Brookhart's acid is the salt of the diethyl ether oxonium ion and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr′4).

Precipitation of sodium chloride thus drives the formation of the oxonium acid compound, which is isolable as a solid.

For example, an electrophilic palladium catalyst, [(2,2′-bipyridine)Pd(CH3)(CH3CN)][BAr′4], is prepared by protonating the dimethyl complex with Brookhart's acid.

This electrophilic, cationic palladium species is used for the polymerization of olefins with carbon monoxide to polyketones in aprotic solvents.

[5] Polyketones, thermoplastic polymers,[6][7] are formed by the copolymerisation of carbon monoxide and one or more alkenes (typically ethylene with propylene).