Custom key binding for Backspace

Have just tried creating custom keybindings and they are great!

I would like to re-assign the backspace key to archive rather than delete. This doesn't seem to work. Is that because it is a defined key that cannot be overridden, or do I have an error? My key binding file for testing looks like this:

{
    "\U0008" = "archive:";   // backspace
    "\UF728" = "archive:";   // ⌦ delete
}

Delete key works as expected backspace does not.

Thanks