The pair in which both species have same magnetic moment (spin only value) is [AIIMS 2006]

Weak field ligands do not allow pairing generally. H2O is a weak field ligand Hence, it will not pair the electrons. In the complexes given in option 2-Cr exist in +2 oxidation state and has a configuration of- 4 0 [3d ,4s ] Fe exist in +2 oxidation state and has a configuration of- 6 0 [3d ,4s ] As both have 4 unpaired electrons, the magnetic moment will be- n(n+2) 4(4+2) 24BM Hence, the value of magnetic moment will be same for both. Hence, the correct option is (b).