CATALYTIC ACTIVITY

Some reactions catalysed by transition elements/their compounds.

process reaction Catalyst

Contact process 2SO2(g)+O2(g)2SO3 V2O5

Haber process N2(g)+3H2(g)2NH3(g) Fe(s)

Hydrogenation CH2=CH2(g)+H2(g)CH3CH3 Ni(s)

Thermal decomposition 2KCLO3(s)2KCL(s)+3O2(g) MnO2(S)

Redox 2I-(aq)+S2O82-(aq)I2(aq)+2SO4

2-(aq) Fe3+(aq)

Homogenous Catalysis

1. In homogenous catalysis, the catalyst and the reactants are in the same physical states.

2. Homogenous catalysis is usually explained by the intermediate product theory.

3. Using the oxidation of iodide ions by peroxodisulphate ions as an example.

S2O82 (aq) + 2I- (aq) ---------------------> 2SO4

2-(aq) + I2(aq)

a) The activation energy for an catalysed reaction is high because it involves the direct collision

of negative charge ions,which tends to repel each each other.

b) However ,in the presence of Fe3+ (aq) as catalyst, the activation energy is lower because it

involves the collision of opposite charged particles.

4. Transition elements and their compounds are good homogenous catalysts because they exhibit

variable oxidation states.

Heterogeneous catalysis

1. In the heterogeneous catalysis, the physical state of the catalyst is different from those of the

reactants.

2. Examples of heterogeneous catalysis are:

Reaction Catalyst

N2(g) + 3H2 2NH3(g)

2SO2(g) + O2 (g) 2SO3(g)

H2(g) + I2 (g) 2HI (g)

CH2 = CH2(g) + H2 (g) CH3CH2(g)

CH3CH2OH(g) CH2 = CH2 (g) + H2O(g)

Fe(s)

V2O5(s)

Ni (s)

Ni (s)

AL2O3(s)

Fe3+

3. Heterogeneous catalysis is usually explained by the adsorption theory.

4. the d-block metals have partly full d orbitals which can be used to form bonds with adsorbed

reactants. Thus they can be effective heterogeneous catalysts.

5. Consider the reaction between hydrogen and iodine.

H2 (g) + I2 (g) 2HI (g)

The nickel atoms at the surface of nickel metal make use of their empty orbitals to form temporary

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

This weakens the covalent bonds in H2 and I2 molecules, thus lowering the activation energy for the

reaction. Furthermore, the H2 and I2 molecules are correctly orientated for new bonds to be formed.

After that, the HI molecules leave the surface of the catalyst, and other H2 and I2 molecules can be

adsorbed.

6. Examples of d-block elements and compounds used as heterogeneous catalysts are shown in table

below:

process Product Catalyst

Haber

Contact

Hydrogenation of oils

Ostwald

Oxidation of propan-2-ol

Ammonia

Sulphuric acid

Margarine

Nitric acid

propanone

Iron

Vanadium (v) oxide

Nickel

Platinum

copper

MAGNETIC PROPERTIES

1. As electric current flows through a wire , magnetic moment is generated. Similarly , the electrons spin

on their axes and are regarded to generate magnetic moment. The electrons occupying the same orbital

have zero magnetic moment as the opposite spins of the two electrons counter the magnetic moment

2. Substances which are weakly repelled by the strong magnetic field are termed as diamagnetic while

those which are weakly attracted by a strong magnetic field are termed as paramagnetic.

3. On the basis of magnetic properties,substances are classified into the following 2 types:

a) Paramagnetic substances: substances which are weakly attracted by the magnetic field

are termed as paramagnetic. These substances lose their magnetism on removing the

magnetic field. Paramagnetism is produced by the presence of unpaired electrons and

most of the transition metal atoms are having unpaired d-electrons, they are

paramagnetic in behavior.

Ni (s)

b) Diamagnetic substances: substances which are repelled by magnetic field are termed as

diamagnetic. It is the property of the completely filled electronic subshells. As all

elements (except hydrogen atom) are having filled electronic shells, some diamagnetism

is shown by all substances . a simple example of a diamagnetic substance is sodium

chloride.

