The structural chemistry of cupric compounds

At one time it appeared that the characteristic behaviour of Cu(ii) was the formation of four coplanar bonds, supposedly dsp as in the case of Ni(ii). This implied that the ninth 3d electron was placed in a 4p orbital  

The stereochemistry of Cu(ii) is summarized in Fig. 25.7. Four or five bonds are formed of length corresponding to normal single bonds. Four bonds are coplanar or directed towards the vertices of a flattened tetrahedron regular tetrahedral 

We show in Table 25.2 examples of these intermediate configurations which are described later in the text for CuCr204 see p. 493. 

Configurations intermediate between square coplanar and regular tetrahedral in Cu(Il) compounds 

The available evidence appears to indicate that having formed four strong coplanar bonds Cu(ii) forms if possible one or two additional weaker bonds perpendicular to the plane of the four coplanar bonds. Naturally there must always be additional neighbours of some kind in directions normal to this plane, and it is necessary to consider their distances from the metal atom in order to decide whether they should be regarded as bonded to Cu(ii). An alternative to (4+1)- or (4 + 2)-coordination is to rearrange the four close neighbours to form a flattened tetrahedral group. We now examine a number of structures in which Cu(ii) has 4-, (4 + 1)-, or (4 + 2)-coordination. 

---