Complexes with coordination numbers one, two or three

As Fig. 3.7 shows, it is possible in principle, if not in practice, to distort a tetrahedral arrangement of ligands so that they eventually assume the square planar structure, and vice versa. This suggests that complexes may exist with structures which are neither tetrahedral nor square planar, but intermediate between the two. Indeed, such is the case with the [CuC ] anion, made by dissolving CUCI2 in concentrated hydrochloric acid. However, caution is needed because symmetry arguments can be invoked 

9 (a) Trigonal bipyramidal D h) and (b) square pyramidal (04, ) modes of five-coordination. 

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