An Introduction to the Chemistry of D-Block Elements

7/30/2019 An Introduction to the Chemistry of D-Block Elements

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Introduction

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Introduction

The position of the d-block elements in the PT is asshown above.

They are called the d-block elements because electronsare being added to the d-subshells.

The elements with proton number 21 (Sc) to 30 (Zn)are known as the first d-block elements.


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Electronic Configuration

The electronic configuration of calcium (proton number20), the element prior to scandium is:1s22s22p63s23p64s2 or [Ar]4s2. Note that, the last twoelectrons of the calcium atom are filled into the 4s

orbital instead of the 3d orbital.This is because, in an isolated atom (without anyelectrons), the 4s sub-shell has a lower energy than the3d sub-shell. Hence, electrons must first fill the 4s

orbital before filling the 3d orbitals.However, once electrons are filled into the 3d orbital,the order is reversed. The 3d orbitals now have lowerenergy than the 4s orbital. Hence, the e.c of Sc (proton

number=21) is [Ar]3d1

4s2

and not [Ar]4s2

3d1


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The e.c for the first transition series (from Sc to Zn) isgiven in the table below.

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Note that the e.c of Cr is not [Ar]3d44s2 as expected.This is because the configuration of [Ar]3d54s1 isenergetically more stable as all the five 3d sub-shellsare completely half-filled.

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Similarly, the e.c of copper is not [Ar]3d94s2 asexpected. This is because the configuration of

[Ar]3d104s1 is energetically more stable as all the five 3dsub-shells are completely filled.

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The e.c of the first transition series can be representedas [Ar]3dx4sy, where x has one of the values one to ten(except 4 and 9) and y is two in all cases except Cr andCu where its value is one.

The e.c of the d-block elements is different from that ofthe Period 3 elements as shown below:

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In the Period 3 elements, each additional electron goesinto the outer sub-shell, while for the d-block elements,each additional electron goes into the inner sub-shell.

The inner sub-shells of the Period 3 elements are all

fully filled, while the outer sub-shells of the d-blockelements are not.


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Transition Elements

Transition element is defined as an element that canform one or more stable ions with an incomplete sub-shell of d-electrons.

Hence, Sc and Zn are not transition elements (even

though they are d-block elements).

The only stable ion of Sc is Sc3+ ([Ar]3d04s0) where the3d orbitals are empty. Zn forms only the Zn2+ ion([Ar]3d104s0) where the 3d orbitals are fully filled.

Hence, the first transition series actually runs from Ti(proton number 22) to Cu (proton number 29).


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Physical Properties

Atomic Radius

The atomic radius of the transition elements is shown inthe table below.

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Physical Properties

Atomic Radius

The d-block elements show little variation in theiratomic radii as compared to the Period 3 elements.

From Sc to Zn, the number of protons in the nucleusand the number of electrons increases. However, eachadditional electron is added to an inner 3d sub-shell.These additional inner electrons are able to shield theouter electrons from the nucleus and to a large extent

cancel out most of the effect of the increase in thenumber of protons in the nucleus.

Hence, the effective nuclear charge increases onlyslightly resulting in only a small change in the atomic

radius.


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Physical Properties

Density

All transition elements have high densities (compared toAl=2.70 gcm-3). This is because of their small atomicsize and the close packing of the atoms in the solid

lattice.

The density increases gradually across the d-blockelements because although the atomic size (and hencethe atomic volume) does not change much, the relative

atomic mass increases gradually.


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Physical Properties

Boiling Point and Melting Point

The variation of the melting point and boiling point isshown in the table below.

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Physical Properties


All transition elements are typical metals (except Zn)with high melting points (in excess of 1000oC) and highboiling point (in excess of 2000oC). This is due to the

presence of strong metallic bonds in the solid lattice.

Due to the small difference in energy between the 3dand 4s sub-shell, the transition elements are able tomake use of the electrons from these two subshells in

the formation of metallic bonds. On the other hand, Ca(a Group 2 metal) is soft with low melting point (850oC)because Ca has only two valence electrons (4s2)available for the formation of the metallic bond.


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Physical Properties


Both the melting point and boiling point drop atmanganese. This is because in manganese, [Ar]3d54s2,the d-subshell is half full, with five unpaired electrons in

each of the d-orbitals. This e.c makes the d-electronsless available to take part in metallic bonding.


