Oxo-hydrido and imido-hydrido derivatives of permethyltantalocene. Structures of (. eta. sup 5 -C sub 5 Me sub 5 ) sub 2 Ta( double bond O)H and (. eta. sup 5 -C sub 5 Me sub 5 ) sub 2 Ta( double bond NC sub 6 H sub 5 )H: Doubly or triply bonded tantalum oxo and imido ligands

The syntheses of the complexes Cp*{sub 2}Ta({double bond}O)H, Cp*{sub 2}Ta({double bond}NH)H and Cp*{sub 2}Ta({double bond}NC{sub 6}H{sub 5})H and the solid-state structures for Cp*{sub 2}Ta({double bond}O)H and Cp*{sub 2}Ta({double bond}NC{sub 6}H{sub 5})H are reported (Cp* = ({eta}{sup 5}-C{sub 5}Me{sub 5})). These are formally 18-electron complexes with the oxo or imido ligands donating only one {pi}-electron pair to tantalum. Since these ligands commonly donate both {pi}-electron pairs (i.e. they triply bond) to transition-metal centers, at issue are the Ta-O and Ta-N bond orders. Cp*{sub 2}Ta({double bond}NC{sub 6}H{sub 5})H crystallizes in the monoclinic space group. The solid-state structure reveals a linear Ta-N-C unit for Cp*{sub 2}Ta({double bond}NC{sub 6}H{sub 5})H. This feature is accommodated on both steric and electronic grounds. Cp*{sub 2}Ta({double bond}O)H crystallizes in the monoclinic space group. The Ta-N and Ta-O bond distances are intermediate between the values expected for double and triple bonds, and the {nu}(Ta-O) for Cp*{sub 2}Ta({double bond}O)H is considerably less than that for other metal oxo compounds. Thus, the tantalum-nitrogen and tantalum-oxygen bond orders for these two compounds are anomalously low.