COLOR OF THE TRANSITION ELEMENTS

Presenter: MS SAMIA SAADIA (Vice Principal)

Subject: CHEMISTRY Class: XII

Defence Authority College for Women - Phase VIII

Colour inTansition Metal ComplexesApplication of Crystal Field Theory

Introduction of Transition Metal Complex Explanation of the subject by Crystal Field Theory (C F T) Distinction Between the d-orbitals splitting in Octahedral

and Tetrahedral complexes Visible Region of the Spectrum Examples References

CONTENTS

TRANSITION ELEMENTS

Introduction• Transition elements are known by this name because they show

their properties which are transitional between highly reactive and strong electropositive elements of s-block which form ionic bonds and highly electronegative elements of p-block elements which form largely covalent compounds.

• B-group elements are transition elements• Transition elements consists of following :-

I. d- block elements

II. f- block elements • d- block elements are placed in the middle (in between the s & p-

block elements) & f- block elements are placed at the bottom of the periodic table separately.

• The elements in which d-orbitals are progressively filled up with electrons are called d-block elements.

• The elements in which the last electron (differentiating) electron

enters (n-1)d orbitals i.e d-orbitals of the penultimate shell.• d- block elements also called outer transition elements.

f- block elements• Transition elements in which f-orbitals are partially filled are

called f-block elements. • These are named as Inner Transition Elements or Rare

Earth elements.

TransitionElements

d-block elements

d-Block covered in XII Syllabus

Color of the Transition Elements

Complex or Coordination Compounds: Metal atom or ion surrounded by oppositely charged ions or neutral

molecules. Polyatomic ion or molecule in which number of ions or molecules are

bonded to a transition metal ion or atom through coordination bonds.

Tendency of Transition metals to form complex compounds: Due to small size of the metal atoms or their cations, have a high charge

density on them. Makes the atom or cation to attract(accept) the lone pair of electrons from

the ligands. Have vacant d-orbitals [(n-1)d orbitals] to accommodate the lone pairs of

electrons donated by ligands to form coordinate bonds (L→M).

Color of the Transition Elements Complex ion: When one or more neutral molecules or negatively charged

ions become attached to central atom by coordination bond. It is charged positively or negatively. Ligand or Coordination group: An ion or neutral molecules surrounding a centre atom A part of complex compound which is directly attached with

the central transition atom by coordination bonds. Transition metals accept electrons from negatively charged

Ions and acts as Lewis acid or electrophile. Legands are electron donors and are called Lewis Base or

Nucleophile. Coordination Number “n”: The number of ligands attached to a centre atom or the

number of coordination bonds formed with in a single central atom in a complex compounds. It is usaully fixed for a metal.

A Complex compound or Complex ion consists ofi. A Cation (+ve)ii. A coordination Sphere is:o Positively charged & Negatively chargedo A coordination Sphere consists of:• Metal atom or Central metal ion• Ligand (Neutral molecules & Anion which can donate pair of

electrons) Strength of Ligands in magnetic field for a common

metal ion: increasing_∆ Cl <F <H2O <NH3 <en < NO2 (N-bonded) <CN

This list is known as Spectrochemical Series Ligands lie on the low end of series are termed as weak

field ligands & on the high end are called strong ligands

Color of the Transition Elements

CRYSTAL FIELD THEORY A model for bonding in transition- metals complexes The interaction between metal ion & ligand is viewed as electrostatic The ligands produce an electric field that causes a splitting in the

energies In a six coordinate octahedral complex, when the ligand approach

the metal ion along x, y, z axis the overall energy of the metal ion plus ligand is lowered (more stability)

When the ligands are drawn towards the metal centre, there is a repulsive interaction between the outermost electrons on the metal & the negative charges on the ligands.This interaction is called crystal field

The crystal field causes the energies of the d-electrons on the metal ion to increase

The repulsive interaction give rise to the splitting of the d-orbitals

Color of the Transition Elements

FIGURE : CRYSTAL FIELD THEORY

Color of the Transition ElementsE

ner

gy

Separated Metal and

Ligands

Electrostatic Attraction

Ligands-d Electrostatic Repulsion

Metal Ion Plus Coordinated Ligands

Splitting of OuterD-orbital in octahedral

field

Color of the Transition Elements

• Central transition atom in complex ion contain partially filled

d-orbital• The color is due to the bonding between transition metal and

ligand i.e electrostatic and is most successfully explained by Crystal Field Theory

• The electrostatic field splits the five degenerated d- orbitals into two sets of energy levels namely t2g (triply degenerate) &

engerade or eg (doubly degenerate)

• T2g includes orbitals lie in between the axis (non-axial)

