OXO-HYDRIDO AND IMIDO-HYDRIDO DERIVATIVES OF PERMETHYLTANTALOCENE - STRUCTURES OF (ETA 5C5ME5)2TA(=O)H AND (ETA-5C5ME5)2TA(=NC6H5)H - DOUBLY OR TRIPLY BONDED TANTALUM OXO AND IMIDO LIGANDS

The syntheses of the complexes Cp*2Ta(= O)H, Cp*2Ta(= NH)H and Cp*2Ta(= NC6H5)H and the solid-state structures for Cp*2Ta(= O)H and Cp*2Ta(= NC6H5)H are reported (Cp* = (eta(5)-C5Me5)). 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*2Ta(= NC6H5)H crystallizes in the monoclinic space group P2(1)/c (No. 14) with Z = 4. The unit cell dimensions are a = 15.324 (3) angstrom, b = 9.630 (3) angstrom, c = 17.707 (5) angstrom; beta = 112.94 (3)-degrees and V = 2406 (2) angstrom 3. The solid-state structure reveals a linear Ta-N-C unit for Cp*2Ta(= NC6H5)H. This feature is accommodated on both steric and electronic grounds. Cp*2Ta(= O)H crystallizes in the monoclinic space group P2(1)/n (No. 14) with Z = 8 and two inequivalent but similar molecules in the asymmetric unit. The unit cell dimensions are a = 13.089 (9) angstrom, b = 19.322 (11) angstrom, c = 15.990 (11) angstrom, beta = 108.61 (6)-degrees and V = 3832 (5) angstrom 3. 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*2Ta(= 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.