Chapter 6 Bonding CFT

8/2/2019 Chapter 6 Bonding CFT

1/39

4/23/2012

CRYSTAL FIELD THEORY The theory is introduced as a result of a failure VBT

in explaining some properties of complex.

The theory is based on assumption that the formationof the bond between M-L is due to electrostaticattraction only, similar to the bond observed in anionic crystal.

The theory states that when a ligand with lone pairelectrons (negative charge) approaching a metal ion(positive charge), all orbitals especially at theoutermost shell in the metal are now being

pressurised. The effect causes the energy level of the orbitals

increase and the five degenerated d-orbital split tovarious energy levels depending to direction of ligandmovement.

1Crystal Field Theory


2/39

Direction of M-L bonds

4/23/2012 Crystal Field Theory 2

x

z

y

x

z

y

x

z

y

octahedral

Tetraherdal

Square planar


3/39

Effect on p- and s-orbitals


L

LL

LL

L

LL

LL

LL

If we assume the incoming ligand approaching themetal ion right on all axes. The pressure effects on sand p-orbitals in all direction are all uniform.


4/39


CRYSTAL FIELD THEORY

Thus both orbitals being pressurized (s- and p-orbitals)

are promoted to higher energy level. The shape and

degeneration of the orbital are unchanged.

Free metal ion Metal ion in complex

E4p

4p

4s

4s


5/39

4/23/2012

Effect on d-Orbital in Metal

d-orbital of metal also will be pressurized when

the ligands approach to metal via axis.

Unlike s and p-orbitals, five d-orbitals in atom are

not identical eventhough they are degenerate.

Orientation of d-orbital can be divided to two

groups

The loops are on axisdx2-y2 & dz2 The loops are out of axisdxy, dyz & dxz.



6/39

4/23/2012

Orientation of d-orbital

Orientation of d-orbitals



7/39

4/23/2012

CFTIN OKTAHEDRALCOMPLEX



8/39

4/23/2012

ENERGY CHANGED OF d-ORBITAL

IN OCTAHEDRAL COMPLEXES

o atau 10Dq

eg

t2g

Orbital -d Bebas Orbital-d Ketika Orbital-d setelahpembentukan pembentukanKompleks Kompleks

0.6

0.4

Free d-orbital d-orbital in aprocess of complexformation

d-orbital afterformation of acomplex

o or 10Dq



9/39

4/23/2012

CRYSTAL FIELD SPLITTING ENERGY

IN OKTAHEDRAL COMPLEX,o Is a separation energy between eg and t2g

energy levels of split d-orbital caused by

interaction with ligand.

The magnitude of the energy depends on

the present of ligand in the complex.

The larger of the energy shows that the

present ligand interacts strongly with d-

orbital of metal..



10/39

4/23/2012

CRYSTAL FIELD SPLITTING ENERGY

(CFSE) IN OKTAHEDRAL COMPLEX,oFor an octahedral complex, the magnitude of

CFSE,o in most of the complexes are as follow;

~ 10,000 cm-1 (monopositive metal ion);

5,00015,000 cm-1 (dipositive metal ion)

10,00030,000 cm-1 (tripositive metal ion)

The results show that the CFSE,o increases as

the oxidation number of a central metal ionincreases.



11/39

4/23/2012

DETERMINATION OF CRYSTAL

FIELD SPLITTING ENERGY , 0 (10Dq) Theoretical :

Using mathematical equation derived based on electrostatic

interaction between the metal and ligand.

Experimental, The value of0 (10Dq) is determined from electronicspectrum of the complex. An band of maximum absorptionnormally observed in ultraviolet and visible regions.

The band is identified by wavelength l or wavenumber

formed as a result of a transition of electron from t2g to egOrbitals.

The transition of this type is known as d-d transition andusually observed in visible region..



12/39


ELECTRONIC SPECTRUM OF

[Ti(H2O)6]3+

Wavelength ofmaximumabsorption

Visible Infraredultraviolet

Wavelength (nm)

300 400 500 600 700

Ab

sorbance


13/39



FIELD SPLITTING ENERGY , 0The energy is calculated using Planck

equation;

E = 0

= hc /lwhere h = Planck constant, c = light velocityand l is wavelength at maximum absorption of acomplex

For majority of transition metal complexes,the absorption occurred in the visible regionreflecting formation of colour.


14/39



FIELD SPLITTING ENERGY , 0As an example, the absorption of [Ti(H2O)6]

3+

complex

The solution of the complex gives violet colour

due to a transition of electron from t2g to egorbitals of Ti3+ ion.

Ti(III) ion has an electronic configuration d1 in

which [Ti(H2

O)6

]3+ complex, this single electron

occupies in any three of t2g orbital

(t2g )1(eg )

0 h (t2g )0(eg )

1


15/39

4/23/2012

THE CHANGES IN ELECTRONIC

CONFIGURATION IN [Ti(H2O)6]3+

Configuration of [Ti(H2O)6]3+ complex

t2g

eg eg

t2g

Keadaanaras KeadaanterujaGround state Excited state



16/39


CACULATING CRYSTAL FIELD

SPLITTING ENERGY, o OF [Ti(H2O)6]3+ The analysis of the complex by uv-vis spectroscopy

reported the wavelength of maximum absorption at l =490 nm or 20300 cm-1 (violet solution) .

