Mixed bis (diene) Complexes of tantalum have been synthesized in a stepwise manner either by reaction of TaC12L (butadiene) of supine (exo) geometry (L= C5Me5, C5H5) with (2, 3-dimethyl-2-butene-1, 4-diy1) magnesium or by reaction of TaC12L (2, 3- dimethylbutadiene) of supine geometry with (2-butenel&diyl) magnesium. In the former reaction, the initial geometry of the coordinated diene changes from supine to prone (endo ...