INTRODUCTION OF D-BLOCK ELEMENTSINTRODUCTION OF D-BLOCK ELEMENTS

Why are they called d-block elements?Why are they called d-block elements?

Their last electron enters theTheir last electron enters the

d-orbital.d-orbital.

Most d-block elements are also Most d-block elements are also called transition metals. This is called transition metals. This is because they exhibit characteristics because they exhibit characteristics that ranges from Group 2 to Group 3 that ranges from Group 2 to Group 3 and have incomplete d- sub shell in and have incomplete d- sub shell in any of their normal oxidation states.any of their normal oxidation states.

Zinc and Scandium are Zinc and Scandium are NOTNOT considered as transition metals. considered as transition metals.

Special properties of d- block Special properties of d- block elementselements

1.1. Variable oxidation states Variable oxidation states

2.2. Complex ion formationComplex ion formation

3.3. Colored compound formationColored compound formation

4.4. Act as catalysts. Act as catalysts.

Variable Oxidation StatesVariable Oxidation StatesD-block elements exhibit variable D-block elements exhibit variable oxidation states.oxidation states.This means that they can form two or This means that they can form two or more different types of cations.more different types of cations. Examples: Examples:

Iron can form both Fe²Iron can form both Fe²⁺ and ⁺ and Fe ³⁺Fe ³⁺Manganese can MnManganese can Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺ and ²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺ and

Mn⁷⁺Mn⁷⁺

How come?How come?

The 3d energy level is very close to The 3d energy level is very close to the 4s energy level.the 4s energy level.

Very small difference in Ionisation Very small difference in Ionisation energies.energies.

Electrons can therefore be removed Electrons can therefore be removed from both the 4s and the 3d energy from both the 4s and the 3d energy levels during ion formation.levels during ion formation.

This leads to one element being able This leads to one element being able to form cations with more than one to form cations with more than one charge or valency.charge or valency.

Complex ion FormationComplex ion Formation

D- block elements are able to form D- block elements are able to form complexes with ligands.complexes with ligands.

This is because;This is because; they have incomplete d-orbital.they have incomplete d-orbital. they act as electron pair acceptors. they act as electron pair acceptors.

What are ligands?What are ligands?

Ligands are species with a lone pair Ligands are species with a lone pair of electrons.of electrons.

Ligands may be negatively chargedLigands may be negatively chargedFF⁻⁻, Cl, Cl⁻⁻, CN, CN⁻, ⁻, etcetc

Or neutral moleculesOr neutral moleculesHH₂₂O,CO,NHO,CO,NH₃₃ etc etc

They donate their lone pair of They donate their lone pair of electrons to form a dative electrons to form a dative (coordinate) covalent bond with the (coordinate) covalent bond with the ions of d-block elements.ions of d-block elements.

Examples of complexes:Examples of complexes:– [Co(NH[Co(NH₃₃))₆₆]]³⁺³⁺– [Ni(CN)₄]²⁻[Ni(CN)₄]²⁻

COLOURED COMPOUNDSCOLOURED COMPOUNDS

WHYWHY??????????

d-orbitals are oriented in five planes.d-orbitals are oriented in five planes.

In an In an isolated atom, all d-orbitals atom, all d-orbitals are are at the same energy

Due to the effect of ligands on them, Due to the effect of ligands on them, d-orbitals are divided into two d-orbitals are divided into two groups, t-orbitals and e-orbitals. groups, t-orbitals and e-orbitals.

What happens????What happens????

Transition metals complexes can be Transition metals complexes can be octahedral (six ligands) or octahedral (six ligands) or tetrahedral with four ligands.tetrahedral with four ligands.

Octahedral shapesOctahedral shapesDue to the orientation of d-orbitals, Due to the orientation of d-orbitals, two of them will point direct on the two of them will point direct on the point of charges (e-orbitals), point of charges (e-orbitals), remaining of them between the remaining of them between the charges (t-orbitals).charges (t-orbitals).

T-orbitals are at a lower energy T-orbitals are at a lower energy than e-orbitals.than e-orbitals.

Tetrahedral shapesTetrahedral shapesIn these shapes two e-orbitals point In these shapes two e-orbitals point between the charges while three t-between the charges while three t-orbitals point more directly on the orbitals point more directly on the chargescharges

t-orbitals are at a higher energy than t-orbitals are at a higher energy than e-orbitals.e-orbitals.

EffectEffectWhen light passes through these When light passes through these compounds, electrons from a lower compounds, electrons from a lower energy d-orbital absorb a photon of energy d-orbital absorb a photon of energy and are promoted to higher energy and are promoted to higher energy d-orbitalsenergy d-orbitals

The energy absorbed is equivalent to The energy absorbed is equivalent to the energy difference between the two the energy difference between the two sets of orbitals.sets of orbitals.

Energy is given by ∆E= hEnergy is given by ∆E= hvv

v is the v is the frequency of light absorbedfrequency of light absorbed

Since light of a certain frequency is Since light of a certain frequency is absorbed, the light that comes out absorbed, the light that comes out looks coloured because it looks coloured because it lacks some colour.

