Capped square antiprismatic molecular geometry

In chemistry, the capped square antiprismatic molecular geometry describes the shape of compounds where nine atoms, groups of atoms, or ligands are arranged around a central atom, defining the vertices of a gyroelongated square pyramid. The symmetry group of the resulting object is C_4v

Capped square antiprismatic molecular geometry
Point group	C4v
Coordination number	9

The gyroelongated square pyramid is a square pyramid with a square antiprism connected to the square base. In this respect, it can be seen as a "capped" square antiprism (a square antiprism with a pyramid erected on one of the square faces).

It is very similar to the tricapped trigonal prismatic molecular geometry, and there is some dispute over the specific geometry exhibited by certain molecules. Examples:

• [SiCo₉(CO)₂₁]^2-, defined by the Co₉ framework, which encapsulates the Si atom
• [Pb(phen)₄(OClO₃)]⁺, defined by the N₈O framework, which encapsulates the Pb²⁺ ion
• [Ge₉]^4-, a zintl ion
• Th(troopolonate)₄(H₂O), defined by the O₉ framework, which encapsulates the Th⁴⁺ ion
• ReH²⁻
  ₉ is sometimes described as having a capped square antiprismatic geometry, although its geometry is most often described as tricapped trigonal prismatic.
• [LaCl(H
  ₂O)
  ₇]⁴⁺
  ₂, a lanthanum(III) complex with a La–La bond.^[1]

Bicapped square antiprismatic molecular geometry

Square antiprisms can be capped on both square faces, giving bicapped square antiprismatic molecular geometry. The bicapped square antiprismatic atoms surrounding a central atom define the vertices of a gyroelongated square bipyramid.^[2] The symmetry group of this object is D_4d.^[3]

Examples:

• B₁₀H₁₂, defined by the B₁₀ framework
• [AsRh₁₀(CO)₂₂]^3- and [SRh₁₀(CO)₂₂]^2-, defined by the Rh₁₀ framework, which encapsulates the main group atoms As and S
• [TlSn₈]^3-, a zintl ion

References

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 917. ISBN 978-0-08-037941-8.
2. King, R. Bruce (1993). Applications of Graph Theory and Topology in In Cluster and Coordination Chemistry. CRC Press. p. 102.
3. Remhov, Arndt; Černý, Radovan (2021). "Hydroborate as novel solid-state electrolytes". In Schorr, Susan; Weidenthaler, Claudia (eds.). Crystallography in Materials Science: From Structure-Property Relationships to Engineering. de Gruyter. p. 270. ISBN 978-3-11-067485-9.