Question

Both Fand [CN]- are negatively charged ligands, yet F - generates a weak crystal field whereas [CN]- generates a strong crystal field. Considering octahedral first-row transition element complexes, show how this observation may be explained in terms of the properties of these ligands as -donors or -acceptors

Answer 1

Solution:

CN- is a strong field ligand (more π-donor) while F- is a weak field ligand (less π-donor). Thefore, CN- forms inner orbital complex having low spin complex and F- ligand forms outer orbital complex having high spin complex.

For example: The complexes of iron formed from ligands CN- and F- are inner and outer orbital complexes due to low spin and high spin complexes respectively.

[Fe(CN)6]^4- is an inner orbital complex having d2sp3 hybridization. It is formed by π-acceptor Fe2+ ion and π-donor CN- ligand.

[Fe(H2O)6]^2+ is an outer orbital complex formed by weak F- ligand (π-donor) and π-acceptor Fe2+ ion having sp3d2 hybridization.