INTRODUCTION OF D-BLOCK ELEMENTS

Why are they called d-block elements?

Their last electron enters the d-orbital.

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

Zinc and Scandium are NOT considered as transition metals.

Special properties of d- block elements

1.

2.

3.

4.

Variable oxidation states Complex ion formation Colored compound formation Act as catalysts.

Variable Oxidation States

 D-block elements exhibit variable oxidation states.

 This means that they can form two or more different types of cations.

 Examples:  Iron can form both Fe² ⁺ and Fe ³ ⁺  Manganese can Mn ²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺ and Mn⁷⁺

How come?

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

Very small difference in Ionisation energies.

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

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

Complex ion Formation

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

This is because;   they have incomplete d-orbital.

they act as electron pair acceptors.

What are ligands?

 Ligands are species with a lone pair of electrons.

 Ligands may be negatively charged  F ⁻ , Cl ⁻ , CN ⁻, etc  Or neutral molecules  H ₂ O,CO,NH ₃ etc

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

Examples of complexes: – [Co(NH ₃ ) ₆ ] ³⁺ – [Ni(CN)₄]²⁻

COLOURED COMPOUNDS

WHY?????

d-orbitals are oriented in five planes.

In an

isolated

atom, all d-orbitals are

at the same energy

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

What happens????

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

Octahedral shapes

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

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

Tetrahedral shapes

In these shapes two e-orbitals point between the charges while three t orbitals point more directly on the charges t-orbitals are at a higher energy than e-orbitals.

Effect

When light passes through these compounds, electrons from a lower energy d-orbital absorb a photon of energy and are promoted to higher energy d-orbitals The energy absorbed is equivalent to the energy difference between the two sets of orbitals.

Energy is given by ∆E= hv v is the frequency of light absorbed

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

lacks some colour.

The colour of the compound is the

complementary

of the one that was absorbed.

Factors affecting the colour

 Oxidation state of the central metal  The nature of the ligand

Exceptions

Sc 3+ and Ti 4+  Since their d-orbitals have no electrons which could be promoted.

Zn 2+ and Cu +  Their d-orbitals are full.

Transition metals and their Ions as catalysts

Catalysts

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

Different catalysts

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

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

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

HETEROGENEOUS

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

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

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

HOMOGENEOUS

 Ions of transition metals show variable oxidation states which allow them to be effective homogeneous catalysts more especially in redox reactions  Such characteristics are of fundamental biological importance in enzyme based reactions

Demonstration of heterogeneous catalysis Nickel used in hydrogenation of ethene to ethane.

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

Nickel catalysis

Other examples   Iron (Fe) in the Haber process: N2 (g)+ 3H 2 (g) ↔ 2NH 3 (g) MnO 2 in the decomposition of hydrogen peroxide 2H 2 O 2 (aq) → 2H 2 O(l) + O 2 (g) • V 2 O 5 2S O 2 in the Contact process; (g) + O 2 (g) ↔ 2SO 3 (g)

First example of homogeneous catalysis

Fe 2+ in heme: Oxygen is transported through the bloodstream by forming a weak bond with the heme group of hemoglobin This group contains a central Fe 2+ ion surrounded by 4 nitrogen atoms The O 2 – Fe 2+ bond is easily broken when oxygen needs to be released

Second example

Co 3+ in vitamin B 12 Part of vitamin B 12 consists of octahedral Co 3+ complex 5 of the sites are occupied by nitrogen atoms leaving the sixth for biological activity

Cobalt in vitamin B12

Sources

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

Course companion Chemistry Geoffrey Neuus pcc.ac.uk

www.chm.bris.ac.uk