Abstract
The authors describe the synthesis of a family of molybdenum-pentadienyl complexes in the eta **1, syn- eta **3, and S- eta **5 configurations as well as their phosphine derivatives. The reaction chemistry of CpMo(CO)//3( eta **1-pentadienyl) with the dienophiles tetracyanoethylene and maleic anhydride is also described. It is shown that for the molybdenum complex, the two pentadienyl groups exist, respectively, in S- eta **5 and unsymmetric U- eta **5 forms; and for the niobium and zirconium analogues, both ligands have symmetric U- eta **5 structures. The metal sizes and electronic influences of molybdenum are thought to be the key factors in its adoption of the unusual 'S' configuration.