Discussion
The number of νS=O stretching vibration band(s) observed in the IR spectrum of the high-spin [Mn(dmso)6]3+ complex (dmso: dimethylsulfoxide) is
Answer: B
The spectrum shows two bands with intensity ratio 1:2.
High-spin Mn(III) is d4, t2g3eg1, with an odd electron in the eg set, so the complex is Jahn-Teller distorted: typically elongated along one axis. The six dmso ligands are then no longer equivalent. Two sit on the long axial bonds and four on the shorter equatorial bonds.
The S=O stretch of the O-bound dmso depends on how strongly the oxygen is held, so the two sets give two separate ν(S=O) bands, with intensities in proportion to the numbers of ligands, 2 axial to 4 equatorial, that is 1:2.
A single band would mean an undistorted octahedron with six equivalent ligands, which a high-spin d4 ion does not have. A 1:1 ratio would need equal numbers in the two sets, and six bands would need every ligand to be different.
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