Center channel behind TV?

I have a 42" TV sitting above a Fireplace. I would like to add a center channel. Question if I put the center channel behind the TV would that work or would the sound be distorted too bad. It is about the only place to put it without it blocking some view. It is a corner unit so there is a big space behind the TV. The center channel that I found is 6" x 18" or so. Picking up a used one.