Which of the following complex has highest ∆o value?  (1) [Fe(H2O)6]2+,   (2) [Fe(H2O)6]3+, (3) [Fe(CN)6]3-,        (4) All have equal values.

Factors Affecting Δ_o:

1. Geometry of the Complex:  Δ_t is approximately 4/9 times of Δ_o . The lower value of Δ_t is due to lesser number of ligands in tetrahedral complex. 

Given three complxes are octahedral.

2. Oxidation state of metal ion: It is observed that the higher the charge on the central metal atom (or oxidation state), the higher the CFSE.

e.g., Δ_o for [Fe(CN)₆]^3- and [Fe (H₂O)₆]³⁺ is greater than [Fe (H₂O)₆]²⁺

3. Nature of ligand: The value of Δ_o depends upon the nature of ligands.

Some ligands in spectro chemical series are given below:

I^––2– < Cl^– < N₃^–, F^– < Urea, OH^– < Oxalate, O^2– < H₂O < NCS^– < EDTA < py, NH₃ < en =SO₃^-2 < bipy, phen < NO₂^– < CH₃^– < C₆H₅^– < CN^– < CO.

For strong field ligands, the order depends on the donor atom and is in the following order:

C-donor > N-donor > O-donor > Halogen donor

 

In given compounds, [Fe(CN)₆]^3- and [Fe (H₂O)₆]³⁺ , [Fe(CN)₆]^3-  has strong field ligand CN^– hence it has greater Δ_o than [Fe (H₂O)₆]³⁺

Complex [Fe(CN)₆]^3- have greater Δ_o than rest of given complexes because the charge on central metal ion is +3 and the central metal ion is bonded to a strong ligand CN^–