Explain crystal field theory of octahedral complexes
Answers
To determine the crystal field stabilization energy (CFSE) of metal complexes.
Theory:
CFT was proposed by the physicist Hans Bethe in 1929. Subsequent modifications were proposed by J. H. Van Vleck in 1935 to allow for some covalency in the interactions.
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Crystal Field Theory was developed to describe important properties of complexes (magnetism, absorption spectra, oxidation states, coordination,). The basis of the model is the interaction of d-orbitals of a central atom with ligands, which are considered as point charges. According to CFT, the attraction between the central metal and ligands in a complex is purely electrostatic.
The theory is developed by considering energy changes of the five degenerate d- orbitals being surrounded by an array of point charges consisting of the ligands. As a ligand approaches the metal ion, the electrons from the ligand will be closer to some of the d-orbitals and farther away from others, causing a loss of degeneracy. The electrons in the d-orbitals and those in the ligand repel each other due to repulsion between like charges. Thus, the d-electrons closer to the ligands will have a higher energy than those further away, which results in the d-orbitals splitting in energy.
This splitting is affected by the following factors:
• The nature of the ligands. The stronger the ligand, the greater is the splitting.
• Oxidation state of the central metal ion. A higher oxidation state leads to larger splitting.
• Size of d orbitals (i.e., transition series).
• Geometry of the complex.
The five d-orbitals are given the symbols dxy, dzx, dyz, dx2-y2 and dz2. In a complex they are all differently aligned relative to the incoming charge. Depending on the geometry of the complex, some of the d-orbitals will point directly towards the ligands, while some will point between them. Those which point at the ligands will experience more repulsion between their own electrons and those of the incoming ligands, than will those which do not point directly at them. Thus, the orbitals pointing at the ligands will be less stable and higher in energy. Now all the d-orbitals are no longer equivalent, giving rise to the phenomenon of orbital splitting, and the difference in energy between the more and less repelled orbitals is called the crystal field splitting parameter.
Angular dependence functions of d-orbitals
The electrons can go into either a high spin or low spin arrangement depending on the magnitude of the crystal field splitting energy.
High spin Low spin
The Spectrochemical Series:
The variation of the magnitude of the crystal field splitting (Δ) with the nature of the ligand follows a regular order, known as spectrochemical series. This series is given below in the order in which they produce increasing value of Δ.
I- < Br- < S22-< SCN- - < N3- < F- < urea, OH- < ox, O2- < H2O < NCS- < py, NH3 < en < bipy, phen < NO2- < CH3-, C6H5-< CN-
The octahedral arrangement of six ligands surrounding the central metal ion is as shown in the figure.
In an octahedral complex, the metal ion is at the centre and the ligands are at the six corners. In the figure, the directions x, y and z point to the three adjacent corners of the octahedran. The lobes of the eg orbitals (dx2-y2 and dz2) point along the x, y and z axis while the lobes of the t2g orbitals (dxy, dzx and dyz)point in between the axes. As a result, the approach of six ligands along the x, y z, -x,-y and –z directions will increase the energy of dx2-y2 and dz2 orbitals (which point towards the ligands) much more than that it increases the energy of dxy, dzx and dyz orbitals ( which point in between the metal-legand bond axis).
Thus, under the influence of an octahedral field, the d orbitals split into triply degenerate orbitals with less energy and another as doubly degenerate orbitals with higher energy. The main energy level between these two sets of orbitals is taken as zero, which is called bari centre. The splitting between these two orbitals is called crystal field splitting. The magnitude of stabilization will be 0.4 Δo and the magnitude of destabilization will be 0.6 Δo.
The magnitude of the splitting of the t2g and eg orbitals changes from one octahedral complex to another. It depends on the identity of the metal ion, the charge on this ion, and the nature of the ligands coordinated to the metal ion.
Answer:
Explain crystal field splitting in octahedral complexes.