Osmium Borallyl\nComplexes: Structure and Bonding of\n[L(PPh<sub>3</sub>)(H)Os(η<sup><i>n</i></sup>‑B<sub>3</sub>H<sub>5</sub>L′<sub>2</sub>)], (L = H, Cp*; <i>n</i>

Structures and bonding of various\nborallyl complexes\nstabilized\nin the coordination sphere of osmium have been demonstrated. For example,\nthe Os-borallyl species [(PPh<sub>3</sub>)(H)<sub>2</sub>Os(η<sup>5</sup>-B<sub>3</sub>H<sub>5</sub>(C<sub>5</sub>H<sub>4</sub>NS)<sub>2</sub>)] (<b>2</b>) has been synthesized from the prolonged\nthermolysis of [(Ph<sub>3</sub>P)Os(κ<sup>2</sup>-<i>N,S</i>-(C<sub>5</sub>H<sub>4</sub>NS))<sub>2</sub>(κ<sup>1</sup>-<i>S</i>-(C<sub>5</sub>H<sub>4</sub>NS))] (<b>1</b>) with\nan excess amount of BH<sub>3</sub>·THF. The structural features\nof <b>2</b> suggest the presence of one linear triborane unit\n[B<sub>3</sub>H<sub>5</sub>(C<sub>5</sub>H<sub>4</sub>NS)<sub>2</sub>] which is stabilized in the coordination sphere of osmium through\nan η<sup>3</sup> bonding mode. Strikingly, the Os center in <b>2</b> possesses unique secondary interactions through two mercaptopyridinyl\nsulfur atoms, along with the primary η<sup>3</sup>-borallyl\nbonding. Furthermore, we have explored the reaction of [Cp*Os(PPh<sub>3</sub>)<sub>2</sub>Cl] (<b>4</b>) with BH<sub>3</sub>·THF\nthat furnished another borallyl species, [Cp*(PPh<sub>3</sub>)(H)Os(η<sup>3</sup>-B<sub>3</sub>H<sub>7</sub>)] (<b>5</b>). Notably, \nspecies <b>2</b> and <b>5</b> are both structurally and\nelectronically linked to <i>arachno</i>-B<sub>4</sub>H<sub>10</sub> in which the “hinge-tip” vertex is replaced\nby osmium. A detailed comparative analysis in combination with theoretical\ncalculation offered valuable insights into the bonding scenarios in\nboth borallyl species and aided us to assess their relative stability.