Polynuclear tantalum oxoalkoxides. Crystal structures of [Ta8O10(OEt)(20)], [Ta7O9(OPri)(17)] and [Ta5O7(OBut)(11)]center dot C6H5Me

By controlled hydrolysis of the parent pentaalkoxides of tantalum [Ta(OR)(5 ), R=Et, Pr-i or Bu-t] in mixed alcohol-toluene solutions the following cry stalline oxoalkoxides were obtained and their structures determined by sing le crystal X-ray diffraction: [Ta-8(mu(3)-O)(2)(mu-O)(8)(mu-OEt)(6)(OEt)(14 )] 1, [Ta-7(mu(3)-O)(3)(mu-O)(6)(mu-OPri)(4)(OPri)(13)] 3 and [Ta-5(mu(3)-O )(4)(mu-O)(3)(mu-OBut)(OBut)(10)]. C6H5Me 4. Each compound contained all of its tantalum atoms in distorted octahedral co-ordination. Using O-17-enric hed (10 atom%) water these compounds were obtained with O-17-enriched oxo a toms and their O-17 NMR spectra measured in toluene solution and related to their molecular structures. Structural aspects of these tantalum oxoalkoxi des are discussed in terms of the steric effects of the alkoxo ligands and pi-electron donation of terminal alkoxo ligands.