In contrast, the "trans" isomer (transplatin) has no useful anticancer activity. Each isomer can be synthesized using the trans effect to control which isomer is produced. For octahedral complexes of formula MX4Y2, two isomers also exist. (Here M is a metal atom, and X and Y are two different types of ligands.) In the "cis" isomer, the two Y ligands are adjacent to each other at 90°, as is true for the two chlorine atoms shown in green in "cis"-[Co(NH3)4Cl2]+, at left. In the "trans" isomer shown at right, the two Cl atoms are on opposite sides of the central Co atom. A related type of isomerism in octahedral MX3Y3 complexes is facial–meridional (or "fac"/"mer") isomerism, in which different numbers of ligands are "cis" or "trans" to each other. Metal carbonyl compounds can be characterized as ""fac"" or ""mer"" using infrared spectroscopy.