Meridional isomers are defined by what arrangement of ligands?

Meridional isomers are characterized by the specific arrangement of ligands around a central metal ion. In octahedral complexes, these isomers feature at least two pairs of identical ligands positioned symmetrically along one of the axes of the coordination sphere. The term "meridional" relates to the arrangement of these ligands in a manner that resonates with how longitudinal lines on a globe are oriented, effectively aligning with the metal ion's meridian. In this context, it means the identical ligands are located in a plane that cuts through the metal ion in a way analogous to lines of latitude or longitude.

Recognizing this structural feature is essential for understanding the geometric isomerism that can occur in octahedral complexes, where different spatial arrangements of ligands can lead to distinct chemical properties. The presence of meridional isomers highlights the significance of ligand positioning in coordination chemistry, leading to compounds with varying reactivity and properties based on the configuration of their ligands.