Synthesis, Reactivity, and ¹²⁵Te NMR Studies of (C₅H₅)RhFe₂Te₂(CO)_x (x = 6, 7)

The square-pyramidal 50e cluster Fe₃Te₂(CO)₉ reacts with CpRh(CO)₂ (Cp = η⁵-C₅H₅) to give a good yield of the heterometallic 52e cluster CpRhFe₂Te₂(CO)₇. This heptacarbonyl undergoes Me₃NO-induced decarbonylation to give two isomers of the hexacarbonyl CpRhFe₂Te₂(CO)₆. Recarbonylation of CpRhFe₂Te₂(CO)₆ formed the same heptacarbonyl in isomerically pure form. In contrast, addition of PPh₃ to CpRhFe₂Te₂(CO)₆ gave two isomers of CpRhFe₂Te₂(CO)₆PPh₃ that differ with regard to the placement of the PPh₃ ligand on iron or rhodium. The iron-phosphine adduct undergoes chemically induced decarbonylation to CpRhFe₂Te₂(CO)₅PPh₃ while the rhodium-phosphine adduct resists decarbonylation. ¹²⁵Te NMR spectroscopy has been used for structure elucidation. The 50e (nido) clusters Fe₃Te₂(CO)₉, Fe₃Te₂(CO)₈(PPh₃), and CpMFe₂Te₂(CO)₆ (two isomers each for M = Co, Rh) all absorb in the range 1123-1081 ppm downfleld of Me₂Te. In dramatic contrast, the 52e (arachno) clusters Fe₃Te₂(CO)₉(PPh₃), CpMFe₂Te₂(CO) and CpRhFe₂Te₂(CO)₆PPh₃ all absorb in the range -825 to -973 ppm. PtFe₂Te₂(CO)₆(PPh₃)₂, which has an arachno-like structure but a nido electron count (50e), absorbs in the arachno range.