Introduction

In this particular article Factor Affecting The Magnitude Of ∆, we are going to discuss various factors that affect the magnitude of ∆. And also the crystal field splitting in lanthanides.

Factor affecting the magnitude of ∆

There are several factors that affect the magnitude of splitting of d-orbitals by the surrounding ligands.

(1) The oxidation state of the metal cation:

The higher the oxidation state of the metal cation, the greater will be the magnitude of ∆.

The higher oxidation state of the metal causes the ligands to approach more closely to it and therefore the ligands cause more splitting of metal d-orbitals. For example

∆₀ for {Co(H₂O)₆}²⁺ = 9200 cm⁻¹

and, ∆₀ for {Co(H₂O)₆}³⁺ = 20760 cm ⁻¹

(2) The same oxidation state of metal cation but the number of d- electrons are different:

In general, for a given series of the transition element, in complexes having the metal cation with the same oxidation states but with the different number of electrons in d-orbitals, the magnitude of ∆₀ decrease with increase in the number of d- electrons. It is due to the facts that the higher number of d- electrons prevent the ligands to come closer the metal cation. For example

For {Co (H₂O)₆}²⁺ , ∆₀ =9200 cm⁻¹ (3d⁷)

For {Ni (H₂O)₆}²⁺ , ∆₀ =8500 cm⁻¹ (3d⁸)

(3) Principal quantum number (n) of the d- orbitals of the metal cation:

In case of complexes having the metal cation with same oxidation states and the same number of d- electrons, the magnitude of ∆ for analogous complexes within a given group increase about 30% to 50% from 3d to 4d and by about the same amount from 4d to 5d. It is because:

(i) On moving 3d to 4d and 4d to 5d, the size of the d- orbitals increases and electron density decrease in them. Therefore the ligands can approach the metal cation with larger d-orbitals more closely.

(ii) There is less steric hindrance round a large metal cation:

For example : For {Co(NH₃)₆}³⁺ , ∆₀ = 23000 cm⁻¹

For {Rh (NH₃)₆}³⁺ , ∆₀ = 34100 cm⁻¹

For {Ir (NH₃)₆}³⁺ , ∆₀ = 41200 cm⁻¹

(4) Nature of ligands:

The ligands are classified as weak and strong ligands. The ligands which cause a small degree of Splitting of d-orbitals are called weak ligands and the ligands which cause a large splitting are called strong ligands. The common ligands have been arranged in order of these increasing crystal field splitting power to cause splitting of d-orbitals from a study of their effects on the spectra of transition metal ions.

I⁻ < Br ⁻ < S²⁻ <SCN ⁻ <Cl ⁻ <N⁻₃ <F ⁻ < urea <OH ⁻ <C ₂H ₅OH <C₂O²⁻₄ <

O²⁻ < H₂O <NCS ⁻ <gly <NH ₃, py< en, SO²⁻₃ <  NH₂OH< bpy, phen <

<NO ⁻₂ <CH ⁻₃, C₆H ⁻₅ <R₃P <CN ⁻ <CO

This order is usually called as spectrochemical series.

The order of the field strength of the common ligands is independent of the nature of the metal cation and the geometry of the complex.

(5) The number of ligands:

The Magnitude of crystal field splitting increases with the increase of the number of ligands. For example ∆₀ > ∆t.

Though the number of ligands in square planar complexes is smaller than that of octahedral complexes, the magnitude of ∆sp is greater than ∆₀. It is because of the facts that square planar complexes are formed by many strong ligands with the d⁸ − metal cation of 3d – series transitions metals cation and 4d or 5d series d⁸ transition metal cation will either weak or strong ligands. The very strong ligands and 4d or 5d series transition metal cation are responsible for higher crystal field splitting. Also, in square planar complexes of d⁸ – metal cation, the dz² orbital with two electrons is stabilized and the vacant dx²-y² orbital is destabilized.

Crystal Field Splitting in lanthanoid complexes.

The seven f- orbitals of lanthanoids in their octahedral complexes are also split liked-orbitals of transition metal complexes. The crystal field splitting of f- orbitals is smaller than that of d- orbitals in octahedral complexes. The 4f- orbitals are buried in deep, therefore, the ligands can not approach close enough to the 4f- orbital to cause much crystal field splitting.

The conclusion of Factor affecting the magnitude of ∆

In order to read this particular article Factor affecting the magnitude of ∆, we are now able to conclude about the topics we cover in this particular article. we have widely discussed Factor affecting the magnitude of ∆ and also the crystal field theory in lanthanoids.

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