Norbornadiene complexes of transition metals: III. Stereospecificity in the generation of cationic complexes [Rh(2,3,8 : 5,6-η-C7H7CHR)(η-C5H5)]+ (R = Me, Ph, [Rh(2,3,8 : 5,6-η-C7H7CH2)(η-C5H5)]+PF6t-