Interconversion of oxo and imido ligands at a dimolybdenum centre: molecular and electronic structure of [{Mo(η-C 5 H 4 Me)(NPh)(µ-NPh)} 2 ]

The tetraimido complexes [{Mo(η-C5H4R)(NR′)(µ-NR′)}2](R = Me, R′= Ph or C6H4Me-p; R = Pri, R′= Ph) have been synthesised from the corresponding oxo compounds [{Mo(η-C5H4R)O(µ-O)}2] and the aryl isocyanates R′NCO. The three mixed imido–oxo derivatives [Mo2(η-C5H4Me)2O4 –x(NPh)x](x= 1–3), presumed to be intermediates in the synthesis of the tetraimido derivatives, have been synthesised independently. The imido–oxo compounds react with excess of phenyl isocyanate to give [{Mo(η-C5H4Me)(NPh)(µ-NPh)}2], for which a single-crystal X-ray structure analysis reveals a trans disposition of the η-methylcyclopentadienyl ligands and a planar Mo2(µ-N)2 core. A molecular-orbital analysis suggests that each NPh ligand acts as a formal three-electron donor. Hydrolysis of [{Mo(η-C5H4R)(NPh)(µ-NPh)}2] proceeds via a stepwise replacement of the two terminal imido groups to give [Mo2(η-C5H4R)2O2 –x(NPh)x(µ-NPh)2](x= 0 or 1). Attempts to replace the bridging imido functionalities of [{Mo(η-C5H4Me)O(µ-NPh)}2] by oxo ligands proved unsuccessful.