Kvadratpyramidal

Kvadratpyramidal (square pyramidal) is a molecular geometry in which a central atom is bonded to five ligands forming a square base with one ligand at the apex, giving a square-pyramidal shape around the central atom. This arrangement arises when there are five bond pairs and one lone pair of electrons around the central atom, corresponding to AX5E in VSEPR terminology and an octahedral electron-domain geometry.

In the idealized model, four ligands define a square plane, while the fifth ligand sits above this plane at the apex of a pyramid. The geometry yields near-90-degree angles between most ligands, particularly between the apex ligand and the base ligands, though real molecules show distortions. The presence of the lone pair causes repulsion that distorts bond angles and lengths from those of a perfect square pyramid, often leading to slightly unequal bond distances among the ligands.

Common examples of square-pyramidal molecules include chlorine pentafluoride (ClF5) and bromine pentafluoride (BrF5), in which the central halogen is surrounded by five ligands with one lone pair occupying the remaining octahedral position. The geometry is also encountered in certain coordination complexes of transition metals and main-group elements, where ligand fields and steric effects influence the exact arrangement.

Related concepts include octahedral and seesaw geometries, which describe other ways five or six electron domains can organize around a central atom. The square-pyramidal shape has implications for molecular dipole moments, reactivity, and ligand binding, and it can interconvert with distorted octahedral geometries under changes in ligation, temperature, or pressure.