What kinds of electron configurations can we get with octahedral and tetrahedral #"Co"^(2+)# complexes?

1 Answer
Mar 24, 2017

Answer:

WARNING! Long answer! The octahedral complexes are either #t_text(2g)^6 color(white)(l)e_text(g)# (low-spin) or #t_text(2g)^4color(white)(l) e_text(g)^3# (high-spin).

Explanation:

Almost all the tetrahedral complexes are #e_text(g)^4color(white)(l) t_text(2g)^3# (high-spin).

Crystal Field Theory

In an octahedral field, the the five degenerate #"d"# orbitals are split into two groups:

  • two high-energy orbitals, designated as #e_g#
  • three low energy orbitals, designated as #t_2g#.

scienceworld.wolfram.com

The difference between the energy levels is #Δ_text(o)#.

The value of #Δ_text(o)# depends on both the metal and the nature of the ligands.

In a tetrahedral field, the energy levels are reversed.

The #"d"# orbitals split into:

  • three #t_2g# high-energy orbitals
  • two #e_g# low-energy orbitals

Octahedral #"Co"^"2+"# complexes

The electron configuration of #"Co"^"2+""# is #"[Ar]3d"^7#.

When we have a strong-field ligand like #"CN"^"-"#, #Δ_text(o)# is relatively large.

The extra electrons occupy the lower #t_(2g)# set of orbitals before they go into the upper #e_g# levels.

High/low-spin
(Adapted from chemed.chem.purdue.edu)

When we have a weak-field ligand like #"H"_2"O"#, #Δ_text(o)# is relatively small.

The electrons fill the orbitals in a Hund's Rule type of order and pair up only when they have no other choice.

#["Co"("H"_2"O")_6]^"2+"# has the maximum number of half-filled orbitals and is called a high-spin state.

upload.wikimedia.org

A high-field ligand like #"CN"^"-"# causes #Δ_text(o)# to be so large that the lower orbitals are filled in before the higher orbitals.

Thus, #["Co"("CN")_6]^"4-"# has a filled set of low-energy orbitals and is called a low-spin state.

Tetrahedral #"Co"^"2+"# complexes

#Δ_text(t)# is about half the size of #Δ_text(o)#, so almost all tetrahedral complexes are weak field/high spin.

scienceworld.wolfram.com

Thus, tetrahedral #"Co"^"2+"# complexes have an #e_text(g)^4color(white)(l)t_text(2g)^3# configuration.

chemiris.labs.brocku.ca