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Physical Properties

Ionization Energy

The table and graph below show the variation of thefirst, second, third and fourth ionisation energies of thed-block elements.

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Physical Properties

Ionization Energy

The first, second, third, and fourth ionisation energyrefers to the following processes:

M(g) M+

(g) + e- first ionisation energy

M+(g) M2+

(g) + e- second ionisation energy

M2+(g) M3+

(g) + e- third ionisation energy

M3+

(g) M4+

(g) + e-

fourth ionisation energy


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Physical Properties

Ionization Energy

There is little change in the first and second ionisationenergy from Sc to Zn. This is because the electrons areremoved from the outer 4s orbitals which is shielded

from the nucleus by the increasing screening effect.However, the second ionisation energy of Cr and Cu arehigher than expected because the second electronremoved from these two elements is from an inner 3d

subshell that is closer to the nucleus. Furthermore, theelectrons are removed from a stable configuration of3d5 (for Cr) and 3d10 (for Cu.)


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Physical Properties

Ionization Energy

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Physical Properties

Ionization Energy

The third ionisation energies of Mn and Zn are higherthan expected because the third electron is removedfrom a stable configuration of 3d5 (for Mn) and 3d10 (for

Zn).*


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Physical Properties

Ionization Energy

On the other hand, the third ionisation energy of iron islower than expected because the Fe3+ ion formed has amore stable configuration of 3d5.

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The fourth ionisation energy of iron is also higher thanexpected because the fourth electron is removed from astable 3d5 configuration.

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Physical Properties

Ionization Energy

Generally, the difference between successive ionisationenergies of the transition elements is not large becauseof the close proximity of the 4s and 3d orbitals.


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General Properties of the TransitionElements

Transition elements

-are hard

-have high melting points and boiling points

-have high densities

-exhibit variable oxidation states in their compounds

-form complexes

-form coloured compounds

-show catalytic properties


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Variable Oxidation States

Due to the small difference in energy between the 3dand 4s subshells, atoms of transition elements canmake use of the outer 4s electrons as well as some or

all of the inner 3d electrons for chemical reactions.Hence, they show variable oxidation states.

e.g. the two important oxidation states of iron are +2and +3. Fe: [Ar]3d64s2; Fe2+:[Ar]3d64s0; Fe3+:

[Ar]3d54s0The oxidation states of the transition elements in theircompounds are shown in the table. The more commonoxidation states are circled.


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The table below lists the more stable oxides andchlorides of the transition elements.

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The table below lists the common aqueous cations andoxo-ions of the transition elements.

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The higher the oxidation states do not exist in the formof free aqueous ions. This is because their high chargedensity would polarise the water molecules that are co-

ordinated to them resulting in the formation of oxo-anions.

The common oxidation state for each element includethe +2 or the +3 or both.

The +3 state is relatively more common at thebeginning of the series, while the +2 oxidation state ismore common towards the end of the series.


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The maximum oxidation state of the elementcorresponds to the maximum number of electronsavailable for bonding. It increases from Ti(+4) to

Mn(+7) as electrons from the 4s and 3d subshells areused for reaction.

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The decrease in the maximum oxidation state from Mnto Cu is due to the decrease in the number of unpaired3d electrons. e.g. Ni [Ar]3d84s2 has two 4s electron and

two unpaired 3d electrons available for bond formation.Hence, the maximum oxidation state for Ni is +4.

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-Relative Stability of the +2 and +3 Oxidation State

-Higher Oxidation States of Transition Elements

l f h


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Complex Ions Formation

Complex ions are ions that are formed when a centralatom or ion is bonded to a group of atoms, moleculesor ions (known as ligands) by co-ordinate bonds.

Ligands are species that have at least one lone-pair ofelectrons that are readily donated to a metal atom orion to form a co-ordinate bond. Ligands are Lewisbases. e.g. The oxygen atom in H2O, nitrogen atom in

NH3, carbon atom in CN- and chlorine in Cl- are knownas donor atoms.

G l f h


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All cations have some tendency to form complex ions asthey tend to attract the lone pair electrons ofsurrounding ligands.

