Expanding Cyanide‐Bridged Weakly Coordinating Anions Through the Brominated Silver Salt Ag[BCNB Br ]

Weakly coordinating anions (WCAs) composed of a cyanide unit bridging two triarylborane Lewis acids are an attractive but largely unexplored complement to established tetraarylborates such as [B(C 6 F 5 ) 4 ] − (BArF 20 ). Here, we report the brominated representative [μ‐(CN)(B(C 6 F 4 Br) 3 ) 2 ] − ([BCNB Br ] − ) through its readily accessible silver salt Ag[BCNB Br ], prepared from AgCN and two equivalents of B(C 6 F 4 Br) 3 in CH 2 Cl 2 . Ag[BCNB Br ] provides convenient access to [BCNB Br ] − in weakly coordinating media and enables the introduction of this WCA through oxidation or halide abstraction. Its utility is demonstrated by the synthesis of a range of [BCNB Br ] − salts containing oxidizing and electrophilic cations. These results show that [BCNB Br ] − is stable under strongly oxidizing conditions, including systems reaching at least +1.14 V vs. FeCp 2 /FeCp 2 + . [BCNB Br ] − represents a bromine‐substituted WCA whose heavier bromine substituents enhance X‐ray scattering and facilitate crystal structure analysis of its salts.