[MnCl6] 3- is high spin and has five unpaired electrons while [Mn(CN)6] 3- has only two unpaired electrons.
A complex may be low spin or high spin depending on the kind of ligand attached to the central metal atom/ion. If the ligand is a weak field ligand, the complex may be high spin (maximum number of unpaired electrons). If the complex is low spin, there are few unpaired electrons (minimum number of unpaired electrons). In that case, the ligand is a strong field ligand.
In the octahedral geometry, [MnCl6] 3- is high spin and has five unpaired electrons since the chloride ion is a weak field ligand. On the other hand [Mn(CN)6] 3- has only two unpaired electrons because the cyanide ion is a strong field ligand.
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