What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.

Spectro-chemical series is a series in which the ligands have been arranged in order of increasing magnitude of splitting they produce. The order is 

I– < Br^– < SCN^– < Cl^– < S^2– < F ^– < OH^– < C₂O₄^2– < H₂O < NCS^–< edta^4– < NH₃ < en < CN^– < CO

The ligand present on the R.H.S of the series are strong field ligand  while L.H.S are weak field ligand. Also, strong field ligand cause higher splitting in the d- orbitals than weak field ligand .

[Fe(CN)₆]^{4 –}

Outer electronic configuration of iron (Z = 26) in ground state is 3d⁶4s². Iron in this complex is in +2 oxidation state. Iron achieves +2 oxidation state by the loss of two 4s electrons. The resulting Fe²⁺ ion has outer electronic configuration of 3d⁶.
Since CN^- is strong field ligand thus cause pairing.


It has been experimentally observed that this complex is diamagnetic as such has no unpaired electron. To account for this two unpaired electrons in the 3d - subshell pair up, thus leaving two 3d-orbitals empty. These two vacant 3d-orbitals along with one 4s-orbital and three 4p-orbitals hybridise to give six equivalent d²sp³ hybridised orbitals. Six pairs of electrons, one from each cyanide ion, occupy the six vacant hybrid orbitals so produced. The resulting complex ion has an octahedral geometry and is diamagnetic due to the absence of unpaired electrons.

The degenerate d-orbitals (in a spherical field environment) split into two levels i.e., e_g and t_2g in the presence of ligands. The splitting of the degenerate levels due to the presence of ligands is called the crystal-field splitting while the energy difference between the two levels (eg and t2g) is called the crystal-field splitting energy. It is denoted by ∆_o.

Fig. d-orbital splitting in an octahedral crystal field.

The formation of complex depend on the crystal field splitting, ∆_o and pairing energy (P).

i)If ∆_o < P, the fourth electron enters one of the eg orbitals giving theconfiguration t_2g³. Ligands for which ∆_o < P are known as weak field ligands and form high spin complexes.

ii) If ∆_o > P, it becomes more energetically favourable for the fourth electron to occupy a t_2g orbital with configuration t^_2g4 e_g⁰. Ligands which produce this effect are known as strong field ligands and form low spin complexes.

Bonding in [FeF₆]^3–
The oxidation state of Fe is +3 and its coordination number is 6. The complex has octahedral geometry and experimental study shows that it is paramagnetic.

The bonding in this entity is explained on the basis of overlap of sp³d² hybrid orbitals of Fe³⁺ ion and six lone pair orbitals of cyanide ligands. Five 3d electrons are unpaired which make it paramagnetic. The outer electronic configuration of Fe³⁺ ion is 3d⁵ which is highly stable and no pairing of electrons takes place in presence of weak field of F^– ions. Here, 4d–orbitals of Fe³⁺ (which are empty) are involved.



The entity is strongly paramagnetic due to five unpaired electrons and is an outer orbital complex.

Fluorine ion is a weak ligand. It cannot cause the pairing of the 3d electrons. As a result, the Co³⁺ ion will undergo sp³d² hybridzation.

Hence, the geometry of the complex is octahedral and paramagnetic.