Application of Planck equation

E = 0 (kJmol-1)

= (hc/l) (Avogadro number)

= (6.6 10-34Js) (3 10-8 ms-1) (6.02 1023)

490 10-9 m= 240 kJ mol-1

Thus, we conclude that two energy levels of t2g and eg areseparated by 240 kJ.


17/39


LIGAND STRENGTH

SERIES CFT has used the o to determine the strength of a ligand

in which the higher is the stronger.

CFT estimates for a ligand with first raw transition metalion and having similar stereochemical, the o decreases in

the order as follow, . Ni2+ < Co2+ < Fe2+ < Co3+ < Fe3+ < Cr3+ < V3+ < Ti3+.

For a same metal ion, the ligand field strength decreases inorder

I-< Br- < Cl-< < SCN-< < N3-< < (CH2H5O)2PS2 < F-


18/39

4/23/2012

, cm-1 For Several OctahedralComplexes

Metal

ion 6F- 6H2O 6NH3 6CN-

Ti3+

17,500(209) 20,100(240) 22,100(264)V2+ 16,100(193) 18,500(221) 23,400(280)

Cr3+ 15,100(180) 17,400(208) 21,600(258) 26,600(318)

Fe3+ 14,000(168) 35,000(420)

Co3+ 13,000(156) 19,100(229) 22,900(274) 34,800(416)

Fe2+ 10,400(124) 33,800(404)

Co2+ 9,300(111) 11,000(132)

Ni2+ 8,500(102) 10,800(129)



19/39

4/23/2012

CRYSTAL FIELD STABILIZATION

ENERGY (ECFSE)

CFT found that the complex with low spin gives higherstability than high spin complex.

Therefore, the theory concludes that in all octahedralcomplexes, the complex is stabilized as electron occupies

in t2g orbital while the complex is destabilized as electronoccupies in eg orbital.

The CFT estimates the magnitude of the stabilizationenergy (later known as crystal field stabilization energyECFSE) of an octahedral complex is calculated usingrelation as follow,

ECFSE = [0.4p - 0.6q] o Where p and q are number of electron occupying in t2g

and eg orbitals respectively.19Crystal Field Theory


20/39



LIGAND FIELD STRENGTH

The CFT has recognized that in a complex the d-orbital ofmetal is no longer degenerate but it splits to severalenergy levels because of interaction between the orbitalwith the surrounded ligand orbitals.

Thus application ofHunds rule in a systematic filling of

electron in an orbital is no longer valid but is determinedby comparing the magnitude of splitting energy, o andelectron pairing energy, Epair.

Epair is an energy required for two electron to formpairing in an orbital.

If o > Epair , the electron tends to form pairing in d-orbital (lowspin)

If o < Epair , the electron tends to singly occupy in the d-orbitalfirst before forming pairing (high spin)obey Hunds rule..


21/39


CFT found that those ligands in earlyspectrochemical series exhibit high spinconfiguration with all metal ions. Thus Epair. The ligands are grouped as astrong ligand (strong field ligand)Such ligandsinclude CN-, CO, NO2 etc.


LIGAND FIELD STRENGTH


22/39

The ligands located between these groups ofligand (in the centre of spectrochemical series)are classified as an intermediate ligand (middle

field ligand). The ligand may give low or highspin complex depending on metal ions present.Such example: NH3, ethylenediamine (en).

I-< Br- < Cl-< < SCN-< < N3-< < (CH2H5O)2PS2 < F-


23/39

4/23/2012


MIDDLE FIELD LIGAND

The configuration of d-orbital with middle field liganddepends on characteristic of central metal ion:

Oxidation state

size It is observed that the increase in oxidation state and size

of metal, the complex formed tends to give low spinconfiguration and vice versa.

Example complex of iron(II) and iron(III) with amonia

[Fe(NH3)6]2+ high spin

[Fe(NH3)6]3+ low spin



24/39

4/23/2012

CRYSTAL FIELD THEORYMAGNETIC PROPERTY OF COMPLEX

CFT has identified that the filling of electron in t2g and egorbitals of octahedral complex depends on o and Epair.

for o > Epair the complex tends to form low spin and high spinwhen o < Epair .

CFT also predicted that the complex with high spin tend toenhance their magnetic property ( increase paramagnetic)while the low spin complex tend to diamagnet.

For complexes with configuration d1-d3, the filling electronin the orbitals for all type of complexes are similar. In thiscases, all complexes will show paramagnetic character.

o and Epair have no significant influence.



25/39

4/23/2012

CONFIGURATION OF d-ORBITAL

(d1-d3) IN OCTAHEDRAL COMPLEXES

d1 d2 d3

t2gt2g t2g

egeg eg



26/39

4/23/2012


For complex with configuration d4, d5, d6 and d7, themagnetic property varies depending on ligand andoxidation state of metal.

