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The centre of a coordination compound is a metal atom or ion, which is surrounded by a number of oppositely charged ions or neutral molecules. These ions or molecules are linked to the metal atom or ion through a coordinate relationship. They do not break down into simple ions when dissolved in water.
The static electric field breaks d- or f-subshell degeneracy when the ligands approach the core metal ion. Because electrons oppose one another, the d electrons closest to the ligands have higher energy than those further away, causing the d orbitals to split. The stability that occurs from ligand binding is known as the crystal field stabilisation energy (CFSE).
Crystal Field Splitting Theory
The breaking of degeneracies of electron orbital states, commonly the d or the f orbitals, due to a static electric field produced by a surrounding charge distribution is described by crystal field theory (CFT). The different spectroscopies of transition metal coordination complexes, mostly the optical spectra, are described using this idea. CFT can explain some magnetic properties, spinel structures, and hydration enthalpies of transition metal complexes, but it can not explain the bonding. After that, CFT was merged with molecular orbital theory to create the more realistic and complicated ligand field theory (LFT), which sheds light on the chemical bonding mechanism in transition metal complexes.
Postulates of Crystal Field Splitting Theory
The following are the fundamental assumptions of crystal field theory.
Crystal field theory is an electrostatic approach to a coordination complex that consists of a core metal ion surrounded by an anion or ligand cage.
The core metal atom is considered to be a cation, with a charge equal to its oxidation number.
The ligands, which are either negatively charged anions or neutral molecules with lone pairs of electrons, are referred to as point charges.
A field of anions surrounds the core metal cation.
The bonding between metal cations and ligands in complexes is solely electrostatic.
Because the electrons of the metal cation and the ligand do not mix, there is no orbital overlap between the metal and the ligands, crystal field theory ignores the covalent interaction between the metal and the ligand.
All 5d orbitals in an isolated metal ion have the same energy and are referred to as degenerate orbitals.
The electrons of metal ions and those of ligands repel each other as the ligand approaches the core metal cation by its negative end. Because of this repulsion, the d-orbital splits into two sets of d-orbitals with differing energies, eg and t2g. Crystal field splitting is the splitting of a d-orbital under the influence of a ligand’s electrostatic field, and the ramifications lie at the heart of Crystal field theory.
Crystal field splitting energy is defined as the difference in energy between the eg and t2g orbitals.
Spectrochemical Series
A spectrochemical series is an arrangement of ligands in order of their capacity to generate splitting.
I– < Br– < S2- < Cl– < F– < OH– < C2O42- < O2- < H2O < NCS– < NH3 < en < CN– < CO
The spectrochemical series is a result of experimentation. The sequence is difficult to explain since it includes both the influence of 𝛔 and 𝚷 bonding.
Octahedral Crystal Field Splitting
Six ligands are connected to the central transition metal in an octahedral complex. The d-orbital is divided into two levels. dxy, dxz, and dyz are the names of the bottom three energy levels (collectively referred to as t2g ). dx2-y2 and dz2 are the names of the two highest energy levels(referred to as eg).
Electrostatic interactions between the electrons of the ligand and the lobes of the d-orbital lead them to separate. The electrons in an octahedral are attracted to the axes. Any orbital with a lobe on the axes advances to the next energy level. This means that in an octahedral, eg has a higher energy level (0.6𝚫o), while t2g has a lower energy level (0.4𝚫o). The distance that electrons must travel from t2g to eg determines the amount of energy that the complex will absorb from white light and thus the hue. The spin state will decide whether the compound is paramagnetic or diamagnetic. The complex is paramagnetic if there are unpaired electrons; the complex is diamagnetic if all electrons are paired.
Tetrahedral Complex
There are four ligands linked to the core metal in a tetrahedral compound. The d orbitals are similarly divided into two energy levels. The dxy, dxz, and dyz orbitals make up the top three. The dx2-y2 and dz2 orbitals make up the bottom two. This is caused by a lack of orbital overlap between the metal and ligand orbitals. The ligands are not directed on the axes, but the orbitals are.
The tetrahedral splitting constant (𝚫t), which is less than (𝚫o) for the same ligands, is the difference in splitting energy: 𝚫t = 0.44 𝚫o
As a result, 𝚫t is usually smaller than the spin pairing energy, resulting in high spin tetrahedral complexes.
Advantages of Crystal Field Theory
The crystal field theory has a number of advantages, which are stated below.
The stability of complexes can be described using this idea. The more crystal field splitting energy there is, the more stable the system will be.
Complexes’ colour and spectrum can be explained using this approach.
The magnetic characteristics of complexes are explained by this hypothesis.
Limitations of Crystal Field Splitting
Crystal field theory has a number of flaws, which are described below.
The notion that the interaction between the metal and the ligand is exclusively electrostatic is far from realistic.
This hypothesis exclusively considers the d-orbitals of the core atom. The s and p orbitals aren’t studied at all.
This idea is unable to explain why some metals exhibit considerable splitting while others exhibit minimal splitting.
Because P bonding is observed in many complexes, this theory fails to explain how it is possible to have p bonding.
The orbitals of the ligands have no significance in this view. As a result, it is unable to explain any ligand orbital characteristics or their interactions with metal orbitals.
Conclusion
The interaction of the ligands with the core metal atom or ion is governed by crystal field theory. The crystal field splitting is also called ligand field splitting. Crystal field splitting is the energy differential between ligand d orbitals. Crystal field splitting explains the colour difference between two equivalent metal-ligand complexes. There is both octahedral and tetrahedral crystal splitting. The splitting energy of the tetrahedral complex is 0.44 times more than the octahedral complex. Even Though there are many advantages for this theory there are also some limitations. One of the main limitations is that CFT only considers the d orbitals only and completely avoids the s and p orbitals.