However, this tendency is exceptionally strong amongstthe transition metal cations. This is because of theirhigh charge density and the presence of ample emptyorbitals in their valence shells.

In the formation of complex ions, the central metal ionaccepts the lone pair electrons from the ligands. Thecentral metal ions act as Lewis acids. Examples ofcomplex ions are [FeCl4]

2-, [Fe(CN)6]4-, and [CoCl4]

2-.

G l P i f h T i i


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In the formation of the tetrachloroferate(II) ion,[FeCl4]

2-, the Fe2+ ion makes use of the empty 4s and4p orbitals to form co-ordinate bonds with the Cl- ions.

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G l P ti f th T iti


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In the formation of the [Fe(CN)6]4- ion, the six electrons

in the Fe2+ ion first undergo pairing. The Fe2+ ion thenmakes use of the empty 4s and empty 4p orbitals and

two of the empty 3d orbitals to form co-ordinate bondswith six CN- ions:

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G l P ti f th T iti


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The formation of the [CoCl4]2-, which is tetrahedral, can

be visualised and is represented as follows.

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G l P ti f th T iti


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Complex formation is a displacement reaction. e.g. thecopper(II) ion exists in solution as the [Cu(H2O)6]

2+ ion.However, on the addition of excess aqueous ammonia,

a dark blue solution containing the [Cu(H2O)2(NH3)4]2+complex ion is formed. In the reaction, ammonia liganddisplaces the water ligand:

[Cu(H2O)6]2+ + 4NH3 [Cu(H2O)2(NH3)4]

2+ + 4H2O

G l P ti f th T iti


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-Classification of Ligands

-Nomenclature of Complex Ions

-The Geometry of Complex Ions (Stereochemistry ofComplexes)

-Stability of Complexes

G l P ti f th T iti


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Colour of Complexes

Most complexes of transition metals are coloured.

The colour of a complex depends on the nature of thecentral metal ion, the oxidation state of the metal ion,

and the type of ligand.

The colour of some complexes are given in the tablebelow:

G l P ti f th T iti


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Colour of Complexes

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G l P ti f th T iti


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Catalytic Properties

According to the Arrhenius collision theory, for areaction to occur, the reaction particles must collidewith one another.

However, only those collision with energy equal to orgreater than the activation energy will result in theformation of products.

A catalyst increases the rate of reaction by lowering the

activation energy of the reaction so that more collisionwill have sufficient energy to overcome the loweractivation energy.

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The energy profile of a catalysed and an uncatalysedreaction is shown below:

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General Properties of the Transition


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Most transition elements and their compounds areimportant catalyst especially in commercial purposes.Their catalytic property is due to their ability to exhibit

variable oxidation states or have empty orbitals in theirvalence shells.

The table below lists some reactions that are catalysedby transition elements or their compounds.

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Homogeneous Catalysis

In a homogeneous catalysis, the catalyst and thereactants are in the same physical states. For example:

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The suggested mechanism for the oxidation of I- byS2O8

2- is as follows:

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The suggested mechanism requires a lower activationenergy because they involve the collision of oppositelycharged particles, whereas, the uncatalysed routeinvolves the collision of negatively charged particles.



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Heterogeneous Catalysis

In heterogeneous catalysis, the catalyst is in a differentphysical state from the reactants. For example:

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Consider the reaction between hydrogen and iodine.The nickel atoms at the surface of nickel metal makeuse of their empty orbitals to form temporary bonds

with the H2 and I2 molecules. This is called adsorption.

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Heterogeneous Catalysis

This weakens the covalent bonds in the H2 and I2molecules, thus lowering the activation energy for thereaction. Furthermore, the H2 and I2 molecules are

correctly orientated for new bonds to be formed. Afterthat, the HI molecules leave the surface of the catalyst,and other H2 and I2 molecules can be adsorbed.

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Isomerism in Complexes

The existence of two or more different compoundshaving the same molecular formula is called isomerism.