The colour of the compound is the The colour of the compound is the complementary of the one that was of the one that was absorbed.absorbed.

Factors affecting the colourFactors affecting the colourOxidation state of the central metalOxidation state of the central metal

The nature of the ligandThe nature of the ligand

ExceptionsExceptions

ScSc33++ and Tiand Ti4+4+

Since their d-orbitals have no electrons Since their d-orbitals have no electrons which could be promoted.which could be promoted.

ZnZn2+2+ and Cu and Cu++ Their d-orbitals are full.Their d-orbitals are full.

Transition metals and their Transition metals and their Ions as catalystsIons as catalysts

Catalysts Catalysts

A substance which alters the rate of A substance which alters the rate of a chemical reaction by providing an a chemical reaction by providing an alternative reaction pathway with a alternative reaction pathway with a lower activation energy.lower activation energy.

Different catalystsDifferent catalysts

Homogenous Homogenous catalystscatalysts

This means the This means the catalyst is in the catalyst is in the same state of same state of matter as the matter as the reactants.reactants.

Heterogeneous Heterogeneous catalystscatalysts

This means the This means the catalysts is in a catalysts is in a different state of different state of matter from that matter from that of the reactantsof the reactants

Why and how transition metals work as Why and how transition metals work as both heterogeneous and homogeneous both heterogeneous and homogeneous

catalysts?catalysts?

HETEROGENEOUS HETEROGENEOUS

By using their 3d and 4s electrons to By using their 3d and 4s electrons to form weak bonds with small reactant form weak bonds with small reactant molecules, they provide a surface molecules, they provide a surface for the reactant molecules to come for the reactant molecules to come together with the right orientation.together with the right orientation.

Electrons from the bonds making up Electrons from the bonds making up the molecules are used to bond to the molecules are used to bond to the metal atoms.the metal atoms.

This is what weakens the bonds This is what weakens the bonds within the molecules thus lowering within the molecules thus lowering the activation energy needed for the activation energy needed for them to reactthem to react

HOMOGENEOUSHOMOGENEOUS

Ions of transition metals show Ions of transition metals show variable oxidation states which allow variable oxidation states which allow them to be effective homogeneous them to be effective homogeneous catalysts more especially in redox catalysts more especially in redox reactionsreactions

Such characteristics are of Such characteristics are of fundamental biological importance in fundamental biological importance in enzyme based reactions enzyme based reactions

Demonstration of heterogeneous Demonstration of heterogeneous catalysiscatalysis

Nickel used in hydrogenation of Nickel used in hydrogenation of ethene to ethane.ethene to ethane.

The nickel metal absorb hydrogen The nickel metal absorb hydrogen and ethene onto its surface and align and ethene onto its surface and align them so that they are in the right them so that they are in the right orientation to react.orientation to react.

Nickel catalysis

Other examplesOther examples

Iron (Fe) in the Haber process:Iron (Fe) in the Haber process:

N2 (g)+ 3HN2 (g)+ 3H2 2 (g) ↔ 2NH (g) ↔ 2NH33(g)(g)

MnOMnO2 2 in the decomposition of in the decomposition of hydrogen peroxidehydrogen peroxide

    2H2H22 O O22 (aq) → 2H (aq) → 2H22O(l) + OO(l) + O2 2 (g) (g)

• VV22OO55 in the Contact process; in the Contact process;

2S O2S O22 (g) + O (g) + O22 (g) ↔ 2SO (g) ↔ 2SO3 3 (g)(g)

First example of homogeneous First example of homogeneous catalysis catalysis

FeFe2+ 2+ in heme: in heme:

Oxygen is transported through the Oxygen is transported through the bloodstream by forming a weak bond bloodstream by forming a weak bond with the heme group of hemoglobinwith the heme group of hemoglobin

This group contains a central FeThis group contains a central Fe2+ 2+

ion surrounded by 4 nitrogen atomsion surrounded by 4 nitrogen atoms

The OThe O2 2 – Fe– Fe2+ 2+ bond is easily broken bond is easily broken when oxygen needs to be releasedwhen oxygen needs to be released

Second exampleSecond example

CoCo3+ 3+ in vitamin Bin vitamin B12 12   

Part of vitamin BPart of vitamin B12 12 consists of consists of octahedral Cooctahedral Co3+ 3+ complexcomplex

5 of the sites are occupied by 5 of the sites are occupied by nitrogen atoms leaving the sixth for nitrogen atoms leaving the sixth for biological activity biological activity 

Cobalt in vitamin B12Cobalt in vitamin B12

SourcesSourcesAtkins P.W and Beran .J. A, General Atkins P.W and Beran .J. A, General chemistry , Scientific American chemistry , Scientific American Books,( 1992). Books,( 1992). http://www.chemguide.co.uk/inorganic/complexions/colour.html

Course companion Chemistry Course companion Chemistry Geoffrey NeuusGeoffrey Neuus

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