Toward Rational Construction of Gold, Gold−Silver, and Gold−Mercury String Complexes:  Syntheses, Structures, and Properties of [Au(Tab) 2 ] 2 L 2 (L = I and PF 6 ), {[(Tab) 2 M][Au(CN) 2 ]} 2 (M = Au and Ag), and {[Hg(Tab) 2 ][Au(CN) 2 ] 2 } [Tab = 4-(Trimethylammonio)benzenethiolate]