Chemistry · Ch 5 — Coordination Chemistry
Optical Isomerism
Optical Isomerism
Coordination compounds that possess chirality exhibit optical isomerism just like organic compounds do. A pair of optically active isomers that are non-superimposable mirror images of one another are called enantiomers; their solutions rotate the plane of plane-polarised light either clockwise or anticlockwise, and the corresponding isomers are labelled 'd' (dextrorotatory) and 'l' (levorotatory) respectively. Octahedral complexes of the type [M(xx)₃], [M(xx)₂AB] and [M(xx)₂B₂] (xx a symmetric bidentate ligand) exhibit optical isomerism. The complex [Co(en)₃]³⁺ shows a simple enantiomeric pair, while the more heteroleptic complex [CoCl₂(en)₂]⁺ has three stereoisomers in total: two optically active cis forms (which are mirror images of each other) an …
What this figure shows. Two octahedral cobalt complexes drawn as non-superimposable mirror images of one another, separated by a vertical mirror line/plane. Each shows Co³⁺ at the centre with three curved bidentate 'en' (H₂N-CH₂-CH₂-NH₂) ligands, each contributing two N donor atoms, wrapped around the metal in a propeller-like arrangement; the two structures are the left-handed and right-handed twist of the same three chelate rings, i.e. …
What this figure shows. Three octahedral cobalt complexes, each with two Cl ligands and two bidentate 'en' ligands (H₂N-CH₂-CH₂-NH₂) around Co³⁺: a mirror-image pair on the left/centre showing the two Cl ligands adjacent (cis) to each other, in two non-superimposable propeller-twist forms -- these are the two optically active cis enantiomers -- and a third structure showing the two Cl ligands on opposite sides (trans), which possesses a plane of symmetry and is therefore optically inactive (achi …
Worked out. A self-check problem asking the student to draw all possible stereoisomers of the complex ion in Ca[Co(NH₃)Cl(Ox)₂], i.e. [Co(NH₃)Cl(Ox)₂]²⁻, where Ox is the bidentate, symmetric oxalate ligand. With two bidentate oxalate ligands (four donor atoms) plus one NH₃ and one Cl (one donor atom each) on an octahedral Co³⁺ centre, this is an [M(xx)₂AB] type per Section 5.5.3, so it exhibits both geometrical (cis/trans placement of NH₃ and Cl relative to each other) and optical isomerism (the cis form is chiral, giving a pair of enantiomers, wh …