Transition metals are hard having high melting point (melting point of Fe = 1535°C) and boiling point (boiling point of Fe = 2800°C).
Reason of High melting and boiling points
1. The strength of bonding between the atoms of metals depends upon the interaction between the valence electrons. 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.
2. 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.
3. 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.
Reason of low melting and boiling points of elements of group IIB
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 low melting point and boiling point of Zn, Cd and Hg 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.
Periodic Trend of melting and boiling points of transition elements
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.
2. Interstitial or Non-Stoichiometric Compound
Formation
VH0.56, TiH1.73, CeH2.69, LaH2.76, CuH22.3.
3. Variable
Oxidation States
General Considerations
Positive Oxidation State
Transition
elements are electropositive, so they
show only positive oxidation states. Transition
elements display 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 due to small energy difference of ns and
(n – 1) d-orbitals.
For example;
Mn shows oxidation states from +3 in [Mn(CO)(NO2)3]
through zero in [Mn2(CO)10] to +7 in [MnO4]1‒.
Common Oxidation State
All
transition metals exhibit two or more oxidation states. The
oxidation numbers of +2 and +3 are more common.
One Oxidation State
Highest Oxidation States
In 3d series, highest oxidation is +7 (Mn). In other d-block transition series, highest oxidation state is +8 (Os, Ru).
Their higher
oxidation states are more stable in their fluorides and oxides. Higher
oxidation states in oxides are normally more stable than fluorides due to
capability of oxygen to form multiple bonds.
Zero Oxidation State
In carbonyl compounds, oxidation state of metal is zero due to synergic effects. The common examples are [Ni(CO)4] and [Fe(CO)5] in which nickel and iron are in zero oxidation state.
Reason of
Variable oxidation states
1.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 bond formation giving rise to their variety of variable oxidation states.
2. Along with ns electrons, one, two or more (n–1) d-electrons may be removed or shared. The (n–1) d-electrons are easily lost as ns electrons.
For example;
(i) 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.
(ii) 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
Maximum oxidation state is equal to the sum of the number of ns electrons and number of unpaired (n–1)d-electrons.
For example
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.
Lower and higher Oxidation
states
1. 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.
2.In their lower
oxidation states, they form ionic compounds and basic oxides but in
their higher oxidation
states they form covalent compounds and acidic oxides.
3. Transition elements
show highest oxidation states when they combine with most electronegative elements F or
O.
4. In higher oxidation states above +4, they form large oxoanions which are covalent e.g. CrO4−, MnO4− etc.
5. Their maximum or
highest 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). In the highest oxidation states
of first five elements, all s and d-electrons are used for
bonding.
6. 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.
7. 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.
Trend of oxidation states
1. The maximum oxidation states increase in each transition
series up to the middle of series and then decreases
afterward.
2. It is because the number of unpaired electrons increases up to the middle of the series and then decreases.
3. 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.
4. Catalytic
Property
Catalyst
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.
For example,
Ni is used
for the hydrogenation of vegetable oils, Fe in the manufacture of ammonia, V2O5
in the conversion of SO2 into SO3 etc.
Examples
of Some industrial Catalysts
Some
industrial reactions which are catalyzed by transition metals and their
compounds are:
1. Finely divided Ni and Pd or Raney
Ni are used in hydrogenation of unsaturated oils.
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.
Reason of
catalytic property
Their catalytic properties of transition metals are mainly due to following reasons:
1. The presence of vacant d-orbitals
providing a suitable surface areas where
gaseous reactants are adsorbed
bringing them closer for the reaction.
2. The tendency to exhibit variable
oxidation states.
3. The ability to form high
energy reaction intermediates with reactants.
4. The presence of void
spaces (defects) in their crystal lattice.
[The availability
of vacant orbitals, allowing reactants (which are gases) to be adsorbed 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].
5. Magnetic
Property or Paramagnetism
The
substances (elements or compounds) which are weakly attracted by
magnetic field are called paramagnetic substances and the
phenomenon is called paramagnetism. Most of the transition elements and
their compounds (Mn2+, Fe2+ etc.) exhibit Paramagnetism
and are paramagnetic due to presence of unpaired electrons. e.g. Cr3+,
Sc2+, K3[Fe(CN)6] etc.
