Crystal field theory

By Dr. Neelam

1. Presented by
Dr. Neelam
Department of chemistry

2. Crystal Field Theory
Thistheoryconsidersonlyelectrostaticinteractionsbetweenthecentral
metalion(C.M.A.)andtheligand.
ItconsiderstheligandsandtheC.M.A.aspointcharges:
C.M.A.=+vepointcharges
Ligands=-vepointcharges
Attractiveforces=decreasesenergy
Repulsiveforces=increasesenergy
Toformacomplexcompoundwiththe–vechargedligandmolecules
approachtheC.M.A.

3. d
xy d
yz d
zx
t
2g
Lobes lying between the axis
Lobes lying along the axis
d
x
2
-
y
2
d
z
2
eg
Thefivedorbitalsinanisolatedgaseousmetalatom/ionhavesameenergy,
i.e.,theyaredegenerate.
d
xyd
yz d
zxd
x
2
-
y
2
d
z
2

4. Splitting of d orbital into two different set of orbitalsin presence of the repulsive
field created by the approaching ligands.

7. The two possibilitesare :
(i) If Δ
0< P weak field ligandsand form high spin complexes.
(ii) If Δ
0> P stongfield ligandsand form low spin complexes.
COLOUR IN COORDINATION COMPOUNDS
According to the crystal field theory the colour is due to the d-d transition
of electron under the influence of ligands.

8. Factors affecting CFSE
nature of the metal ion
CFSE ∝principle quantum no. Of metal
oxidation state of metal
CFSE ∝oxidation state of C.M.A.
nature of the ligands
CFSE ∝strength of ligand
nature of the complex
Octahedral complex > tetrahedral complex

9. Limitations of crystal field theory
(1)Itconsidersonlythemetaliond-orbitalsandgivesnoconsiderationat
alltoothermetalorbitals(suchass,px,pyandpzorbitals).
(2)Itisunabletoaccountsatisfactorilyfortherelativestrengthsof
ligands.ForexampleitgivesnoexplanationastowhyH
2Oisastronger
ligandthanOH
–
inthespectrochemicalseries.
(3)Accounttothistheory,thebondbetweenthemetalandligandsare
purelyionic.Itgivesnoaccountonthepartlycovalentnatureofthemetal
ligandbonds.
(4)TheCFTcannotaccountfortheπ-bondingincomplexes.
(5)Thetheoryfailedtoexplaincolorincompoundhavingd
0
configuration
Forex.KMnO
4,K
2Cr
2O
7

10. 10