Rotational versus Migration Isomerism in [{(<i>t</i>BuC₅H₄)₂NbH(μ₃-Te)(μ₄-Te)Fe₂(CO)₆}·Cr(CO)₅], a Sterically Crowded Polymetallic Telluride Complex

The reaction of [Cp‘2Nb(Te2)H] (1) (Cp‘ = tBuC5H4) with [Fe2(CO)9] gives [{Cp‘2NbH(Te)2}Fe2(CO)6] (2). Compound 2 reacts with [Cr(CO)5THF] to give [{Cp‘2NbH(Te)2Fe2(CO)6}·Cr(CO)5] (3) in nearly quantitative yield. The crystal structure of 3 reveals that the Fe2(CO)6 unit in this complex (and consequently in 2) is inserted in a perpendicular manner into the original NbTe2 moiety of 1, while the incoming Cr(CO)5 fragment is attached at the central Te atom and in vicinity (vic) to the Nb−H moiety. 1H NMR investigations of solutions of 3 at −80 °C show that only the sterically crowded vic-3 has been separated by recrystallization. At −60 °C a rapid exchange is observed for the ring protons, which may be explained by rotation of the Cp‘ rings around the Cp‘−Nb axes. Above 0 °C an additional set of C5H4, tBu, and NbH resonances appears, which may be assigned to migration of the Cr(CO)5 fragment from the central to the lateral Te atom. The resulting opp-3 is in a 55:45 equilibrium with vic-3 at 20 °C.