ISOMERISM IN COMPLEXES

1. Three types of isomerism occur in transition element complexes :

Geometrical isomerism

Optical isomerism

Structural isomerism

Geometrical isomerism

1. Geometrical isomerism ( or cis-trans isomerism ) is shown by

a) Square planar complexes with the formula Ma2b2

b) Octahedral complexes with the formula of Ma4b2 and Ma3b3

c) Octahedral complexes with the formula of M(x—x)2b2

[a and b are monodentate ligands , and (x—x) are bidentate ligands]

2.i) Complexes of Ma2b2

cis isomer trans isomer

ii) Example: Diamminedichloroplatinum (II), [Pt(NH)3]2Cl2]

cis isomer trans isomer

3.i) Complexes of Ma2b

Cis isomer trans isomer

a

NH3

b bM M

a

ab

b

Cl

NH3

Cl NH3

Cl NH3

ClPtPt

b

a aM M

a

aa

ab a

b

b

a

a

ii) Example: tetraamminedichlrochromium (III),[Cr(NH3)4Cl2]

Cis isomer trans isomer

4.i) Complexes of M(x—x)2b2

Cis isomer trans isomer

ii) Example: Bis-ethane-1,2 diamminedichlorochromium (III), [Cr(NH2CH2CH2NH2)2Cl2]+

5.i) Complexes of Ma3b3

fac-isomer mer-isomer

Cl

NH3

Cl

NH3H3N

H3N NH3

NH3

CrCr

NH3

NH3

Cl

Cl

b xM M

xb

bx

x

x

b

xxx

Cl

Cl

NH2

NH2NH2

H2N NH2

NH2

CrCr

NH2

NH2

CH2

CH2

CH2

CH2

CH2

CH2 CH2

CH2

Cl

+ +

trans isomerCis isomer

M

a

a

a

bb

bM

a

a

a

bb

b

In the fac-isomer, the three identical groups ( a or b ) are in the same face of the octahedron. In the

mer-isomer, the three identical groups ( a or b ) are not found in the same face of octahedron.

ii) Example: Triamminetrichlorochromium (III) , [CrCl3(NH3)3]

Fac-isomer mer-isomer

Optical Isomerism

1. Optical isomerism occurs in octahedral complexes which do not have a plane of symmetry. The

complex cannot be divided into two equal halves through any plane.

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

3. Optical isomers are also called enantiomers. They are optically active. One isomer will rotate plane

polarized light in the clockwise direction ( the dextro or + isomer ), while the other will rotate plane

polarized light in the anti-clockwise direction ( the laevo or – isomer )

4.i) Complexes with the formula of M (x—x )3

Cr

Cl

Cl

NH3H3N

H3N

Cl

Cr

Cl

Cl

Cl

NH3

H3N

H3N

MM

XXX

X X

XX

X

XXX

X

ii) Example : Tris-ethanedioatecobaltate (III)

5.i) Complexes with the formula of M(x—x)2b2

ii) Example: Bis-ethane-1,2-diamminedichlorochromium (III),

6. EDTA complexes,

Co

o

c

o

o

o

oo

o

oo

o

o

o

c

cc

c

c

c

c

c

c Co

o oo o

o

o

oo

oo

o

o

MM

bbX

X X

bb

X

XX

XX

Cr

Cl

NH2

H2N

H2N

H2N

Cl

CH2

CH2

CH2

H2C Cr

H2N NH2

Cl

NH2

NH2ClH2C

CH2

CH2

+

CH2

+

c c

For example, [Ni(EDTA)]4-

Structural isomerism

1. Structural isomerism occurs in complexes having the same molecular formula but are different with

respect to the type of ligands that are bonded to the central ion.

2. An interesting example is a compound of chromium having the molecular formula of CrCl3 ●6H2O.

3. There are three compounds having the molecular formula of CrCl3 ●6H2O. One is dark grenn, the other

two are light green and purple respectively.

4. They can be differentiated by the number of moles of silver chloride precipitated when excess

aqueous silver nitrate is added separately to one mole of each of the compounds.

The result of the experiment is shown in the table below.

Isomer Colour Number of moles of AgCl

precipitated

I Dark green 1

II Light green 2

III Purple 3

5. Since chlorine atoms that are covalently bonded to the central Cr 3+ ion will not be precipitated, the

isomers are different in terms of the number of chlorine atoms bonded to the chromium (III) ion. The

structural formulae of the three isomers are given in the table in the next page.

