Stereoisomerism: geometrical isomers

   Distinguishing between cis- and trans-isomers of a square planar complex or between mer- and fac-isomers of an octahedral complex is most unambiguously confirmed by structural determinations using single-crystal X-ray diffraction. Vibrational spectroscopy  may also be of assistance. For example, illustrates that the asymmetric stretch for the PtCl₂ unit in [Pt(NH₃)₂Cl₂] is IR active for both the trans- and cis-isomers, but the symmetric stretch is IR active only for the cis-isomer.

Fig. 1.1 The trans- and cis-isomers of the square planar complex [PtCl₂(NH₃)₂] can be distinguished by IR spectroscopy. The selection rule for an IR active vibration is that it must lead to a change in molecular dipole moment.

   The existence of ions or molecules in different structures (e.g. trigonal bipyramidal and square-based pyramidal [Ni(CN)₅]^3-) is just a special case of geometrical isomerism.   In the cases of, for example, tetrahedral and square planar [NiBr₂(PBzPh₂)₂] (Bz = benzyl), the two forms can be distinguished by the fact that they exhibit different magnetic. To complicate matters, square planar [NiBr₂(PBzPh₂)₂] may exist as either trans- or cis-isomers.