Pairing Energy

Pairing Energy

The energy required to force the two unpaired electrons in one orbital is called the pairing energy.
In other words, pairing energy is the energy required to place two electrons in the same orbital.
If the crystal field splitting parameter (Δ) is small because of weak-bonding ligands, then the pairing energy will be larger and the complex will be high-spin.
If the crystal field splitting parameter (Δ) is large because of strong-bonding ligands, then the pairing energy will be smaller, and the complex will be low-spin.

If Δ > P, it favours the low spin complexes.
If Δ < P, it favours the high spin complexes.
If Δ = P, high spin and low spin complexes are equalyy exist.

When more than one electrons are paired, pairing energy becomes the mean pairing energy. It may be obtained from the analysis of electronic spectra.

In general, for 4d and 5d series transition metal complexes, magnitude of Δ is greater than that of pairing energy. So, favours always low spin complexes
For 3d elements, a typical value of pairing energy is about 15,000 cm^-1.
3d complexes are high spin with weak field ligands and low spin with strong field ligands.
High valent 3d complexes (e.g., Co³⁺ complexes) tend to be low spin (large Δ)

Correct relationship between pairing energy (P) and C.F.S.E. (Δ_o) in complex ion [Ir(H₂O)₆]³⁺ is
A. Δ_o > P
B. Δ_o < P
C. Δ_o = P
D. No Relation

Hints: H₂O is a weak filed ligand. So, the crystal field splitting caused by H₂O is small.

For Mn⁺³ pairing energy is 28000 cm⁻¹, Δ_o for [Mn(CN)₆]³⁻ is 38500 cm⁻¹ then which of the following is not correct.
A. Complex will be coloured
B. Complex will be low spin complex
C. Net CFSE = -33600 cm^-1
D. Complex will be colourless

Hints: Δ_o > P. So, it is a low spin complex.

It has two unpaired elecrons in t_2g orbitals so, it is paramagnetic and colored.
CFSE = −1.6 x 38500 + 28000 = −33600cm⁻¹

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