Ni

C

O

N

O

O

O

N

O

O

OCCH2

C

C

CCH2 Ni

N N

OO

O

O

OO

O

O

CCH2

CH2

CH2

O

CH2CH2

4- 4-

CH2

CO

CH2

CH2

CH2 CH2

Isomer Formula Structure of complex ion

I [Cr(H2O)4Cl2]+ ● Cl -

●2H2O

II [Cr(H2O)5Cl]2+

●2Cl -●H2O

III [Cr(H2O)6]3+● 3Cl -]

STABILITY OF COMPLEXES

1. Complexes ions have different stabilities depending on the types of ligands.

ClCl

Cr

OH2

H2O

OH2H2O

OH2

Cr

OH2

H2O

OH2H2O

Cl

OH2

Cr

OH2

H2O

OH2H2O

OH2

2. If the ligands are easily displace, the complex is not stable. On the other hand, ligands that are

stubborn and hard to be displace give rise to strong and stable complexes.

3. When concentrated hydrochloric acid is added to aqueous copper(II), the solution changes from blue

to yellow. From the colour change, it is due that the chloride ions is a stronger ligand than water and an

exchange of ligands has occurred.

[Cu(H2O)6]2+ (aq) + 4Cl- (aq) [CuCl4]2- (aq) + 6H2O (l)

4. The equilibrium constant of the complex ion in solution is called the stability constant. All complexes

dissociate partially in solution according to the equation:

M(aq) + nL(aq) MLn(aq) where M = Transition metal ion

L = Ligand

n = Number of ligands

The equilibrium constant, K, of the reaction at constant temperature is given as:

K is the stability constant used to measure the stability of the complex ion with respect to its constituent

species. The larger the value of K, the higher the concentration of MLn in solution and thus the more

stable the complex.

5. Stability constant have a wide range of values. As such, thet are better expressed as their logarithm,

log10 K or simply lg K.

6. The table below shows the stability constant of some complex ions.

Complex ion Stability constant (K) lg K

[Fe(Cn)6]3- 1 x 1042 42

[Fe(Cn)6]4- 1 x 1037 37

[Co(NH3)6]3+ 4.5 x 1033 33.7

[AlF6]3- 6.7 x 1019 19.8

[Cu(NH3)4]2+ 1.4 x 1013 13.0

[CuCl4]2- 3.98 x 105 5.6

7. The stability of a complex depends on the following factors.

a) the oxidation state of the central metal ion

The higher the charge density of the central ion, the more stable is the complex.

eg : Iron (II) in [Fe(Cn)6]4- lg K = 24

Iron (II) in [Fe(Cn)6]3- lg K = 31

b) the size of the halide ion

The strength of the halide ion ligands increase in the order Cl- < Br- < I-.

Eg : [CdCl4]2- < [CdBr4]2- < [CdI4]2-

lg K = 2.3 lg K = 3.7 lg K = 6.1

c) the presence of polydentate ligands

The stability of complex increase significantly with the presence of polydentate ligands. As such,

hexadentate ligand like EDTA will produce a far more stable complex than monodentate and

bidentate ligsnds.

eg : [Co(NH3)6]2+ < [Co(H2NCH2CH2NH2)]2+ < [Co(EDTA)]2- g

lg K = 4.9 lg K = 10.6 lg K = 19.0

Even though four of lone pairs in EDTA are from nitrogen atoms as in ammonia, the binding effect is

much stronger than ammonia ligands. EDTA liberates six water molecules during bonding, resulting in a

strong ‘clawing’ effect on the complex and thus it is stable entropically.

USES OF TRANSITION METAL AND THEIR COMPOUNDS

Titanium

1. Titanium has the same mechanical strength of steel but it is lighter and does not corrode.

2. The main titanium ores are rutile, TiO2, and ilmenite which is a combination of rutile and iron(II) oxide,

FeTio3.

3. Extraction of titanium can be tedious because it combines strongly with non-metals like nitrogen,

oxygen and carbon. Titanium can be extracted by Kroll process which consists of four stages:

(a) Purification of oxide

The first stage involves the removal of impurities like clay and granite that come with the ore. Pure

titanium(IV) oxide, being amphoteric is obtained from rutile by dissolving it in concentrated sulphuric

acid. Titanyl sulphate, [TiO][SO4], is first formed.

However, upon dilution Ti(OH)4 is precipitated. It is then filtered off, washed and heated strongly to

obtain the oxide.

