When a molecule has an #sp^3d# hybridization with 2 lone pairs, the molecular geometry is T-shaped. Why is the T-shape preferable over a trigonal planar shape? (ie, why are the lone pairs positioned that way?)

1 Answer
Dec 20, 2017

Well, as always, it boils down to lone-pair-bonding-pair repulsions being greater in a hypothetical trigonal planar geometry.


With 5 electron groups, a T-shaped molecular geometry would have

https://chem.libretexts.org/

  • two #120^@# interactions (equatorial-equatorial)
  • one #90^@# interaction (equatorial-axial)

relative to each lone pair, whereas a hypothetical trigonal planar molecular geometry would have

Modified from https://chem.libretexts.org/

  • three #90^@# interactions (equatorial-axial)

relative to each lone pair.

Thus, the electrons are not maximally-spaced, and the T-shaped molecular geometry for a trigonal bipyramidal electron geometry is favored.