Three types of isomerism occur in transition elementcomplexes:

-Geometrical isomerism

-Optical isomerism

-Structural isomerism


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Geometrical Isomerism

Geometrical isomerism (or cis-trans isomerism) isshown by:

-Square planar complexes with the formula of Ma2b2-Octahedral complexes with the formula of Ma4b2 andMa3b3-Octahedral complexes with the formula of M(x-x)2b2[a and b are monodentate ligands, x-x are bidentateligands]


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Complexes of Ma2b2*


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Complexes of Ma4b2

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Geometrical IsomerismComplexes of M(x-x)2b2

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Geometrical IsomerismComplexes of Ma3b3*


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Optical IsomerismOptical isomerism occurs in octahedral complexes whichdo not have a plane of symmetry. The complex cannotbe divided into two equal halves through any plane.

Optical isomers occur in pairs. One is the mirror imageof the other and they are not superimposable.

Optical isomers also called enantiomers. They areoptically active. One isomer will rotate plane polarised

light in the clockwise direction (the dextro or + isomer),while the other will rotate plane polarised light in theanti-clockwise direction (the laevo or isomer).


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Optical IsomerismComplexes with the formula of M(x-x)3*


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Optical IsomerismComplexes with the formula of M(x-x)2b2*


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Optical IsomerismEDTA complexes-[Ni(EDTA)]4-

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Structural IsomerismStructural isomerism occurs in complexes having thesame molecular formula but are different with respectto the type of ligands that are bonded to the central

ion.An interesting example is a compound of Cr having themolecular formula of CrCl3.6H2O. There are threecompounds having the molecular formula of CrCl3.6H2O.

One is dark green, the other two are light green andpurple respectively.


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Structural IsomerismThey can be differentiated by the number of moles ofsilver chloride precipitated when excess aqueous silvernitrate is added separately to one mole of each of the

compounds. The result of the experiment is shown inthe table below.

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Structural IsomerismSince chlorine atoms that are covalently bonded to thecentral Cr3+ ion will not be precipitated, the isomers aredifferent in terms of the number of chlorine atoms

bonded to the chromium(III) ion. The structuralformulae of the three isomers are given in the table.

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Structural Isomerism


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The Uses of Cr, Co, Mn and Ti

The Uses of TitaniumTi has the same mechanical strength of steel but withtwo added advantages: It is lighter and it does notcorrode. Ti is used in the making of aircraft body, space

capsules and nuclear reactors.Titanium(IV) oxide is used as white pigments in paints.It has the following advantages compared to otherwhite pigments such as lead white PbCO3, Pb(OH)2,

BaSO4 and CaSO4. It is non-toxic, it does not darkenwhen exposed to air containing hydrogen sulphide.

Titanium(IV) oxide is also used as fillers for plasticsand rubber.


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The Uses of ChromiumCr is used to harden steel and to increase its resistanceto corrosion.

Stainless steel is an alloy of steel consisting of 18% Cr,

1% Ni, and 0.4% C.Alloy of Cr with Vanadium and tungsten is used in highspeed cutting tools.


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Uses of Cobalt and ManganeseThe major use of Co and manganese is in alloy making.

Alloy of Co and samarium (Sm) is used to makepermanent magnets.

Alloy of manganese and steel is used to make highspeed cutting tools and railway points.


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Summary

The d block elements have incomplete inner sub-shells.Transition elements are elements that can form at leastone stable ion with an incomplete d-subshells.

Going across the first transition series:

-the atomic radius does not change much

-there is little changes in the first and second ionisationenergies

-there is a gradual increase in the density of the

elements.


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Summary

All transition elements have the following characteristicproperties:

-exhibit variable oxidation states

-formation of complexes

-formation of coloured compounds

-catalytic properties

Going across the series of +2 oxidation state becomesmore stable than the +3 oxidation state.

Generally, lower oxidation states are reducing agents,while the higher oxidation states are oxidising agents.


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Summary

Complex ions:-The bonds between the ligand and the centralion/atom are co-ordinate bonds.

-The general shape of the complex ion depends on its

co-ordination number.Co-ordination number: 2; General shape: Linear

Co-ordination number: 4; General shape: Tetrahedronor square planar

Co-ordination number: 6; Octahedron

-They exhibit structural, geometrical (cis, trans, fac andmer) and optical isomerism


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Summary

Complex ions:-Splitting of the 3d orbitals by ligands is responsible forthe colour of complex ions.

In homogeneous catalysis, the catalyst and the

reactants are in the same physical state.In heterogeneous catalysis, the catalyst and thereactants are in different physical state.

Documents

An Introduction to the Chemistry of D-Block Elements