Geometrical Isomerism

Question

A square planar complex of formula [Pt(NH3)2Cl2] is known to exist in two distinct forms; what type of isomerism does this illustrate?

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Geometrical isomerism arises when ligands can occupy different relative positions around a metal centre without breaking any bonds, and it is common in square planar and octahedral complexes of the type MA2B2. In square planar [Pt(NH3)2Cl2] the two ammonia and two chloride ligands can sit either adjacent to each other, giving the cis form, or directly opposite, giving the trans form. These two arrangements are distinct, non-interconvertible isomers with different physical properties; the cis isomer is the anticancer drug cisplatin. Optical isomerism is ruled out because the planar molecule has a plane of symmetry and is superimposable on its mirror image. Linkage isomerism requires an ambidentate ligand such as nitrite, which is absent here. Ionisation isomerism needs an exchangeable counter ion outside the sphere, also absent. This is the textbook NCERT example of cis-trans isomerism. Plausibility check: a four-coordinate planar MA2B2 system geometrically permits exactly two arrangements, matching the observed existence of two forms.

Geometrical Isomerism

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