Coordination Compounds: Effect of an Octahedral Field on the D Orbitals (For CBSE, ICSE, IAS, NET, NRA 2022)

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Crystal Field Theory

  • Crystal field theory is a model of the electronic structure of transition-metal complexes that considers how the energies of the d orbitals of a metal ion are affected by the electric field of the ligands.
  • According to this theory, the ligands in a transition-metal complex are treated as point charges.
  • A ligand anion simply becomes a point of negative charge.
  • A neutral ligand, with its electron pair that it donates to the metal atom, is replaced by a partial negative charge.
  • In an electric field of these negative charges, the five d orbitals of the metal atom no longer have exactly the same energy.

Effect of an Octahedral Field on the D Orbitals

  • All five d orbitals of an isolated metal atom have the same energy.
  • If the atom is brought into the electric field of several point charges, these d orbitals may be affected in different ways and therefore may have different energies.
  • Figure shows the shapes of the five d orbitals.
Effect of an Octahedral Field on the D Orbitals
  • The orbital labelled has a dumbbell shape along the z-axis, with a collar in the x – y plane surrounding this dumbbell.
  • This shape represents the volume most likely to be occupied by an electron in an orbit.
  • The other four d orbitals have “cloverleaf” shapes, each differing from one another only in the orientation of the lobes in space.
  • The “cloverleaf” orbital has its lobes along the x- axis and y-axis.
  • Orbitals , , and have their lobes directed between the two set of axes designated in the orbital label. for example, has lying its lobes between x-axis and y-axis.
  • A complex ion with six ligands will have the ligands arranged octahedrally about the metal atom.
  • The bonding in a complex is due to the attraction of the positive metal ion for the negative charges of the ligands.
  • But an electron in a d-orbital of the metal atom is repelled by the negative charge of ligands.
  • When ligands approach along the , , and axes, electrons in the orbital will be repelled but from the diagram we can see that the effect will be greater for the and orbitals because these two orbitals have lobes lying along the line of approaching ligands.
Effect of an Octahedral Field on the D Orbitals
  • The 3d levels are split into an upper group of two e. g. and a lower group of three (trebly degenerate and labelled ) , (doubly degenerate and labelled.
  • Now consider the example of a transition metal ion with only one 3d electron surrounded octahedrally by six ligands, e. g. .
  • Then, this single 3d electron will normally occupy one of the three degenerate lower levels .
  • In order to transfer this electron into an upper level (e. g.) radiation of the appropriate frequency must be supplied.

  • Transition metal ions are colored because radiation in the visible spectrum is of the right frequency to promote this electronic transition.

For a given transition series, the value of radiation depends upon:

  • Charge carried by the central transition metal ion.
  • The nature of the ligand.
  • The nature of transition metal ion

For transition metal ions carrying the same charge, the value of radiation increase in the order.

Magnetic Properties by Crystal Field Theory

  • Consider the octahedral complexes and .
  • The electron configuration of is , and there are two possible ways to distribute the five d electrons among the d orbitals.
Magnetic Properties by Crystal Field Theory
  • The d orbitals are split into two groups i.e.. and .
  • If the value of is small, then high-spin complex is formed.
  • If the value is large, then complex will be of low spin type.
  • High-spin complexes are more paramagnetic than low-spin complexes.

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