The 3-dimensional structure of molecules is extremely important: it frequently determines how and with what they react, and it is always related to their physical characteristics. The compound cis-platin is known to be effective against certain types of tumors, whereas trans-platin has no antitumor characteristics. [Notice that many prefixes (iso, n-, sec-, tert-, cis-, trans- are italicized. Many variables and constants are also italicized.] As shown in Figure 37, these two compounds have the same structural formula--Pt(NH3)Cl2--but differ in the arrangement of the NH3 and Cl groups around the platinum. Because all of the groups lie in the same plane, the geometry of both compounds is referred to as square planar (even though it is not a perfect square).

structures of cis- and trans-platin

Figure 37. The structures of cis- (left) and trans-platin (right).