Identifying the origins of the ring-size specificity of transition metals for polysulfide dianions.

The propensity for early transition metal polysulfide sulfur ring complexes to form LxMS5 in preference to LxMS4 has been investigated. Examination of known crystal structures reveals the M-S torsion angles for the LxMS4 rings tend to cluster between 0-40°, whereas LxMS5 rings cluster between 60-70°, suggesting differing interactions across the M-S bond might be responsible. DFT studies focused on Cp2TiS4/5 using energy decomposition analysis reveal the effect is indeed associated with the Ti-S bonds. A Natural Bond Orbital (NBO) analysis identifies this as a stabilising localised donor-acceptor interaction between a lone pair on the sulfur bound to the metal with a vacant d-orbital acceptor on the metal, with the overlap between this pair of orbitals being favoured for LxMS5. This can only occur if the metal valence electron shell contains 16 electrons. This orbital stabilisation is absent for 18-electron metal sulfur ring complexes, which show either a much-reduced preference for LxMS5 or an inverted preference instead for LxMS4. The as-yet unknown 18-electron system Cp.cht.TiS4/5 (cht = cyclohexatrienyl) is suggested as a verification of this hypothesis.