There is only one compound that is named 1,2-diiodoethane, but there are two distinct compounds that can be named 1,2-diiodoethene. Why? Because 1,2-diiodoethane has more hydrogen atoms. There is a rotation about the C-C bond in 1,2-diiodoethane, and there isn’t about the C-C bond in 1,2-diiodoethene. There is a rotation about the C-C bond in 1,2-diiodoethene, and there isn’t about the C-C bond in 1,2-diiodoethane. Because the compounds have different molar masses