Olation and Structure

The ability of cis hydroxoaqua-metal complexes to undergo olation and form the corresponding diols is correlated with their dimeric structure, having two μ-(H₃O₂) bridging ligands between the two metal atoms. A hypothesis is put forward that olation may take place only if the cis complex has this dimeric structure. The hypothesis is tested by means of the only cis hydroxoaqua complex that does not have a dimeric structure, [Cr(pico)₂(H₂O)OH]S₂O₆, compound 1 (pico = 2-picolylamine). This complex does not olate. Replacement of the dithionate counter ion by iodide yielded the complex [(pico)₂Cr-(H₃O₂)₂Cr(pico)₂]I₄'2H₂O, compound 2. The structure of 2 was determined by a single-crystal X-ray study, which established that it had the normal dimeric structure. Compound 2 olates, as predicted by the hypothesis, and produces the diol [(pico)₂Cr(OH)₂Cr(pico)₂]⁴⁺. The structure of this ion was determined by X-ray study of [(pico)₂Cr(OH)₂Cr-(pico)₂](S₂O₆)₂*272H₂O, compound 3. A two-step mechanism of olation, in the solid state, is proposed. A mono-ol is produced in the first step, followed by the formation of the diol. This mechanism predicts retention of the racemic AA,AA configuration of compound 2 in the product 3 and was confirmed by the structures of 2 and 3. The well-known fundamental feature of olation, namely that only cis isomers olate and trans isomers do not, is explained, for the first time, by the proposed mechanism and is correlated with the structure of the reagents and products.