Is the conversion of sodium perrhenate to the tetrabutylammonium salt a double replacement reaction?
Yes, the conversion of sodium perrhenate to a tetrabutylammonium salt is a double replacement reaction; this reaction is actually a salt metathesis reaction, which is an alternative name given to a reaction in which counterions (cations and anions) exchange partners.
The general form for a metathesis reaction looks like this
In your case, the reaction between sodium perrhenate, or
The reaction takes place in aqueous solution (aq), and the tetrabutylammonium perrhenate salt will precipitate from solution.
The counterions that will exchange partners are