Ti(OH)4 (s) TiO2 (s) + 2H2O (l)

(b) Formation of titanium(IV) chloride

The second stage is to produce titanium(IV) chloride by heating the oxide with chlorine and carbon to

about 900°C.

TiO2 (s) + 2C (s) + 2Cl2 (g) TiCl4 (l) + 2CO2 (g)

Titanium(IV) chloride produced is a colourless, covalent liquid (boiling point 136°C) which fumes strongly

in moist air. It is then separated and purified through fractional distillation. It is kept in a bronze

container containing argon to prevent hydrolysis by moist air.

TiCl4 (l) + 2H2O (g) TiO2 (s) + 4HCl (aq)

(c) Reduction of titanium(IV) chloride

The titanium(IV) chloride stored under inert condition is then reduced by molten magnesium at 1000°C

to produce the titanium metal.

TiCl4 (g) + 2Mg (l) Ti (s) + 2MgCl2 (s)

(d) Purification

Titanium produced by the Kroll process needs to be processed further to remove impurities like

unreacted magnesium and side-product magnesium chloride. Magnesium chloride cam be recycled

through electrolysis, producing chlorine gas which can in turn be used to produce titanium(IV) chloride.

4.Titanium is used in the making of aircraft body, space capsules and nuclear reactors. It is added to

steel in the form of alloy with iron to remove combined oxygen and nitrogen. Titanium(IV) oxide is used

as white pigments in paints. It is also used as ‘filters’ for plastic and rubber.

Chromium

1. Chromium is a bright, shiny metal which forms a transparent yet stable oxide layer on its surface.

2. The only commercial important chromium ore is chromite, FeCr2O4.

3. If pure chromium is needed, the ore is first concentrated and converted to chromium(III) oxide. The oxide is then reduced by Thermite process, in which a powdered mixture of chromium(III) oxide and aluminium is ignited.

Cr2O3 (s) + 2Al (s) 2Cr (l) + Al2O3 (s)

The reaction is highly exothermic and the molten chromium metal produced settles at the bottom of the refractory container and collected through a pipe.

4. If it is used to produce ferrochrome alloy, then the concentrated chromite ore is reduced by carbon in an electric arc furnace.

FeCr2O4 (s) + 4C (s) Fe (l) + 2Cr (l) + 4CO (g)

Molten chromium is then mixed with nickel to produce heat-resistant alloys or with iron and nickel to produce stainless steels.

5. Chromium is used to harden steel and increase its resistance to corrosion. It is used in electroplating. Alloy of chromium with vanadium and tungsten is used in high speed cutting tools.

6. Dichromate such as K2Cr2O7 are oxidizing agents and are used in quantitative analysis and also in tanning leather.

7. Chromium is used by the aircraft and other industries for anodizing aluminium. The refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stable crystalline structure.

Cobalt

1. Cobalt is a hard silvery-white metal. Cobalt is harder than iron, but melts at a lower temperature and is somewhat heavier.

2. Most cobalt is obtained as a by-product in smelting nickel or copper ores.

3. Cobalt is used in magnet steels and stainless steels. It is used in alloys used in jet turbines and gas turbine generators and electroplating because of its appearance, hardness, and resistance to oxidation.

4. Cobalt-60, an artificial isotope, is an important γ ray source, and is extensively used as a tracer and a radio therapeutic agent. Single compact sources of 60Co are readily available.

5. Different kinds of compounds cobalt are used as pigment in pottery, glass enamels and paints. Some compounds of cobalt are used as driers, substances that promote drying in paints, varnishes and printing inks.

Manganese

1. Manganese is a pinkish-gray, chemically active element. It is a hard metal and is very brittle. It is hard to melt, but easily oxidized. Manganese is reactive when pure, and as a powder it will burn in oxygen, it reacts with water (it rusts like iron) and dissolves in dilute acids.

2. Manganese shows a variety of oxidation states (+2 to +7). The most stable oxidation state is +2 which is always ionic. While +4 may be ionic or covalent, +6 and +7 are always covalent.

3. Steel becomes harder when it is alloyed with manganese. Manganese dioxide is used to manufacture ferroalloys and dry cell batteries. It is used to "decolorize" glass and to dry black paints. Manganese sulfate (MnSO4) is used as a chemical intermediate and as a micronutrient in animal feeds and plant fertilizers.

