Trigonal pyramidal molecular geometry

Trigonal pyramidal molecular geometry
ExamplesNH3
Point groupC3v
Coordination number3
Bond angle(s)90° to 109.5°
μ (Polarity)>0

In chemistry, a trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base. When all three atoms at the corners are identical, the molecular symmetry is C3v. In the AXE method of VSEPR theory this geometry is classified as AX3E1.


Selected inorganic cases

Inversion of ammonia

With three hydrogen ligands and a nonbonded pair of valence electrons, ammonia is a prototypical trigonal pyramid. The molecule undergoes rapid intramolecular inversion. More so than ammonia, phosphine (PH3) is highly pyramidal, with H-P-H angles near 93°. Inorganic compounds related to ammonia and phosphine are numerous, common examples are chloramines and phosphorus trichloride.

xenon trioxide (XeO3), the chlorate ion, ClO−3, and the sulfite ion, SO3^{−2}, are examples of pyramidal inorganic species with multiple bonds.

Selected organic cases

In organic chemistry, atoms at the center of a trigonal pyramid are sometimes described as being sp3 hybridized. For example, amines (like ammonia) feature pyramidal nitrogen centers. Nitrogen in such cases is described as being sp3 hybridized.

Amides (RC(O)NR'2) almost always feature trigonal planar nitrogen. Distortions from that geometry, which have been parameterized, relate to the hydrolytic stability of the amide (RC(O)-NR'2). The significance of this pyramidalization is shown by the mode of action of the penicillins.

Phosphines, with the formula PR3, are pyramidal but less so than phosphine (PH3) itself. Unlike ammonia and amines, phosphines do not readily invert, i.e. pyramidal inversion is subject to a high barrier. One consequence of their rigidity is that chiral phosphines are optically stable.

Examples of pyramidal organic compounds with multiple bonds are sulfoxides.

Radicals

Although they are often invoked, radicals with the formula ·AX3 can adopt pyramidal structures. The methyl radical (·CH3) is flexible but trigonal planar, but trifluoromethyl radical is pyramidal.

See also

References

  1. ↑ Cleeton, C. E.; Williams, N. H. (1934). "Electromagnetic Waves of 1.1 cm Wave-Length and the Absorption Spectrum of Ammonia". Phys. Rev. 45 (4): 234. Bibcode:1934PhRv...45..234C. doi:10.1103/PhysRev.45.234.
  2. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. doi:10.1016/C2009-0-30414-6. ISBN 978-0-08-037941-8.
  3. ↑ Winkler, F.K.; Dunitz, J.D. (1971). "The non-planar amide group". Journal of Molecular Biology. 59 (1): 169–182. doi:10.1016/0022-2836(71)90419-0. PMID 5283751.
  4. ↑ Yamada, Chikashi; Hirota, Eizi; Kawaguchi, Kentarou (1981). "Diode laser study of the ν2 band of the methyl radical". The Journal of Chemical Physics. 75 (11): 5256–5264. doi:10.1063/1.441991.
  5. ↑ Fessenden, Richard W.; Schuler, Robert H. (1965). "ESR Spectra and Structure of the Fluorinated Methyl Radicals". The Journal of Chemical Physics. 43 (8): 2704–2712. doi:10.1063/1.1697199.