Jahn-Teller distortion square

The series of 3d elements from scandium to iron as well as nickel preferably form octahedral complexes in the oxidation states I, II, III, and IV. Octahedra and tetrahe-dra are known for cobalt, and tetrahedra for zinc and copper . Copper(II) (d9) forms Jahn-Teller distorted octahedra and tetrahedra. With higher oxidation states (= smaller ionic radii) and larger ligands the tendency to form tetrahedra increases. For vanadium(V), chromium(VI) and manganese(VII) almost only tetrahedral coordination is known (VF5 is an exception). Nickel(II) low-spin complexes (d8) can be either octahedral or square. 

The Cupric, Cu2+ or Cu(II) State, 3d9 The most important and stable oxidation state for copper is divalent. There is a well-defined aqueous chemistry of the Cu2+ ion, which generates the familiar blue solution when complexed with water. A large number of copper coordination compounds exist and these have been studied extensively. A strong Jahn-Teller distortion is associated with the 3d9 electronic configuration of this ion. This implies that a regular tetrahedron or octahedron about the Cu2+ ion is never observed, except in the rare occurrence of a dynamic Jahn-Teller effect. The tetragonal distortion about an octahedron can lead to a square-planar coordination which is often observed in Cu(II) oxides. 

This latter distortion is associated with the strong Jahn-Teller effect of the Cu2+ ion. The Jahn-Teller distortion can result in the formation of an elongated octahedron (4 short + 2 long) a square pyramid (4 short + 1 long) or quite commonly a square plane (4 short). 

This effect is observed, for example, in inorganic complexes, where the metal ion is octahedral and the six ligands might be expected to occupy positions at equal distances at the corners of an octahedron, in an ideal structure. In the actual structure however, the octahedron may be distorted with four ligands in a square planar coordination and the remaining two ligands at opposite corners at longer distances. Many oxides exhibit Jahn-Teller distortions. 

Nonexistent atoms used for the definition of a structure if the program does not allow a specific connectivity (e. g., coordination numbers greater than four) or if certain interactions and geometries (e. g., Jahn-Teller distortions, planarity of a system such as a square planar nickel(II) complex) can not be modeled in another way. 

Note that in fact cyclobutadiene does not have degenerate, singly-occupied molecular orbitals, as a Jahn-Teller type (actually a pseudo-Jahn-Teller) distortion lowers its symmetry from square to rectangular and leads to a closed-shell paired-electron molecule [4]. 

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