4. Manganese metal is used as a brick and ceramic colorant, in copper and aluminum alloys, and as a chemical oxidizer and catalyst. Potassium permanganate (KMnO4) is used as a bactericide and algicide in water and wastewater treatment, and as an oxidant in organic chemical synthesis.

GEOMETRY OF COMPLEX IONS

These shapes are for complex ions formed using monodentate ligands - ligands which only form one

bond to the central metal ion.

You will probably be familiar with working out the shapes of simple compounds using the electron pair

repulsion theory. Unfortunately that doesn't work for most complex metal ions involving transition

metals. The answer is just to learn the shapes you need to know about. As you will see, it isn't difficult.

6-co-ordinated complex ions

These are complex ions in which the central metal ion is forming six bonds. In the simple cases we are

talking about, that means that it will be attached to six ligands.

These ions have an octahedral shape. Four of the ligands are in one plane, with the fifth one above the

plane, and the sixth one below the plane.

The diagram shows four fairly random examples of octahedral ions.

4-co-ordinated complex ions

These are far less common, and they can take up one of two different shapes.

Tetrahedral ions

These are the ones you are most likely to need for A' level purposes in the UK. There are two very similar

ions which crop up commonly at this level: [CuCl4]2- and [CoCl4]2-.

The copper(II) and cobalt(II) ions have four chloride ions bonded to them rather than six, because the

chloride ions are too big to fit any more around the central metal ion.

A square planar complex

Occasionally a 4-co-ordinated complex turns out to be square planar. There's no easy way of predicting

that this is going to happen. The only one you might possibly come across at this level is cisplatin which

is used as an anti-cancer drug.

Cisplatin is a neutral complex, Pt(NH3)2Cl2. It is neutral because the 2+ charge of the original platinum(II)

ion is exactly cancelled by the two negative charges supplied by the chloride ions. The platinum, the two

chlorines, and the two nitrogens are all in the same plane.

Bonding In Complex Ions

1.In complex compounds, transition metals ions use empty orbitals (3s, 4s and 4p) to

form coordinate bonds with ligands. Ligands like chloride ion donates their lone pair of electrons and

occupy the available empty orbitals. For example, in complex ion [CuCl4]2- the copper(II) ion has four

empty 4s and 4p orbitals to accommodate the four lone pair of electrons from the chloride ions.

With four equivalent bonds, [CuCl4]2- ion has a tetrahedron shape.

Bonding in tetrahedral complex,[CuCl4]2-

2. For complex ions to form an octahedral, the single electron in the 3d orbitals paired up leaving six

empty orbitals. Initially, the iron(II) ion is paramagnetic (can be attracted by a magnetic field) caused by

unpaired electrons in the d suborbitals. After all the electrons in the d suborbitals are paired up, the six

empty orbitals are filled up by the lone electrons from the water or cyanide ligands. The complex ion is

no longer influenced by magnetic fields and is now called diamagnetic. For example, in

hexacyanoferrate(II) ion, [Fe(CN)6]4- ,

Types Of Ligands

Cl -

Cl -

Cl -

Cl Cu 2+

Cl

Cl

Cl Cu

2-

1. The molecules or ions that surround the centre metal ions through dative bonds are called ligands.

Ligands play the role of Lewis bases as they act as electron donors. On the other hand, the centre metal

ions are Lewis acids, acting as electron recipients.

2. Depending on the number of electron pairs donated, lagands are classified as monodentate,

bidentate or polydentate.

3. H2O, CN-, Cl- and NH3 are monodentate ligands donating only one donor pair of electron for

coordinate bonds.

4. Two known bidentate ligands are ethylenediamine (in short, en) and oxalate ion (in short, ox),

whereby there are two pairs of donor electrons in each ligand.

Figure below shows complexes with these two bidentate ligands.

5. One popular example of polydentate ligand is ethyenediamminetetraacetate ion(EDTA), a

hexadentate. There are six donor pairs forming a very stable complex ion.

The stability of the complex formed with polydentate ligands is due to chelating effect. Unlike cyanide

ion, EDTA is non-toxic. As such, it is useful madically as it can be used to treat metal poisoning such as

lead and cadmium. EDTA acts as sequestering agent, forming complex with the toxic lead ions and thus

dissolving it. It can be then removed from the blood and tissues by excreting from the body.