For many applications, the counterion simply provides charge and lipophilicity that allows manipulation of its partner ion.  The counterion is expected to be chemically inert.  For counteranions, inertness is expressed in terms of low Lewis basicity.  The counterions are ideally rugged and unreactive. For quaternary ammonium and phosphonium countercations, inertness is related to their resistance of degradation by strong bases and strong nucleophiles.