Formation of thiolene versus dithiolene species from TTBEO. Crystal structures of [Au(ddtCO2Et)(PPh3)] and [Sn(ddtCO2Et)2Me2] {TTBEO = 2,5,7,9-tetrathiabicyclo[4.3.0]non-1(6)-en-8-one; ddtCO2Et = 3-(ethoxycarbonylsulfanyl)-5,6-dihydro-1,4-dithiine-2-thiolate}