I thank the Chemistry Departments of Tsinghua and Siegen Universities for support. I am grateful to S. Brandt (Siegen), G. Frenking (Marburg), A. Karachalios (Mainz), H. Kragh (Aarhus), M. Schmidt (Ames), and particularly to K. Ruedenberg (Ames) for many constructive comments and to A. Schirrmacher (Berlin) and H. Schönherr (Siegen) for help with the graphics.