Tmux with non-alphanumeric prefix

In tmux 1.6 a backslash at the end of a configuration line acts as a line continuation character. You can arrange to get the backslash to the command itself in several ways:

Simply make sure it is not the last character. Put a space after it, or a space and a comment:

set-option -g prefix C-\ # (not a line continuation!)

Wrap it in single quotes:

set-option -g prefix 'C-\'

Wrap it in double quotes (and escape it, since backslash is special inside double quotes):

set-option -g prefix "C-\\"

Your C-' was failing because the single quote was starting a quoted string (the error message is not so helpful here).

Also, there is no standard control character or sequence for Control-', so tmux would have complained (bad key: C-') even if you had double quoted it to get it past the initial parsing stage; there are only a handful of non-alphabetic control characters: @[\]^_?.