Synthesis and Catalysis of Novel Mono‐ and Bis(diene) Complexes of Niobium and X‐Ray Structures of Binuclear (Nb(μ‐Cl)(C5H5)(s‐cis‐butadiene))2 and Mononuclear Nb(C5H5)(s‐cis‐2,3‐dimethylbutadiene)2.