What is the geometry and hybridization of "AuCl"_4^-?

1 Answer
May 26, 2017

Warning! Long Answer. The "AuCl"_4^(-) ion is square planar and "dsp"^2 hybridized.

Explanation:

CRYSTAL FIELD THEORY

Crystal field theory postulates that

  • "d" orbitals pointing directly at an axis are most destabilized by electrostatic interactions with a ligand.
  • "d" orbitals pointing away from an axis are least destabilized by electrostatic interactions with a ligand.

Crystal FieldCrystal Field
(From www.slideshare.net)

For example, in an octahedral field, the diagram above shows that:

  • the "d"_(z_2) and "d"_(x^2-y^2) orbitals are most destabilized.
  • the "d"_text(xy), "d"_text(xz), and "d"_text(yz) orbitals are least destabilized.

In a square planar complex, the four ligands are only in the xy plane, so any orbital in the xy plane has a higher energy level.

The absence of ligands along the z-axis relative to an octahedral field stabilizes the "d"_(z^2), d_(xz), and d_(yz) levels, and leaves the "d"_(x^2-y^2) level the most destabilized.

When you work it out, there turns out to be four different energy levels in a square planar field:

upload.wikimedia.orgupload.wikimedia.org

The "d"_(x^2-y^2) level is at such a high level that it remains unoccupied in almost all "d"^8 complexes.

bb("AuCl"_4^-)

(a) The electron configuration of "Au" is "[Xe] 6s"^1 "4f"^14 "5d"^10.

(b) The electron configuration of "Au"^"3+" is "[Xe] 6s"^0 "4f"^14 "5d"^8.

dsp2 hybridizationdsp2 hybridization

(c) As the 4 "Cl"^(-) ions, which are weak-field ligands, approach the "Au"^"3+" ion in preparation for bonding, the "5d" electrons pair up since repulsions are minimal and the crystal-field splitting energy is relatively small.

(d) The "Au"^"3+" ion can then combine its vacant "5d"_(x^2-y^2), 6s", "6p_x" and "6p_y orbitals to create four new equivalent, hybridized "dsp"^2 orbitals.

These orbitals can accept a lone pair from each of the chloride ions and form "Au-Cl" bonds.

The bonds point to the corners of a square, forming a square planar molecular geometry.

Square planarSquare planar
(From www.chemtube3d.com)