“…This is the most relevant detail of the molecular structure ( Figure 6) because, frequently, in diene mono(cyclopentadienyl)niobium and -tantalum complexes [57a,57d,58] the diene ligand forms a metallacyclopent-3-ene system with a preferred supino conformation, although there are examples with a prono conformation. Furthermore, the diene ligand can adopt an s-trans conformation, as observed in bis(cyclopentadienyl)zirconium, [59] mono(cyclopentadienyl)tantalum [60] and cationic bis(cyclopentadienyl)tantalum [61] complexes. In our case, the crystallographic data do not correspond to the same bonding system and, therefore, we propose a moiety consisting of a ''diene'' C31ϪC32ϪC33ϪC34 fragment linked to the metal atom through the C33ϪC34 atoms, forming a niobacyclopropane ring, and the donor vinyl substituent C31ϪC32.…”