Rotational versus Migration Isomerism in [{(tBuC₅H₄)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.