Unless you mean something out of the ordinary, you probably need at least "purlins" to support the metal between the 2x6 rafters. If you could provide a sketch of your proposed framing scheme it might help. With typical rafter spacing that might mean a 2x4 laid flat every 1-3 feet or so (depending on what the roofing metal can "self-support" across, roof loadings, etc.)
Normal metal roofing has no structural strength to speak of if simply attached to rafters in the direction where water drains properly. If attached to rafters in the direction it has strength, it tends to leak because water pools in-between the ribs.
Plywood sheathing can serve the same function as purlins do, but normally costs a bit more.
If, in fact, your proposed framing is one where you are using 2x6 as purlins, then you should not need additional wood.