STRUCTURE OF METALLATHIACYCLES - PLANAR VS NONPLANAR GEOMETRIES - A THEORETICAL AND EXPERIMENTAL INVESTIGATION

Since the previously reported results of Fenske-Hall molecular orbital calculations suggested that the bent metallacycle geometries observed in transition-metal-inserted thiophene complexes do not result from e lectronic properties, molecular mechanics calculations were employed t o explore how steric factors could influence ring geometries. These ca lculations have identified steric factors responsible for the observed ring deformations in (C5Me5)Rh(PMe3)(eta(2)-C,S-2,5-Me2C4H2S) and ben zothiophene and dibenzothiophene analogs, The results of the molecular mechanics calculations have been supported by X-ray structural charac terization of the products formed via insertion of the reactive 16-ele ctron metal fragment [(C5Me5)Rh(PMe3)] into the C-S bonds of unsubstit uted thiophene, benzothiophene, and dibenzothiophene. The parent thiop hene complex, unlike the previously reported ring-substituted analog ( and as predicted by the molecular mechanics calculations) is found to adopt a planar six-membered ring structure, The benzothiophene inserti on product shows a moderate degree of bending of the metallathiacycle, whereas the dibenzothiophene molecule is strongly bent.