For these complexes, there are two possible electronarrangement: High spin configuration where Epair > oobserved in complex with

low field ligand. The complexes enhancing towards paramagnetic.

Low spin configuration where Epair < oobserved in complex withlow field ligand. The complexes enhancing towards diamagnetic.

High spin d6 paramagnet

t2g

eg

t2g

eg

Low spin d6 diamagnet



27/39



For complexes with configuration d8-d10, the filling

electron in the orbitals for all type of complexes are

similar.

o

and Epair

have no significant influence.

In this case, d8 and9 in all complexes exhibit low

paramagnetic while d10 is totally diamagnetic.

t2g

eg

t2g

eg

t2g

eg

d8 d9 d10


28/39

4/23/2012

PROBLEM I

CALCULATING ECFSE

Question

Determine ECFSE for two d6 complexes

[Fe(CN)6]4- low spin

[Fe(H2O)6]2+ high spin

Answer

For [Fe(CN)6]4- - configuration t2g6 eg0

Bagi [Fe(H2O)6]2+ - configuration t2g4 eg2

ECFSE ([Fe(CN)6]4- ) = 0.4 x 6 - 0 = 2.4 o

ECFSE ([Fe(H2O)6]2+) = 0.4 x 4 - 0.6 x 2 = 0.2 o



29/39

4/23/2012

Incoming ligand approaching metal

through between axis.29Crystal Field Theory

CFT EFFECT ON d-ORBITAL OF

TETRAHEDRAL COMPLEXES

x

z

y


30/39

4/23/2012

CFT EFFECT ON d-ORBITAL OF

TETRAHEDRAL COMPLEXES

t0.6

0.4

Orbital-d bebas Orbital-d ketika

pembentukankompleks

Orbital-d setelah

pembentukankompleks

Freed d-orbital d-orbital during

complexformation

d-orbital after

complexformation



31/39

4/23/2012

SPLITTING ENERGY, tINTETRAHEDRAL COMPLEXES

t is a CF splitting energy of d-orbital in tetrahedralcomplexes.

The magnitude of t

is a different between theorbital energy level of t2g and eg and it is varieddepending on ligand present.

However, the splitting oft is small compared to .In all cases the ratio is

t = 0.45 Therefore, all tetrahedral complexes exhibit high spin

complex t< Epair.31Crystal Field Theory


32/39

4/23/2012

THE ECFSEEFFECTINTETRAHEDRAL COMPLEXES

ECFSE = [0.4p - 0.6q]t Where p and q are the number of electron

occupying t2g and eg orbitals respectively

OR

ECFSE = q (0.27

o)

p (0.18

o) where o is splitting energy of a similar

complex in octahedral field.



33/39

4/23/2012

PROBLEM 2,CALCULATING E

CFSEQUESTION Calculate the ECFSE of [CoCl4]

2- complex (Co(II) has d7configuration)

ANSWER ECFSE = [0.4p- 0.6q]t

= [(0.4 x 4) - (0.6 x 3)] t

= - 0.2t

ECFSE = q (0.27 o) p (0.18 o)= [(3 x 0.27) (4 x 0.18)] o= + 0.09 o

eg

t2g



34/39

4/23/2012

THE ECFSEEFFECTIN SQUAREPLANAR COMPLEXES

The energy level of d-orbitalincrease in the following orderas a result of interaction with

ligand in square planar field:

dx2-y2dxy

dz2dxz dan dxz

z

x

y

E


x

z

y

Square planar


35/39

4/23/2012

THE ECFSEEFFECTIN SQUAREPLANAR COMPLEXES

Orbital-d bebas Orbital-d ketika

pembentukankompleks

Orbital-d setelah

pembentukankompleks

dx2-y2

dz2

dxy

dxz dyz

sp

Freed d-orbitald-orbital during

complexformation

d-orbital aftercomplexformation



36/39

4/23/2012

sp - SQUARE PLANARCOMPLEXES

sp is a crystal field splitting energy of square planarcomplexes

It is defined as a different between dx2-y2 with dxy

energy levels.

Those complexes (ML4) involved are derived from d8

metal ion with strong ligand. Such as CN-, CO, NO2,

PH3.

It is found that the square planar complex is morestable than tetrahedral.



37/39

4/23/2012

CFT EFFECTCOMPARISONIN ALL COMPLEXES



38/39

4/23/2012

ADVANTAGEOUS OF CFT The theory has successfully explains how electron configuration in d-

orbital of all complexes are arranged.

The theory has produced aspectrochemical series which explain theligand strength.

The theory has able to explain the colour formation in complex due to

a transition of electron from t2g toeg. This type of transition is knownas d-d transition.

Able to estimate stablity of the complex based on magnitude ofowhere as o increase the stability of the complexes increases.

The theory uses a calculated ECFSE to estimate relative stability among

complexes.



39/39

Documents

Chapter 6 Bonding CFT