• eg includes orbitals lie on the axis (axial)

• This splitting causes unpaired electrons to transit triply degenerate from one set to another

• The ∆E0 i.e. energy difference is too small and equivalent to the wavelength that electron jump from an orbital to the other

A lower energy trio, dxy, dyz and dzx designated as “t2g” (Triply degenerate)

eg

t2g

dx2-y2dzxdxy dyzTd

dz2

dxy dyz dzx

dx2-y2 dz2A high energy pair, dx2-y2 and dz2 designated as “eg” (Doubly degenerate)

Color of the Transition Element

Splitting of d orbitals in an Octahedral Field

Splitting of d orbitals in an Octahedral Field

Example: [Co(NH3)6]2+

According to C F T, the interaction between metal ion ligands in a complex is electrostatic

In free metal ions, all five d-orbitals have equal energies & are called degenerate orbitals

Lone pairs present on the ligands attract the positively charge metal ion When six ligands (strong field ligand) approach the set d-orbitals along x, y, z axis

(dx2-dy2 ,dz2), this direction of approach produces an octahedral complex Stronger repulsion occur between the two orbitals & approaching ligands This raises the energies of these d-orbitals than their energies in free ion The energies of other d-orbitals which are between the axis i.e dxy, dyz & dxz are

not raised As a result the d-orbitals of central metal ion split into two sets eg (higher energy) &

t2g (lower energy) Strong field ligand, create a splitting of d-orbitals energies i.e large enough to

overcome the spin-pairing energy The d-electrons then preferentially pair up in the lower energy orbitals producing a

low- spin complex

Splitting of d-orbitals in an Octahedral Field

Splitting of d orbitals in a Octahedral Complex

Example: [Co(NH3)6]2+

The strongest repulsion occurs between these two d-orbitals and approaching ligands.In free metal ions all five d-orbitals have equal energies and are called degenerate orbitals.When six ligands (NH3) approach the set of d-orbitals along x, y & z

axis (dx2- y,

2 dz2) the direction of approach produces an octahedral

complex.This raises the energies of these d-orbitals more than the energies in free ions.The energies of the other d-orbitals which are between the axis i.e dxy, dyz & dxz are not raised.As a result the d-orbitals of central metal ions split into two sets eg (higher energy) and t2g(lower energy). As NH3 is strong field ligand, so

low spin complex is formed.

Splitting of d orbitals in a Tetrahedral Field

Example:[Co(NH3)4]2+

Example:[Co(NH3)4]2+ In free metal ions, all five d- orbitals have equal energies and are called

degenerate orbitals When four ligands(eg NH3) approach the set of d-orbitals along x, y, and

z axis dxy, dyz & dxz. This approach produces tetrahedral complex Stronger repulsion occur between these three d- orbitals and

approaching ligands This raises the energies of these d- orbitals which are on the axis As a result the d- orbitals of central metal ion split into two sets t2g

(higher energy) & eg (lower energy) When the ligand exert a weak crystal field, the splitting of the d- orbital

is small The electrons then occupy the higher energy d-orbitals in preference to

pairing up in the lower energy set, producing a high- spin complex

Splitting of d orbitals in a Tetrahedral Field

Color of the Transition Elements

d-Orbital SplittingThe magnitude of the splitting of the d-orbitals in a transition metal complex depends on three things:• The geometry of the complex•The oxidation state of the metal• The nature of the ligands

The Nature of the Ligands• Some ligands only produce a small energy separation among the d-orbitals while others cause a wider band gap.• Ligands that cause a small separation are called weak field ligands and those that cause a large separation are called strong field ligands.• The ordering of their splitting ability is called the spectrochemical series. Increasing

Cl < F< H2O < NH3 < en < NO2(N-bonded) < CN

Oxidation State of the metal atom/ion.

eg Cu1+(White), Cu2+(Blue)

Geometry of the complex.

eg [Zn(NH3)4]2+(White), [Fe(CN)]63-(Violet)

Nature and approach of the ligand.

eg [Cu(H2O)6]2+(Blue), [Cu(NH3)2]2+(Deep Blue)

Colour and Variation in Color

Colour and Variation in Color

eg [Ti(H2O)6]3+ absorbs blue green color at 4900A-

5000A & transmits Red-Purple color but [Ti(F)6]3-

absorbs yellow color at 5750A- 5900A& transmits violet color.

Reason: changes weaker ligand smaller crystal field splitting at range of longer wave length.

Effect of the nature of ligand when metal shows same oxidation state

Examples: A comparison of the visible absorption maxima for a number of cobalt (III) complexes shows the effects of ligands on the d-orbital band gap.

