4.14.2: Magnetic Moments
- Page ID
- 36219
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oct d3,d8 tet d2,d7 ←-----------------------------------→ oct d2,d7 tet d3,d8
Three peaks are predicted in their electronic spectra, namely:
- 4T2 ← 4A2 ν1
- 4T1(F) ← 4A2 ν2
- 4T1(P) ← 4A2 ν3 ** the band observed in the visible region
The energy of the third transitions is approximately ν3 = 6/5 Δ + 15B (where B is the Racah parameter and ignoring configuration interactions). For Co(II) tetrahedral complexes B has generally been found to be about 750 cm-1. Hence Δ for the three complexes above can be calculated to be roughly, Δ = (ν3 - 15x750) x 5/6, or:
Copyr2Cl2 (16260-15x750) x 5/6 = 4175 cm-1
Copyr2Br2 (15870-15x750) x 5/6 = 3850 cm-1
Copyr2I2 (14925-15x750) x 5/6 = 3063 cm-1
For three unpaired electrons the spin only magnetic moment is predicted to be 3.87 BM. Using a value for the free ion spin orbit coupling constant (λ) of -172 cm-1 then a better approximation of the magnetic moment can be obtained by using the third formula above. This would give a value of:
- Copyr2Cl2 3.87*(1+ 688/4175) = 4.49 B.M. found 4.42 BM
- Copyr2Br2 3.87*(1+ 688/3850) = 4.57 B.M. found 4.50 BM
- Copyr2I2 3.87*(1+ 688/3063) = 4.76 B.M. found 4.48 BM
In the case of the series;
CoI42-, CoBr42-, CoCl42-, Co(NCS)42-
the magnetic moments have been recorded as
4.77, 4.65, 4.59, 4.40 BM
showing even more clearly the inverse effect of the spectrochemical series on the magnetic moment.