REACTIONS OF (ETA-5-C5ME5)TA(PME3)(H)2(ETA-2-CHPME2) WITH HETEROCUMULENES AND THE INTERCONVERSION OF TAUTOMERIC FORMAMIDO COMPLEXES OF TANTALUM

The complex (eta5-C5Me5)Ta(PMe3)(H)2(eta2-CHPMe2) (1) reacts with aryl isocyanates, RNCO (R = C6H5, 4-MeC6H4) to afford the hydrido-formamido complexes (eta5-C5Me5)Ta(H)(eta7-OCHNR)(eta2-CHPMe2) (R = C6H5 (2), 4-MeC6H4 (3)) which exist in two tautomeric forms in solution. The interconversion of these isomers proceeds via an intramolecular rotation of the (OCHNR) moiety analogous to the equilibration of syn and anti hydrogens of an eta3-allyl ligand. Compound 1 undergoes insertion of two equivalents of CO2 to afford (eta5-C5Me5)Ta(eta1-OCHO)(eta2-CHPMe2) (4), a complex containing eta1 and eta2-formamato ligands.