Traditional weakly coordinating anions, such as perchlorate, tetrafluoroborate, and hexafluorophosphate, will nonetheless coordinate to very electrophilic cations, making these counterions unsuitable for some complexes.  The highly reactive species [CpZr(CH)], for example, has been reported to abstract F from PF.  Starting in the 1980s, new types of weakly coordinating anions began to be developed.  BAr′ anions are used as counterions for highly electrophilic, cationic transition metal species, as they are very weakly coordinating and unreactive towards electrophilic attack.  One common method of generating these cationic species is via protonolysis of a dialkyl complexes or an olefin complex.  For example, an electrophilic palladium catalyst, [(2,2′-bipyridine)Pd(CH)(CHCN)][BAr′], 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.