Subject regular octahedron

Stereochemistry.3,21 The d9 configuration makes Cu11 subject to Jahn-Teller distortion (page 592) if placed in an environment of cubic (i.e. regular octahedral or tetrahedral) symmetry, and this has a profound effect on all its stereochemistry. With only one possible exception, to be mentioned below, it is never observed in these regular environments. When 6-coordinate, the octahedron is severely distorted, as indicated by the data in Table 25-H-2. 

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