Reason of
Paramagnetism
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.
transition elements and many of
their compounds have unpaired electrons in d-orbitals, they are attracted by
magnetic field and so they are paramagnetic.
Magnetic
moment
Magnetic moment of unpaired electron is due to spin and orbital angular momentum. The magnetic moment (μs) is related to the number of unpaired
electrons (n) by the following spin-only formula:
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.
Reason of
Diamagnetism
The substances
which are slightly repelled by magnetic field are called diamagnetic
substances and the phenomenon is called diamagnetism. e.g. Zn,
Cd, Hg, Ti4+, V5+, Sc3+ etc.
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.
d-block elements and ions having d0 and d10
configuration are 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.
Ferromagnetism
and ferromagnetic materials
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 and the phenomenon is called ferromagnetism.
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.
Anti-ferromagnetism and anti-ferromagnetic substances
If the alignment of the magnetic dipoles is compensatory so as to give zero net moment, the material is termed as anti-ferromagnetic and the
phenomenon is termed as anti-ferromagnetism.
e.g.
CoO, NiO, MnO, MnO2, Co3O4,
Fe2O3, Cr2O3, V2O3
etc are the examples of anti-ferromagnetic.
6. Colour
Formation in Compounds
Chemistry
of Colour Formation in Compounds
1. Colour in compounds or ions is produced
due to absorption and emission of radiation in the visible region of spectrum (4000oA to 7000oA).
2. Colour in general, is associated with the incomplete electron sub-shell and the ability of the compound or ion to promote an electron from one energy level to another by absorption of light of a particular wavelength.
3. Colour in transition metals ions is associated with d-d transition of
unpaired electrons from “t2g” to “eg” set of
energy levels and this is achieved by absorption of light in the visible spectrum, rest of the transmitted light is no longer white i.e. it is coloured.
4. Metal ions that contain partially filled d-subshell usually form colored complex ions
5. metal ions with empty d-subshell or d0
configuration (Sc3+,Ti4+,etc.) or with
filled d-subshells or d10 configuration (Zn2+,
Cd2+, etc.) usually form colorless complexes. No d-d
transition occurs if d-orbitals
are empty or fully filled and therefore, such ions are colourless.
Summary
of Colour Formation
1. d-d splitting
2. d-d- transition
3. Excitation of electron by absorbing
light
4. De-excitation of electron by
transmitting light giving colour
Exceptions
(i) AgBr, Agl, have fully filled d-orbitals but are coloured due to transference of electron cloud from Br– or I– to Ag+ (d10) when white light is incident on AgBr/Agl. During this process also characteristic wave length of visible light is absorbed.
(ii) Similarly MnO4– (purple), CrO42– (yellow) and Cr2O72– (orange) are coloured due to charge transfer from oxide ions to the central metal ions although they have no d-electrons.
(iii) Permanganate (Mn+7)
has purple colour due to ligand-metal charge transfer (LMCT). Mn+7
has no unpaired electron in d orbital but its ligand i.e. oxygen gives it
electron from p-orbital due to which it gets unpaired electrons in d-orbital.
This charge transfer takes place when photons from visible spectrum is absorbed
leading to purple color.
Reason of
Colour of Transition Metal Compounds on Molecular Level
1. Colour in transition metal compound is
associated with partially filled (n–1)d sub-shell i.e. colour is attributed by the unpaired
d-electrons.
2. In an isolated atom or ion of a
transition metal, all the five d-orbitals are degenerate having same energy.
3. On complex formation under the
influence of combining ligands, in a
complex ion the five d-orbitals are
split up into two levels of different energies namely lower energy trio
called t2g and higher energy
pair called eg and this splitting is termed as crystal field splitting (Crystal Field Theory).
4. [The difference or
energy gap between the two energy levels corresponds to the visible region of
light; wavelength = 380-760 nm and depends upon the nature of the combining
ligands].
5.
In transition metal
compounds, electrons from the lower energy d-orbital are promoted to a higher
energy d-orbital within the d-sub-shell by absorption of light of
characteristic wavelength in the visible region and this electronic transition
or shift is called d-d transition. The rest of the light transmitted is
no longer white i.e. it appears coloured, so the complementary colour of the
absorbed light is transmitted and the ion shows colour.
