STRUCTURES OF RHENIUM COMPLEXES: HOW CRYSTALLOGRAPHY CAN BE AN ESSENTIAL, BUT SOMETIMES MISLEADING, TOOL. Céline Pearson and André L. Beauchamp, Département de chimie, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, Québec, H3C 3J7.

Crystallography is the most powerful identification method for complexes with a paramagnetic metal centre. As part of our project aimed at the synthesis of rhenium(III) dimers containing purine ligands, rhenium(III) and (IV) monomers are being considered as intermediates to these dimers. The paramagnetism of these products makes the use of nmr very difficult as an identification tool. Nevertheless, ¹H nmr is being used to characterize the products in solution and since the major identification rules applicable to diamagnetic compounds no longer hold for such systems, reliable methods are needed to corroborate the results. Several crystal structures were solved, which generally supported the conclusions based on spectroscopic results. However, in the case of a product whose spectroscopic and other properties corresponded to ReCl₄(MeCN)(Me₂-adenine), the crystal structure showed that nucleophilic attack had taken place on the ligand during the reaction.

In another reaction, a by-product formed as green crystals insoluble in most solvents. The structure solved easily to a R factor of 2% and the molecules appeared to be the rhenium(VI) monomer trans-ReO₂Cl₂(PPh₃)₂. Bond lengths were consistent with this formulation and all thermal ellipsoids looked normal, but the presence of an unusual infrared band remained puzzling. Very careful analysis helped elucidate the identity of this intriguing structure, which turned out to contain a Re(V) centre.