[Mn(H2O)6]2+⇒ CFSE value is zero because of d5 configuration with Weak Field Ligands in coordination number 6 [Cr(H2O)6]2+.
[Cr(H2O)6]2+⇒CFSE value is −0.6Δ0 because of d4 configuration with Weak Field Ligands in coordination number 6.
For : K3[Fe(CN)6],μ=1(1+2)=3 B.M.
For : Na4[Fe(CN)6],μ=0 B.M.