From sigma to pi coordination: Tetraisopropylcyclopentadienyl dichlorides [(C(5)HR(4))ECl(2)] (R=CHMe(2)) of phosphorus, arsenic, antimony, and bismuth

[(C(5)HR(4))PCl2] (1a), [(C(5)HR(4))PBr2] (1b), [(C(5)HR(4))AsCl2] (2), and [(C(5)HR(4))SbCl2] (3), have been obtained from the corresponding pnicogen trihalides and sodium tetraisopropylcyclopentadienide (R = CHMe(2)). The molecular structures of 1, 2, and 3 as well as the related structure of [(C(5)HR(4))BiCl2] (4) display increasing pi character of hetero atom to cyclopentadienyl bonding with increasing metallic character of the hetero atom.