increasing difficulty of removing a (n – 1) d-electrons as the
nuclear charge increases.
The stability of higher oxidation state (+3
or even higher) relative to lower oxidation state (+2) increases from top to
bottom down the group with increasing atomic number (nuclear charge). It
reflects the increasing readiness of removing a (n – 1) d-electrons as the
nuclear charge decreases due to increasing atomic size.
4. Catalytic Property
Catalysts are the chemical substances that
accelerate or retard the rate of reaction either by forming unstable
intermediate compound or by change of oxidation state. Many transition elements
(V, Fe, Co, Ni, Mo, Pd, W, Pt etc) and a variety of transition metal compounds
especially their oxides (V2O5, MnO2, Fe2O3,
ZnO, Cr2O3, FeSO4 etc.) exhibit remarkable
catalytic properties and are wonderful and effective catalysts for
hydrogenation, oxidation, dehydration etc. Some industrial reactions which are
catalyzed by transition metals and their compounds are:
1. Finely
divided Ni or Pd are used in hydrogenation of unsaturated oils.(Sabatier-Senderen’s
Reaction)
2. Finely
divided Fe and Mo are used in manufacture of ammonia (Haber’s process)
3. Pt or
Pt-Rh alloy is used for oxidation of ammonia into NO (Ostwald’s Process).
4. Pt or
V2O5 is used for oxidation of SO2 into SO3
in the manufacture of H2SO4. (Contact Process)
5. ZnO-Cr2O3
(Zinc chromite) is used in the synthesis of methanol (Petret Process)
6. Ziegler-Natta
catalyst (TiCl4.AlR3)) is used in polymerization of
olefins.
7. Fenten’s
reagent (FeSO4 + H2O2) is used for oxidation
of alcohols to aldehydes.
Their catalytic properties of transition
metals are mainly due to following reasons:
1. The
presence of vacant d-orbitals.
2. The
tendency to exhibit variable oxidation states.
3. The
ability to form high energy reaction intermediates with reactants.
4. The
presence of defects (void spaces) in their crystal lattice.
[The availability of vacant orbitals, allowing reactants
(which are gases) to be absorbed on their surfaces and bringing them closer for
the reaction e.g. Ni is used in hydrogenation of vegetable oils into vegetable
ghee. In addition they have ability to form Activated Complex of lower
activation energy thereby providing low energy pathway for the reaction by
either forming unstable Intermediate Compound or by change of oxidation states.
e.g. V2O5 acts as catalyst in oxidation of SO2
into SO3. Here vanadium undergoes oxidation state change from +5 to
+4].
V2O5
|
+
|
SO2
|
¾¾®
|
V2O4
|
+
|
SO3
|
|
2V2O4
|
+
|
O2
|
¾¾®
|
2V2O5
|
5. Magnetic Property of transition elements
Most of the transition elements and their
compounds exhibit Paramagnetism and are paramagnetic. The substances which are
weakly attracted by magnetic field are called paramagnetic substances. Paramagnetism is due to the presence of
unpaired electrons in an atom, ion or molecule, because there is a magnetic
moment associates with spinning electron. The spin and revolution of an
electron creates a magnetic field around it and it behaves as a small magnet,
and hence is attracted by another magnets. Since transition elements have
unpaired electrons in d-orbitals, they are attracted by magnetic field and so they
are paramagnetic.
The magnetic moment (μs) increases
with an increase in the number of unpaired electrons. Since number of unpaired
electrons first increases from 1-5 (till VIIB) and then decreases gradually and
finally become zero from left to right, therefore, Paramagnetism first
increases in any transition elements series around the middle of the series and
then decreases. Thus paramagnetic behaviour is the strongest for Fe3+
and Mn2+ both containing 5 unpaired electrons each and decreases on
both sides of the first transition series.
The substances which are weakly repelled by
magnetic field are called diamagnetic substances. The diamagnetism is due to
the presence of paired electrons. When the electrons are paired in an orbital,
then magnetic moments of all paired electrons cancelled out and the substance
becomes diamagnetic. Zinc and Cadmium are have d10 configuration
containing all the paired electrons, so they are not attracted in a magnetic
field and are diamagnetic.
Certain transition elements like Iron, Cobalt
and Nickel and their compounds such as CrO2 which can be magnetized
showing strong Paramagnetism and are strongly attracted in magnetic field are
called ferromagnetic materials. They show permanent magnetism even when the
magnetic field is removed. [CrO2 is ferromagnetic material used in
the manufacture of audio and video tapes]. Ferromagnetism is due to a spontaneous
alignment of the magnetic dipoles in the same direction. However, for each
ferromagnetic material there is a critical temperature above which no
ferromagnetism is observed. This temperature is called the Curie temperature.
6. Complex Formation
Transition metals form complexes or
co-ordination compounds in which metal atom or ion (central atom) is surrounded
by a number of oppositely charged ions or neutral molecules called
Ligands. Ligands donate its lone pairs
to central metal atom (centre of coordination) and are attached to the metal
atom by co-ordinate bond, hence the name co-ordination compounds.
Transition metals form complexes due to 3
reasons:
1. Small size of
their atoms.
2. Highly
effective nuclear charge of their cations.
3. Presence of
vacant d-orbitals available for chemical bonding.
Parts of Complex
1. Central Atom
It is an atom or ion
of transition metal bounded by ligand through co-ordinate bond.
2. Ligands
Groups surrounding the central metal atom by
donating its lone pair of electrons are called ligands. They are Lewis
Base. They may be negative, neutral or
positive.
3. Co-ordination Number
The no. of ligands
bounded or attached to the central atom are called co-ordination number.
4. Complex Ion
Group comprising of
central atom and its ligands carrying +ive or –ive charge is called Complex
Ion.
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