Decreasing order of unpaired electrons in low spin octahedral complexes
Q. Consider the transition metal ions Mn3+, Cr3+, Fe3+ and Co3+ and all form low spin octahedral complexes. The correct decreasing order of unpaired electrons in their respective d-orbitals of the complexes is
A. Cr3+ > Mn3+ > Fe3+ > Co3+
B. Mn3+ > Fe3+ > Co3+ > Cr3+
C. Fe3+ > Co3+ > Mn3+ > Cr3+
D. Cr3+ > Fe3+ > Co3+ > Mn3+
Correct Answer: Cr3+ > Mn3+ > Fe3+ > Co3+

Explanation

First, determine the electronic configuration of each metal ion.

Cr (Z = 24):
Cr3+ → [Ar] 3d3
Low spin octahedral configuration: t2g3 eg0
Number of unpaired electrons = 3

Mn (Z = 25):
Mn3+ → [Ar] 3d4
Low spin octahedral configuration: t2g4 eg0
Number of unpaired electrons = 2

Fe (Z = 26):
Fe3+ → [Ar] 3d5
Low spin octahedral configuration: t2g5 eg0
Number of unpaired electrons = 1

Co (Z = 27):
Co3+ → [Ar] 3d6
Low spin octahedral configuration: t2g6 eg0
Number of unpaired electrons = 0

Thus, the decreasing order of unpaired electrons is:

Cr3+ > Mn3+ > Fe3+ > Co3+

This is a core concept from Crystal Field Theory and is very important for JEE Main 2026, JEE Advanced and IIT JEE.

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