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 Mes NH) 2 ‐C 2 H 4 )[OTs] 2 ( 1 2+  2[OTs] − ; L Mes =1,3‐dimesityl imidazolin‐2‐ylidene, OTs= p ‐toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2‐(L Mes N) 2 ‐C 2 H 4 )BH 2 [OTs] ( 2 +  [OTs] − ) . The boronium cation 2 +  [OTs] − reacts with elemental sulfur to give the thioxoborane salt (1,2‐(L Mes N) 2 ‐C 2 H 4 )BS[OTs] ( 3 +  [OTs] − ). The hitherto unknown compounds 1 2+  2[OTs] − , 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 BS double bond.