6. The colour of the
transition metal ions is due to the electronic transitions or shifts between the available d-orbitals. The colour
of the ion is complementary to the colour absorbed.
e.g.
(i).Cu2+ ions (3d9) or [Cu(H2O)6]2+ ions absorb red light from the visible range for promotion of 3d electrons reflecting excess of blue light and appear deep blue (which is complementary colour to red).
(ii). Hydrated Co2+ ions absorb radiation in the blue-green region, and therefore, appear red in sunlight.
(iii). [Ti(H2O)6]3+ ion is purple as
it absorbs green light and transmits purple colour.
7. The colour of an
ion may be affected by altering the environments of the ions. For example;
hydrated copper sulphate (CuSO4.5H2O) is blue while
anhydrous form is white.
8. The colour of
co-ordination compounds or complexes also depends upon the nature of ligands as
different ligands affect the energy levels of the d-orbitals. For example:
9. A same transition
metal ion has different colour in different oxidation states. This is because
different oxidation states of the same transition metal ions possess different
number of electrons and thus different amount of energy would be absorbed and
emitted in electronic transitions with consequent manifestation of different
colours of ions.
e.g.
Cr3+
ion is deep green while Cr2+ ion is blue.
Colours
of Hydrated Ions of First Transition Series
7. Complex Formation
Coordination
Compound or Complex Compound or Transition Metal Complexes
Coordination compounds are neutral substances (i.e. uncharged) in which at least one ion is present as a complex.
The
compounds containing complex molecules or complex ions and counter
ions (anions or cations as needed to produce a neutral compound) capable
of independent existence in which the central metal
atom or ion is surrounded by a definite number of oppositely
charged ions (Cl−, CN−, OH− etc.) or neutral
molecules (NH3, H2O, CO, NO, -en etc) or positively
charged ions called Ligands (electron pair donor) which donate
their lone pairs of electrons to central metal cation or atom
(center of co-ordination) to form Metal-Ligand (M←L) co-ordinate
bond are called complex compound or co-ordination compounds or
transition metal complexes.
e.g. [Co(NH3)5Cl]Cl2
[Fe(en)2(NO2)2]SO4 K3[Fe(CN)6]
OR
A compound
in which a metal atom or ion mostly transition metal (central
atom) is surrounded or coordinated
by a definite number of oppositely charged ions (Cl−,
CN−, OH− etc) or neutral molecules (NH3,
H2O, CO, NO, -en etc) or positively charged ions called Ligands (electron
pair donor) which donate their lone
pairs of electrons to central metal cation or atom (center of co- ordination) to form Metal-Ligand
(M←L) co-ordinate bond (along with some counter ion) and which retains its identity in the
solid as well as in solution is called a complex compound or co-ordination compounds or transition metal complexes.
Complex compounds are the addition compounds which retain their identity even in the solution having properties entirely different from those of the constituent simple ions and are capable of independent existence. The solution of complex compound does not give test of individual ions present in it.
Reason of calling co-ordination
compounds
Since metal-ligand bond is co-ordinate, such compounds are termed as co-ordination compounds.
Formation
of Complex Compound
The
formation of a complex compound is simply a Lewis acid-base addition
reaction in which transition metal atom or cation accept an
electron pair thereby acting as Lewis acid while the anion or
electron-rich neutral molecules donate lone pair of electrons
thereby acting as Lewis base or ligand.
Reason of
Complex Formation
All transition metals exhibit a characteristic property of complex compound formation. The complex formation tendency of the transition metals is attributed by two reasons;
(i) firstly transition metal cations due to their small size and highly effective nuclear charge with high positive charge density which makes it easy for their cations to accept the lone pair of electrons from the ligands.
(ii) Secondly, availability
of vacant inner (n–1)d-orbitals of appropriate energy in their
cations facilitates
the acceptance of lone pair of electrons from ligands.
Parts of Complex
1. Central metal Atom or ion (consisted of transition metal surrounded
by a number of ligands)
2. Ligands or electron pair donor (which is negatively charged, positively
charged or neutral)
3. Simple cation or anion(Counter ion)
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