Abstract
Abstract Variable temperature proton magnetic resonance studies on dimeric niobium and tantalum penta-methoxides,-ethox-ides and isobutoxides have been carried out. The activation energy for scrambling of the terminal and bridging alkoxy groups was found to be fairly constant (10 to 12 kcals mole −1) and the free energy activation ΔG + + increased by small amounts as the degree of chain branching in the alkoxy ligand increased. For the tantalum derivatives the variation in entropy of activation was found to be quite dependent on ligand bulk, and it has been suggested that steric effects play an important role in the activation process. Terminal site scrambling was found to occur at a faster rate than terminal-bridging scrambling, as reflected in a lower activation energy. Some observations on chemical shifts of terminal and bridging sites are made, the importance of a neighbour-anisotropy effect being indicated.
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