trigonalbipyramidal

Trigonal bipyramidal is a molecular geometry in which five substituents are arranged around a central atom. In an ideal trigonal bipyramid, three ligands occupy the equatorial positions in one plane, separated by 120°, while two ligands occupy axial positions above and below the plane, aligned 180° apart.

Bond angles are 120° between equatorial ligands, 90° between axial and equatorial ligands, and 180° between the two axial ligands. Distortions occur when ligands differ in size or electronegativity; bulky ligands often prefer equatorial sites to minimize 90° interactions.

Electron-domain versus molecular geometry: TBP describes the arrangement of five electron domains around a central atom. If some domains are lone pairs, the resulting molecular geometries differ: AX4E1 yields a see-saw geometry, AX3E2 yields a T-shaped arrangement, and AX2E3 yields a linear geometry.

Fluxional behavior: many trigonal-bipyramidal molecules, such as PF5 and PCl5, exhibit Berry pseudorotation, a low-energy interconversion that exchanges axial and equatorial ligands without breaking bonds. This leads to rapid ligand exchange and apparent equivalence of certain positions at room temperature.

Occurrence and relationships: the trigonal-bipyramidal geometry is common for main-group elements in five-coordinate environments and for many transition-metal complexes. Some five-coordinate species adopt distorted TBP shapes, including see-saw or near-linear arrangements depending on ligand identity and electronic structure.

See also: square pyramidal geometry, a contrasting five-coordinate arrangement, and sp3d hybridization commonly invoked in valence bond descriptions of TBP structures.