Abstract. X-ray studies revealed a strong trans-influence in titanium tetrachloride-ethyl benzoate complexes. From FTIR spectra, a very strong complexation is found in a 2:1 complex and in a surface complex on magnesium chloride supported Ziegler-Natta catalysts. In both cases the ester is proposed to be trans to a bridging chlorine. Key words:Ziegler-Natta, ethyl benzoate, titanium tetrachloride, transinfluence, complex.A recent important development in Ziegler-Natta catalysis is the development of the magnesium chloride supported highly active catalyst for the polymerization of olefins. The complexity and the excellent activity of the catalysts challenged our understanding of the system. We therefore initiated a basic research of the compounds which constitute the catalysts in order to obtain a more reliable comprehension of the structure of the active center, In this context the combination of IR-spectroscopy and single crystal X-ray scattering measurements proved to be a powerful tool to elucidate structural information of possible catalyst precursors.Accurate X-ray scattering measurements on single crystals of the 1:2 and 2:2 complexes between titanium tetrachloride and ethyl benzoate revealed important trends in bonding behaviour [1]. A significant transinfluence is observed. Terminal Ti-C1 bonds (2.31 A) trans to another terminal Ti-C1 are significantly longer than terminal Ti-C1 bonds (2.22--2.24 ,~) trans to an ester group or a bridging chlorine. A cis influence is also seen, as the ester complexation through carbonyl is enhanced by an increasing number of weakly bonded neighbours in the octahedral Ti-entity.

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