Question

What are strong field and weak field ligands? Give one example of each.

Answer 1

The ligands are then classified as (a) strong field and (b) weak field ligands. Strong field ligands are those in which donor atoms are C, N or P. Thus CN^-, NC^-, CO, HN₃, EDTA, en (ethylenediammine) are considered to be strong ligands. They cause larger splitting of d orbitals and pairing of electrons is favoured. These ligands tend to form low spin complexes. Weak field ligands are those in which donor atoms are halogens, oxygen or sulphur.

For example,

F^-, CI^-, Br^-, I^-, SCN^-, C₂O₄^2-. In case of these ligands the A0 parameter is smaller compared to the energy required for the pairing of electrons, which is called as electron pairing energy. The ligands then can be arranged in order of their increasing field strength as

I^- < Br^- < CI^- < S^2- < F^- < OH^- < C₂O₄^2- < H₂O < NCS < EDTA < NH₃ < en < CN^- < CO.