Color of the Transition Elements

VISIBLE REGION OF THE SPECTRUM:This excitation involves absorption of light (photons) of certain wave length in the region of visible light(3800A0-7800A0) and release of energy is observed in the form of colors ‘VIBGYOR’

• This is called d-d transition which is responsible for the color

If we add the colors on opposite

sides of the wheel together, white light is obtained.

We only detect colors when one or more of the wavelengths in the visible spectrum have been absorbed, and thus removed, by interaction with some chemical species.

When the wavelengths of one or more colors is absorbed, it is the colors on the opposite side of the color wheel that are transmitted.

Color of the Transition Elements

 Red  +  Yellow  makes  Orange  Yellow  +  Blue  makes  Green  Blue  +  Red  makes  Violet 

If red, yellow, orange, blue and violet are absorbed... only one color is transmitted:

GREEN.

If violet, red, and orange are absorbed… then blue, green and yellow are transmitted ... And the middle color which is perceived...

GREEN

Why go Green?

Pictorial Presentation of the concept

V

B

GY

O

R

White Light

Black Only

a) Sample absorbs all , black color perceived

a) Sample does not absorb any color of light. White light is perceived

Pictorial Presentation of the concept

White lightWhite light

Sample

Grass and leaves appear green because chlorophyll absorbs wavelengths in the red and blue portion of the visible spectrum. The wavelengths in between (green) are transmitted.

Everyday Life Examples

Chlorophyll (Green matter of leaves)

Vitamin B12 (Cobalamin) contains Co (III) :i. The antipernicious anemia vitamin owning to its

curative effect in anemiaii. Isolated from liver & the compound is usaully referred

to as Cyanocobalaminiii. Presence of Cobalt atom in the trivalent stateiv. The only organic compound which contains cobaltv. It appears as a red crystalline compound containing

nitrogen, phosphorus & cobalt

Everyday Life Examples

S.N Pigment Color Metallic ion associated

Pigment found in

Animals

1. Haemoglobin (Hb) Red Fe Erythrocytes Mammals Birds Reptiles Amphibians Fish

Plasma Annelids Molluscs

2. Chlorocruorin(Cr) Green Fe Plasma Annelids Polychaetes

3. Haemocyanin Blue Cu Plasma Mollusca:a. Gastropodsb.Cephalopods Crustaceans

4. Haemerythrin Red Fe Corpuscles Annelids

Everyday Life Examples

Examples of Transition Metal Complexes: When light passes through a solution containing transition metal complexes, we

see those wavelengths of light that are transmitted. The color of the transmitted light is called complementry color of the absorbed

light The solutions of most octahedral Cu (II) complexes are blue. The visible

spectrum for an aqueous solution of Cu (II), [Cu(H2O6]2+, shows that the absorption band spans the red-orange-yellow portion of the spectrum and green, blue and violet are transmitted.

Color of the Transition Elements

• The absorption band corresponds to the energy required to excite an electron

from the t2g level to the eg level.

• The energy possessed by a light wave is inversely proportional to its wavelength.

• The Cu(II) solution transmits relatively high energy waves and absorbs the low

energy wavelengths.

• This indicates that the band gap between the two levels is relatively small for this

ion in aqueous solution.

Excitation of d electron in Cu(II) Complex

Color of the Transition ElementsExamples:

Anhydrous Cobalt Compounds: They absorbs light for the excitation of their d-electrons. When red color is absorbed from light, then the transmitted light consists of

wavelengths corresponding to other colors of white light in which the blue color predominates and the cobalt compounds thus appear blue green.

Hydrated Cobaltic Compounds: absorbs light of a different wavelength blue green and

therefore appears red in color.

  Co, [Co(NH3)6]2+   appears blue because the incoming ligand interacts with the

d-orbital of transition metal ion. Thus interaction results in the  splitting of the energies of d-orbital. It causes the unpaired electrons to transit from one set to another. This excitation involves absorption & release of energy is observed in the form of blue color in the visible region of spectrum.

Physical state: Anhydrous and hydrated species of a metal atom or ion absorb at different wavelengths that is why exhibit different colors.

Examples: Anhydrous Cobalt Compounds absorb (6250A-7500A) red

color from the light& appear blue green. Hydrated Cobalt Compounds are peach red in colour. They

absorb (4900A-5000A) blue green color from the light & transmit red color

Approach of ligand: In a solution, if ligand is Ammine, then it appears blue.

Oxidation state: Cu(I) is white but Cu(II) are blue-green. Fe(II) is light green but Fe(III) shows rust colour. Cr(III) is deep green but Cr(VI) appear orange yellow.

Colour Change

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References