Question

Calculate CFSE of K2[Fe(NCS)5] ?

Answer 1

In [Fe(CN)6]4−, iron has 3d6,4s2 system in ground state but in excited state it loses two electrons in the formation of ions and two electrons from 4s, so thus Cobalt gets 3d6 configuration. Now it is of low spin complex due to CN ligands so all 6 electrons will go to t2g orbitals. and 0 electrons will be in eg orbital. By applying formula,

 Δ = no. of electrons in t2g⋅(−0.4)+ no. of electrons in eg(0.6)

            = 6(−0.4)+0(0.6)

            = −2.4Δ0