Distribution of d- electrons in octahedral complexes

In this particular article High spin and low spin complexes, we are going to discuss the distribution of d-electron in octahedral complexes, High spin, and low spin complexes, pairing energy etc.

The distribution of d- electrons in t₂g and eg orbitals depend upon the magnitude of ∆₀. When the magnitude of ∆₀ is relatively strong than it is assumed that all the five d-orbitals are degenerate. The distribution of d- orbitals in t₂g and eg sets takes place. According to Hund’s rule of maximum multiplicity i.e., the pairing of electrons will take place only when each of five d-orbitals is singly filled. Thus in complexes having the small value of ∆₀ the first three electrons occupy the t₂g  orbitals, 4th and 5th electrons occupy the t₂g orbitals and the last two electrons occupy the eg orbitals. This can be shown as:

In these complexes, no pairing of electrons will take place. I.e., the arrangement of d- electrons remain the same as in free metal cation.

The complexes in which the magnitude of ∆₀ is large, the energies of t₂g and eg orbitals differ significantly and the distribution of d-electrons in t₂g and eg orbitals does not obey the Hund’s rule of maximum multiplicity. In these complexes, the first six electrons occupy the t₂g orbitals and the remaining four electrons occupy the eg orbitals. This can be shown as:

High spin and low spin complexes

In these complexes, the pairing of electrons takes place in t₂g orbitals for d⁴, d⁵, d⁶ and d⁷ configuration. Whether the value of ∆₀ is small or large, there is no difference in the d-electron configuration for d¹, d², d³, d⁸, d⁹ and d¹⁰ systems i.e., for these systems, there is no pairing of electrons in octahedral complexes.

High spin and low spin complexes

In this session, we are going to discuss strong field and weak field in detail-

Weak Field or High Spin Free Complexes

In weak field octahedral complexes of 3d – series transition metals with oxidation numbers ≤ + 3, the value of ∆₀ is small. And there will be no pairing of d- electrons. These complexes have the maximum number of unpairing electrons. These complexes having the maximum number of unpaired electrons are called high spin or spin free complexes. The term high spin or spin free is used. Because these complexes have the same number of spin as in d- orbitals of free metal cations.

Strong Field or Low Spin or Spin Paired Complexes

In strong field octahedral complexes of 3d- series transition metals with oxidation number, in general, ≥ + 2, the value of ∆₀ is large. In the strong field Complexes of d⁴, d⁵, d⁶ and d⁷ – configurations pairing of d-electrons will take place in t₂g orbitals according to Hund’s rule. These Complexes having the maximum number of paired electrons are called low spin or spin Paired Complexes. The term low spin or spin Paired is used. Because these complexes have the number of paired electrons than that of the free metal cation.

It is to be noted that weak field octahedral complexes always are not the high spin complexes. The metal cation of 3d- transition series with the oxidation number of ≥ +4. And The 4d – and 5d series transition metal cations always form low spin Complexes with weak ligands. For example, {Ni F₆}²⁻ ion state of Ni is +4 is low spin and diamagnetic, though F⁻ is a weak ligand. {Rh(H₂O)₆}³⁺ is low spin and diamagnetic, though H₂O is a weak ligand. An exception is observed for 3d- series transition metals in which CO³⁺ forms low spin complexes with H₂O and O²⁻, though H₂O and O²⁻ are the weak ligands.

Pairing Energy

The energy required to force the two unpaired electrons in one orbital is called the pairing energy. When more than one electrons are paired, P becomes the mean pairing energy. It may be obtained from the analysis of electronic spectra.

If ∆₀ >P, it favors the low spin complexes

If ∆₀ <P, it favors the high spin complexes

And, If ∆₀ =P, high spin, and low spin Complexes equally exist.

In general, for 4d and 5d- series transition metal complexes, the magnitude of ∆₀ is greater than that of P.

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