The calculated spin-only magnetic moments B M of the anionic and cationic species of Fe H 2 O 6 + 2 and Fe CN 6 4 - respectively, are:

Step-by-step solution

In both cases iron exhibits + 2 oxidation state. Electronic configuration of F e + 2 ion = [ A r ] 3 d 6 In [ F e ( H 2 O ) 6 ] 2 + ,   H 2 O is a weak field ligand and hence, no pairing up. ∴ number of unpaired electron n = 4 ⇒ Spin magnetic moment = 4 ( 4 + 2 ) = 24 = 4.9   B M In [ F e ( C N ) 6 ] 4 - , C N - is a strong field ligand and hence, pairing will occur. ∴ number of unpaired electron n = 0 ⇒ spin magnetic moment = zero