Clean off the particleboard and the metal with a soft sponge and a small amount of warm water and liquid dish soap. This helps improve the appearance of both and prevents foreign materials like dirt or grease from interfering with a bonding agent if you use one.
Drill the metal into the particleboard if you're able to. This is especially useful if attaching handles or other metal pieces that have screw holes pre-punched into them. Place the screw through the opening in the metal, and then drill the screw down into the particleboard. If the metal doesn't already have holes in it, use a drill bit designed for drilling metal and lubricate the bit with oil.
Fasten the metal to the particleboard with epoxy glue if the metal pieces are lighter-weight. Epoxy glue has a resin and a hardener that form a bonding agent when mixed. It works on both metal and wood and is generally long-lasting. Heat the glue with a hair dryer once it is applied to speed up the drying time. If the metal is too heavy, the epoxy may break off after drying.
Hammer the metal into place using hammer-in insert nuts if there are large enough openings in the metal to accommodate them. The metal nuts are designed to work well with particleboard and have teeth in the sides that grip the particleboard internally after being nailed into place with a hammer.