## Pairing Energy:

The energy required to force the two unpaired electrons in one orbital is called 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 complex
If ∆

_{0}< P, it favors the low spin complex
If ∆

_{0}= P, high spin and low spin complex equally exist
In general, for 4d and 5d series transition metal complexes, the magnitude of ∆

_{0 }is greater than that of P.- In weak field octahedral complex of 3d-series transition metal with oxidation number less than equal to +3, the value of is ∆
_{0 }small and there will be no pairing of electrons. Therefore in weak field complexes of d^{4}, d^{5}, d^{6}, and d^{7 }configuration, there is no pairing of electrons. These complexes have 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-orbital of free metal cations. - In strong field octahedral complex of 3d-series transition metal with oxidation number, in general, greater than equal to +2, the value of ∆
_{0}is large. In strong field complexes of d^{4}, d^{5}, d^{6}, and d^{7 }configurations, the pairing of d-electrons will take place in according to Hund's rule. These complexes have 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 more number of paired electrons(or spin)than that of the free metal cation. - It is to be notated that week field octahedral complexes are always not the high spin complexes. The metal cation of 3d-transition series with the oxidation number of greater than equal to +4 and 4d and 5d series transition metal cations always form low spin complexes with weak ligands.
- For example, [NiF
_{6}]^{2- }ion (oxidation state of Ni is +4) is low spin and diamagnetic, though F^{-}is a weak ligand. [Rh(H_{2}O)_{6}]^{3+}is low spin and diamagnetic, though is a weak ligand. - An exception is observed for 3d-series transition metals in which Co
^{3+}form low spin complexes with H_{2}O and O^{2-}though H_{2}O and O^{2-}are weak ligands.

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