Osmium Borallyl Complexes: Structure and Bonding of [L(PPh3)(H)Os(ηn‑B3H5L′2)], (L = H, Cp*; n = 5, 3; L′ = C5H4NS, H) (Cp* = η5‑C5Me5)

Structures and bonding of various borallyl complexes stabilized in the coordination sphere of osmium have been demonstrated. For example, the Os-borallyl species [(PPh3)(H)2Os(η5-B3H5(C5H4NS)2)] (2) has been synthesized from the prolonged thermolysis of [(Ph3P)Os(κ2-N,S-(C5H4NS))2(κ1-S-(C5H4NS))] (1) with an excess amount of BH3·THF. The structural features of 2 suggest the presence of one linear triborane unit [B3H5(C5H4NS)2] which is stabilized in the coordination sphere of osmium through an η3 bonding mode. Strikingly, the Os center in 2 possesses unique secondary interactions through two mercaptopyridinyl sulfur atoms, along with the primary η3-borallyl bonding. Furthermore, we have explored the reaction of [Cp*Os(PPh3)2Cl] (4) with BH3·THF that furnished another borallyl species, [Cp*(PPh3)(H)Os(η3-B3H7)] (5). Notably, species 2 and 5 are both structurally and electronically linked to arachno-B4H10 in which the “hinge-tip” vertex is replaced by osmium. A detailed comparative analysis in combination with theoretical calculation offered valuable insights into the bonding scenarios in both borallyl species and aided us to assess their relative stability.