pseudooctahedral

Pseudooctahedral is a term used in coordination chemistry to describe a six-coordinate metal center whose ligand arrangement closely resembles octahedral geometry but is not perfectly octahedral. The overall arrangement is near octahedral, yet noticeable distortions reduce the symmetry from the ideal Oh.

In pseudooctahedral species, the six ligands occupy positions that are approximately the corners of an octahedron, but there can be deviations such as unequal metal–ligand bond lengths or angles. Common distortions include elongation or compression along a particular axis, leading to symmetries such as D4h, C4v, or lower.

Causes of pseudooctahedral geometry include electronic and steric factors. Jahn-Teller distortions in certain electronic configurations (for example, some d8, d9, or high-spin d4 cases) can elongate or compress along one axis. Steric crowding from bulky ligands, bite angles imposed by chelating ligands, and crystal packing effects in the solid state can also produce pseudo-octahedral shapes.

Characterization of pseudooctahedral coordination is typically through X-ray crystallography, which reveals deviations from ideal bond lengths and angles. Quantitative measures, such as distortion indices or continuous shape measures, help distinguish near-octahedral from more severely distorted geometries.

Relationship to other terms: pseudooctahedral is a descriptive label for near-octahedral, six-coordinate environments. It is related to, but distinct from, simply “distorted octahedral”; authors may use the terms interchangeably in some contexts, while others reserve pseudooctahedral for specific patterns of distortion.

See also: octahedral coordination, distorted octahedral, Jahn-Teller effect, coordination chemistry.