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] (<b>1</b>) (Cp‘ = <i>t</i>BuC₅H₄) with [Fe₂(CO)₉] gives [{Cp‘₂NbH(Te)₂}Fe₂(CO)₆] (<b>2</b>). Compound <b>2</b> reacts with [Cr(CO)₅THF] to give [{Cp‘₂NbH(Te)₂Fe₂(CO)₆}·Cr(CO)₅] (<b>3</b>) in nearly quantitative yield. The crystal structure of <b>3</b> reveals that the Fe₂(CO)₆ unit in this complex (and consequently in <b>2</b>) is inserted in a perpendicular manner into the original NbTe₂ moiety of <b>1</b>, while the incoming Cr(CO)₅ fragment is attached at the central Te atom and in vicinity (<i>vic</i>) to the Nb−H moiety. ¹H NMR investigations of solutions of <b>3</b> at −80 °C show that only the sterically crowded <i>vic</i>-<b>3</b> 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, <i>t</i>Bu, 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 <i>opp</i>-<b>3</b> is in a 55:45 equilibrium with <i>vic</i>-<b>3</b> at 20 °C.