In the last 20 years structures with pentacoordinated central atoms have been the subject of increasing interest. This development was stimulated by both the progress of dynamic NMR spectroscopy and graph theory. Bond theoretical arguments show that only three of the five theoretically possible pentacoordinated structures having at least one symmetry element can be adopted, i.e., the trigonal bipyramid, the rectangular pyramid and the monocapped tetrahedron (Figure 1). These basic structures have been found in different compounds.^1-5