Crystal Field Theory, Overview, Octahedral Complex, Crystal Field Stabilization Energy (For CBSE, ICSE, IAS, NET, NRA 2022)

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Crystal Field Splitting in Tetrahedral Complex

  • The splitting of fivefold degenerate d orbitals of the metal ion into two levels in a tetrahedral crystal field is the representation of two sets of orbitals as . The electrons in and orbitals are less repelled by the ligands than the electrons present in orbitals. As a result, the energy of orbital set are raised while that of the orbitals are lowered.
  • There are only four ligands in complexes and therefore the total negative charge of four ligands and hence the ligand field is less than that of six ligands.
  • The direction of the orbitals does not coincide with the directions of the ligands approach to the metal ion.
Crystal Field Splitting in Tetrahedral Complex

Crystal Field Splitting in Tetrahedral Complex

  • Thus, the repulsions in tetrahedral coordination compound yield two energy levels:
    • – set of three orbitals with higher energy
    • – set of two orbitals with lower energy
  • The crystal field splitting in a tetrahedral complex is intrinsically smaller in an octahedral filed because there are only two thirds as many ligands and they have a less direct effect of the d orbitals. The relative stabilizing effect of e set will be and the destabilizing effect of set will be +

Crystal Field Stabilization Energy

  • In a chemical environment, the energy levels generally split as directed by the symmetry of the local field surrounding the metal ion. The energy difference between the and levels is given as or It states that each electron that goes into the lower level stabilizes the system by an amount of and the electron that goes into level destabilizes the system by . That is the is lowered by and the level is raised by
  • For example, the net change in energy for and systems will be zero as shown below.

The decrease in energy caused by the splitting of the energy levels is called the “Ligand Field Stabilization Energy (LFSE) ” .

Crystal Field Stabilization Energy Table

The crystal field stabilization energies for some octahedral and tetrahedral complexes of 3d metal ions are tabulated below.

Crystal Field Stabilization Energy Table
Electronic ConfigurationOctahedral ComplexTetrahedral Complex
Weak Field Strong Field Weak Field Strong Field
  • Thus, the crystal field splitting depends on the field produced by the ligand and the charge on the metal ion.
  • An experimentally determined series based on the absorption of light by coordination compound with different ligands known as spectrochemical series has been proposed. Spectrochemical series arranges ligands in order of their field strength as:

  • Filling of d-orbitals takes place in the following manner; the first three electrons are arranged in level as per the Hund՚s rule. The fourth electron can either enter into level giving a configuration of or can enter the orbital giving a configuration of This depends on two parameters magnitude of crystal field splitting, and pairing energy, P. The possibilities of two cases can better be explained as:
    • Electron enters in the level giving a configuration of . Ligands producing this configuration are known as strong field ligands and form low spin complexes.
    • Electron enters in the level giving a configuration of Ligands producing this configuration are known as weak field ligands and form high spin complexes.

Question

What is the Basis of Crystal Field Theory?

Answer:

  • 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.

What is Crystal Field Theory and Its Limitations?

Answer:

  • The theory rules out the possibility of having p bonding.
  • This is a serious drawback because is found in many complexes.
  • The theory gives no significance to the orbits of the ligands.
  • Therefore, it cannot explain any properties related to ligand orbitals and their interaction with metal orbitals.

What Are the Important Features of Crystal Field Theory?

Answer:

  • Crystal field theory (CFT) is a bonding model that explains many important properties of transition-metal complexes, including their colors, magnetism, structures, stability, and reactivity.
  • The central assumption of CFT is that metal – ligand interactions are purely electrostatic in nature.

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