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 [Cp2Zr(CH3)]+, for example, has been reported to abstract F− from
PF6. Starting in the 1980s, new types of weakly coordinating anions began to be developed. BAr′4 anions are used as counterions for highly electrophilic, cationic transition metal species, as they are very weakly coordinating and unreactive towards electrophilic attack.
Potential application Polyketones,
thermoplastic polymers, are formed by the
copolymerisation of
carbon monoxide and one or more
alkenes (typically
ethylene with
propylene). The process utilises a
palladium(II)
catalyst with a
bidentate ligand like
2,2′-bipyridine or
1,10-phenanthroline (phen) with a non-coordinating BARF counterion, such as [(phen)Pd(CH3)(CO)]BArF4. The preparation of the catalyst involves the reaction of a dimethyl palladium complex with Brookhart's acid in
acetonitrile with loss of
methane and the catalytic species is formed by uptake of carbon monoxide to displace acetonitrile. Use of monodentate
phosphine ligands also leads to undesirable side-products but bidentate phosphine ligands like
1,3-bis(diphenylphosphino)propane have been used industrially. and
carbon monoxide to a
polyketone. Examples of defects from double insertions are highlighted in . ==References==