Topography of cyclodextrin inclusion complexes. 12. Structural chemistry of linear .alpha.-cyclodextrin-polyiodide complexes. X-ray crystal structures of (.alpha.-cyclodextrin)2.LiI3.I2.8H2O and (.alpha.-cyclodextrin)2.Cd0.5).I5.27H2O. Models for the blue amylose-iodine complex