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The spectrochemical series is a classification system for ligands based on how well they interact with metal ions. The nature of the ligands is a critical component in determining whether a transition metal complex is high-spin or low-spin. The strength of the ligand’s interaction with the metal affects the d orbital energy splitting. Ligands that interact relatively weakly modify the d orbital energy levels very little, whereas ligands that interact strongly change the d orbital energy levels much more. When two comparable complexes with two different ligands are compared, a change in a band of the UV-Vis spectra occurs, giving rise to the spectrochemical series.
Spectrochemical Series:
A spectrochemical series is a list of ligands organised by “strength” and a list of metal ions organised by oxidation number, group and element. The ligands change the energy difference between the d orbitals for a metal ion, which is known as the ligand-field splitting parameter in ligand field theory or the crystal-field splitting parameter in crystal field theory.
I− < Br− < S2− < SCN– (S–bonded) < Cl− < N3− < F−< NCO− < OH− < C2O42–< O2−< H2O < acac− (acetylacetonate) < NCS– (N–bonded) < CH3CN < gly (glycine) < py (pyridine) < NH3 < en (ethylenediamine) < bipy (2,2’-bipyridine) < phen (1,10-phenanthroline) < NO2− (N–bonded) < PPh3 < CN− < CO
Ligands placed on the left end of this spectrochemical series are considered weaker ligands because they are unable to produce forced pairing of electrons within the 3d level, resulting in high spin outer orbital octahedral complexes. Low spin ligands, on the other hand, are stronger ligands that form inner orbital octahedral complexes after forced pairing of electrons within the 3d level.
According to “the spectrochemical series,” It is fundamentally backwards from what it should be for a realistic prognosis based on the assumptions of crystal field theory.
Crystal Field Splitting
Crystal field theory (or CFT) explains the significance of the spectrochemical series, explaining how ligands affect the energy levels of degenerate orbitals (orbitals with more than one distinct, measurable energy state) in coordination complexes.
It is assumed that metal electrons occupy appropriate atomic orbitals of the metal, and ligands are considered to be point negative charges. An electric charge is generated as a result of electrostatic interaction between metals and their ligands. Electrostatic interactions include the following:
It is the electrostatic attraction between negative electrons of the ligands and the positive metal nucleus.
Electrostatic repulsion between the electrons in the metal’s valence shell and those in the ligand.
The first step (a) results in a decrease in system energy and the second step (b) results in an increase in system energy. A degenerate orbital is split in half as a result.
Split size is referred to as the crystal field splitting parameter (CFSP) and is symbolised by Δ. In a strong-field ligand, electrons are strongly repelled from metals, producing large splits. Electrons are only partly repelled by weak-field ligands, which results in smaller splits. Partly dependent on the ligand is the size, but the geometry of the complex determines the size and number of energy splits most of all. To gain a better understanding of how ligand strength affects complexes, we will review octahedral and tetrahedral splits.
Difference between weak and strong field ligands
There was an establishment of the Spectrochemical series in 1938, which is a list of ligands listed by strength or a list of metal ions listed by oxidation number, group and element. In crystal field theory, it is known that ligands alter the energy difference between the d orbitals. This process is called crystal field splitting.
Weak field and strong field ligand
- crystal fields are less likely to split with weak field ligands. Strong spin complexes are more likely to form.
An example would be chloride ions and fluoride ions.
- Ligands with strong field properties split the crystal field more easily. These compounds have low spins.
Examples include Co and Cyanide ions.
Strong field ligands | Weak field ligands |
These ligands are used in octahedral complexes where the crystal field stabilisation energy *0 is greater than the pairing energy (p). | Some ligands are used in octahedral complexes where the stabilisation energy *0 is less than the pairing energy (p). |
These ligands contain C, N, and P as donor sites. | Atoms of X, O, and S serve as donors. |
Low spin complexes are also formed by these ligands. | Also known as high spin complexes, these compounds consist of these ligands. |
It is mostly diamagnetic or comparatively less paramagnetic when complexes form. | There are usually many paramagnetic complexes formed. |
Conclusion
The spectrochemical series, which dates back to 1938, is a list of ligands organised by strength, or a list of metal ions organised by oxidation number, group and elements. Ligands modify the energy difference between d orbitals in crystal field theory, a process called crystal field splitting.
Metals accept electrons, while ligands tend to donate electrons. Ligands fall into two categories: weak field ligands and strong field ligands. Weak field ligands, on the other hand, are those ligands that have low crystal field splitting and thus lead to high values. Strong field ligands produce a large crystal field splitting and the result is low spin values.
