Isolation of a Three‐Coordinate Boron Cation with a Boron–Sulfur Double Bond

The reaction of the bulky bis(imidazolin‐2‐iminato) ligand precursor (1,2‐(L^MesNH)₂‐C₂H₄)OTs]₂ ( 1 ²⁺ 2OTs]^?; L^Mes=1,3‐dimesityl imidazolin‐2‐ylidene, OTs=p‐toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2‐(L^MesN)₂‐C₂H₄)BH₂OTs] ( 2 ⁺ OTs]^?)_. The boronium cation 2 ⁺ OTs]^? reacts with elemental sulfur to give the thioxoborane salt (1,2‐(L^MesN)₂‐C₂H₄)BSOTs] ( 3 ⁺ OTs]^?). The hitherto unknown compounds 1 ²⁺ 2OTs]^?, 2 ⁺ OTs]^?, and 3 ⁺ OTs]^? were fully characterized by spectroscopic methods and single‐crystal X‐ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2 ⁺ and 3 ⁺. The theoretical, as well as crystallographic studies reveal that 3 ⁺ is the first example for a stable cationic complex of three‐coordinate boron that bears a B?S double bond.