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4. FACTORS AFFECTING THE CRYSTAL FIELD SPLITTING

FACTORS AFFECTING THE CFS

There are various factor which affect the value of CFS. Which are as follow with their description

  1. Oxidation state of the metals 

Crystal field splitting increases on increasing in oxidation state of the metal. This is because the ligands with their electron density can approach the higher-charge (smaller sized) metal-ion more closely and interact more strongly with the d orbitals. Thus the complex [Co(NH3)6]+3 in which Co in 3+ oxidation state has large crystal field splitting and [Co(NH3)6]2+ with Co in 2+ oxidation state possesses small crystal field splitting .

Principal Quantum no./ Size of the d orbital

Crystal field splitting increases with increase in the principal quantum no. of the d- orbital. As the Principal Quantum number increases the distance of the d-orbital of the metal from its nucleus increases. Such an orbital interacts more strongly with the ligands giving large splitting. In genral, crystal field splitting is greatest for complexes containing 5d metal and least for those containing 3d metal. Thus, crystal field splitting increases on going down a group of transition series. This is shown below......

Complex
Kind of d-orbital present
🔺0  Splitting (cm -1)
[Co(NH)3]+3
3d
23000
[Rh(NH)3]+3
4d
34000
[Ir(NH)3]+3
5d
41000


Geometry of complexes

As mentioned earlier, the crystal field splitting energy of tetrahedral complexes (🔺t)  is nearly half the value for octahedral complexes (🔺t = -4/9 ðŸ”º0 ). In other words, splitting energy for tetrahedral complexes, in general, is small as compared to the pairing energy P. The tetrahedral complexes are, therefore, mostly high spin complexes.
 The splitting order of the complexes is 
🔺sp >  ðŸ”º0  > ðŸ”ºt

Nature of the ligands

The crystal field splitting depends on the nature of the ligands . The greater the ease with which the ligands can approach the central ion, the greater is the crystal field splitting caused by it. Therefore, the ligands with large negative charge and small size will give large crystal field splitting as they can approach the central ion closely.
The variation of the magnitude of crystal field splitting ðŸ”ºwith the nature of the ligands follows a regular order constituting a series known as the spectrochemical series. This series is given below in the order in which the ligands produce increasing value of ðŸ”º:
I- < Br- < Cl- < F- < Ox2- ⋍ H2O ⋍ py ⋍NH3 < en < NO2< CN- < CO
According to the above order, if the ligand is an I- ion, the difference of energy ðŸ”º induced in the two sets of d orbitals of the central ion will be minimum and if the ligand is a carbonyl group, CO, the difference of energy induced will be maximum.
We may illustrate the above concepts with reference to the two octahedral complexes of cobalt (III) mentioned above. Co3+  ion has six d electron. Since  ........ ion is a weak field ligand, the value of ðŸ”º0  is small. In other words, the electron go to the eg orbitals before getting paired with electrons in t2g orbitals. Thus, there are four unpaired electrons. Hence, it is paramagnetic, i.e., it is a high spin complex.
    Ammonia molecule is a strong field ligand. The crystal field splitting energy 🔺0    is, therefore, much greater. As a result, the electrons get paired in t2g orbitals instead of going to the eg orbitals. Since there are no nupaired electrons in this complex, it is diamagnetic.


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