Trigonal Pyramidal Arrangement

The trigonal pyramidal arrangement of amines and ammonia is slightly flattened, with a lone pair of electrons above the nitrogen atom. In the quaternary ammonium ions, this area can be occupied by the fourth substituent.

In chemistry, there exists both Trigonal pyramidal molecular geometry and Trigonal bipyramidal molecular geometry. Let us discuss these two in this section.

Trigonal Pyramidal Molecular Geometry

The trigonal pyramid is a molecular geometry that resembles a tetrahedron that has one atom at the apex and three atoms at the trigonal base corners. The molecule belongs to point group C₃v because all three atoms present at the corners are equal. A few ions and molecules having trigonal pyramidal geometry are given as the xenon trioxide (XeO₃), pnictogen hydrides (XH₃), and the sulfite ion, SO₂⁻³. In organic chemistry, molecules that hold a trigonal pyramidal geometry are at times described as sp³ hybridized. The AXE method for the VSEPR theory indicates that the classification is AX₃E₁.

Trigonal Pyramidal Geometry in Ammonia

The nitrogen in the ammonia molecule has 5 valence electrons and bonds with three hydrogen atoms for octet completion. This would result in the regular tetrahedron geometry with each bond angle equal to around ≈ 109.5°. But, the three hydrogen atoms can be repelled by the electron lone pair in a method that the geometry can be distorted to a trigonal pyramid with bond angles of 107°. By comparison, boron trifluoride is a flat compound, adopting a trigonal planar geometry due to the fact that the boron does not hold a lone pair of electrons. Also, in ammonia, the trigonal pyramid undergoes a rapid nitrogen inversion.

Axial (or Apical) and Equatorial Positions

The five atoms bound to the central atom are not identical, and two types of positions are identified. For example, in phosphorus pentachloride, the phosphorus atom also shares a plane with three chlorine atoms in equatorial positions at an angle of 120°, with two additional chlorine atoms above and below the plane (either axial or apical positions).

An axial location is claimed to be more crowded by the VSEPR principle of molecular geometry since the axial atom contains three adjacent equatorial atoms (on a single central atom) at a bond angle of 90°. At the same time, an equatorial atom contains only two neighbouring axial atoms at a bond angle of 90°. For the molecules having five identical ligands, the axial bond lengths tend to be longer due to the ligand atom being unable to approach the central atom closely. For eg, the axial PF bond length in PF₅ is 158 pm, while the equatorial is 152 pm, and the equatorial and axial bond lengths in PCl₅ are 202 and 214 pm, respectively.

In the mixed halide of PF₃Cl₂, the chlorine compounds occupy two of the equatorial positions, indicating that fluorine holds a greater tendency or apicophilicity to occupy an axial position. Generally, the ligand apicophilicity increases with electronegativity and with pi-electron withdrawing ability, as the sequence, which can be given as Cl < F < CN. All of these effects reduce the electron density in the bonding area near the central atom, reducing the importance of crowding in the axial position.

Related Geometries with Lone Pairs

In addition, the VSEPR principle assumes that a lone pair of valence electrons would replace a ligand at the central atom, leaving the typical structure of the electron configuration unchanged with the lone pair occupying one position now. For molecules having five pairs of valence electrons along with both lone pairs and bonding pairs, the electron pairs are arranged in a trigonal bipyramid still, but either one or more equatorial positions is not attached to the ligand atom. Hence, the molecular geometry is different only for the nuclei.

The seesaw molecular geometry type can be found in the sulfur tetrafluoride (SF₄) with a central sulfur atom that is surrounded by four fluorine atoms occupying two equatorial and two axial positions and one lone equatorial pair as well, corresponding to an AX₄E molecule, present in the AXE notation.

And a T-shaped molecular geometry can be found in the chlorine trifluoride (ClF₃), an AX₃E₂ molecule with the fluorine atoms, available in two axial positions and one equatorial position, and two lone equatorial pairs as well. Finally, the triiodide ion (I⁻³) also depends upon the trigonal bipyramid, but the exact actual molecular geometry is resulted as linear with the terminal iodine atoms in two axial positions only and three equatorial positions, which are occupied by lone pairs of electrons (AX₂E₃). Another example of the same type of geometry is given by XeF₂ and xenon difluoride.