Transcript introduction of d-block elements 1-1-

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 Mn²⁺, 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
H₂O,CO,NH₃ 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(NH₃)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 torbitals 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
Sc3+ and Ti4+
Since their d-orbitals have no electrons
which could be promoted.
Zn2+ 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)+ 3H2 (g) ↔ 2NH3(g)
 MnO2 in the decomposition of
hydrogen peroxide
2H2 O2 (aq) → 2H2O(l) + O2 (g)
•
V2O5 in the Contact process;
2S O2 (g) + O2 (g) ↔ 2SO3 (g)
First example of homogeneous
catalysis
Fe2+ in heme:
Oxygen is transported through the
bloodstream by forming a weak bond
with the heme group of hemoglobin
This group contains a central Fe2+
ion surrounded by 4 nitrogen atoms
The O2 – Fe2+ bond is easily broken
when oxygen needs to be released
Second example
Co3+ in vitamin B12
Part of vitamin B12 consists of
octahedral Co3+ 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