INTERCONVERSION OF OXO AND IMIDO LIGANDS AT A DIMOLYBDENUM CENTER - MOLECULAR AND ELECTRONIC-STRUCTURE OF [(MO(ETA-C5H4ME)(NPH)(MU-NPH))2]

The tetraimido complexes [{Mo(eta-C5H4R)(NR')}2] (R = Me, R' = Ph or C6H4Me-p; R = Pr(i), R' = Ph) have been synthesised from the corresponding oxo compounds [{Mo(eta-C5H4R)O(mu-O)}] and the aryl isocyanates R'NCO. The three mixed imido-oxo derivativesd [Mo2(eta-C5H4Me)2O-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(eta-C5H4Me)(NPh)(eta-NPh)}2], for which a single-crystal X-ray structure analysis reveals a trans disposition of the eta-methylcyclopentadienyl ligands and a planar Mo2(mu-N)2 core. A molecular-orbital analysis suggests that each NPh ligand acts as a formal three-electron donor. Hydrolysis of [{Mo(eta-C5H4R)(NPh)(eta-NPh)}2] proceeds via a stepwise replacement of the two terminal imido groups to give [Mo2(eta-C5H4R)2O2-x(NPh)x(mu-NPh)2](x = 0 or 1). Attempts to replace the bridging imido functionalities of [{Mo(eta-C5H4Me)O(mu-NPh)}2] by oxo ligands proved unsuccessful.