From σ to π Coordination: Tetraisopropylcyclopentadienyl dichlorides [(C5HR4)ECl2] (R = CHMe2) of phosphorus,arsenic, antimony,and bismuth

[(C₅HR₄)PCl₂] ( 1a ), [(C₅HR₄)PBr₂] ( 1 b ), [(C₅HR₄)AsCl₂] ( 2 ), and [(C₅HR₄)SbCl₂] ( 3 ), have been obtained from the corresponding pnicogen trihalides and sodium tetraisopropylcyclopentadienide (R = CHMe₂). The molecular structures of 1, 2 , and 3 as well as the related structure of [(C₅HR₄)BiCl₂] ( 4 ) display increasing π character of hetero atom to cyclopentadienyl bonding with increasing metallic character of the hetero atom.