Coordination geometry

Coordination geometry is the spatial arrangement of atoms or ligands around a central atom in a coordination complex or crystal structure. It is shaped by coordination number, bonding, and the surrounding structure.

Coordination number: The number of atoms or ligands directly bonded to a central atom in a coordination entity. It counts the neighbors whose positions define a coordination geometry.
Crystal field theory: A model that explains metal-ion energy levels by treating surrounding ligands as electric charges or dipoles. It predicts how ligand arrangement splits a metal ion’s d orbitals.
Tetrahedral molecular geometry: A molecular shape with four atoms or groups directed toward the corners of a tetrahedron around a central atom. Four-coordinate complexes often adopt this arrangement.
Trigonal prismatic molecular geometry: A six-coordinate molecular shape with ligands at the vertices of two parallel, aligned triangles. It competes with octahedral geometry as a possible six-coordinate arrangement.
Ligand: An atom, ion, or molecule that donates electron density to a central atom in a coordination complex. Ligands occupy the positions arranged around the central atom.
Ligand field theory: A theory describing bonding and electronic structure in transition-metal complexes using molecular orbitals. It relates geometry to metal–ligand bonding and electronic stabilization.
Square-planar molecular geometry: A molecular shape with four atoms or groups arranged at the corners of a square in one plane around a central atom. Many four-coordinate d8 metal complexes favor this alternative to tetrahedral geometry.
Berry pseudorotation: A rearrangement in which a trigonal-bipyramidal molecule interchanges axial and equatorial positions through a five-coordinate pathway. It makes some five-coordinate complexes dynamically interconvert between apparent geometries.
Chelation: The binding of a multidentate ligand to one metal center through two or more donor atoms. A ligand’s multiple attachment points can constrain the geometry it forms.
Jahn–Teller effect: A distortion of a nonlinear molecule that lowers its energy by removing an electronic degeneracy. It can distort ideal coordination geometries, especially in certain transition-metal complexes.

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