Chemistry · Ch 5 — Coordination Chemistry
Stereoisomers and Geometrical Isomerism
Stereoisomers and Geometrical Isomerism
Stereoisomers share the same chemical formula and the same connectivity between the central metal atom and its ligands, differing only in the spatial arrangement of the ligands in three-dimensional space; they are further classified as geometrical isomers and optical isomers. Geometrical isomerism specifically arises in heteroleptic complexes because of the different three-dimensional arrangements possible around the central atom, and it is seen in both square planar and octahedral complexes. In square planar complexes of the form [MA₂B₂] and [MA₂BC] (A, B, C monodentate), placing the two similar groups on the same side versus opposite sides of the metal gives the cis and trans isomers respectively; the same cis-trans pattern is shown by [M(xy)₂] where xy is an unsymmetrical bidentate ligand, and a [MABCD] square planar complex shows geometrical isomerism too, with three distinct isomers possible depending on which ligand is treated as the fixed reference point. Octahedral complexes of the type [MA₂B₄] and [M(xx)₂B₂] show the same cis-trans pattern using the numbered-position scheme of Figure 5.5, while [MA₃B₃] complexes show a different pair of isomers -- facial (fac), where the three A ligands occupy one triangular face and the three B ligands the opposite face, and meridional (mer), where the three A ligands instead run along an imaginary semicircular meridian from on …
A reference table pairing each square planar type with its cis and trans structures. MA₂B₂: cis-[Pt(NH₃)₂Cl₂] (the two Cl on the same side) versus trans-[Pt(NH₃)₂Cl₂] (the two Cl on opposite corners) -- the cisplatin/transplatin pair. MA₂BC: cis-[Pt(NH₃)₂(Br)(Cl)] versus trans-[Pt(NH₃)₂(Br)(Cl)], distinguished by whether the two identical NH₃ ligands sit adjacent or across from each other. M(xy)₂: cis and trans forms of a platinum-oxalate-type complex where xy is an unsymmetrical bidentate ligand with one N-donor and one O-donor end (drawn as two five-membered chelate rings), the cis form placing like donor atoms adjacent and the trans form placing them across. MABCD: three distinct isomers of [Pt(NH₃)(Br)(Cl)(NO₂)]⁺, each obtained by holding one ligand fixed as reference and permuti …
What this figure shows. Two equivalent drawings of an octahedron with the central metal M at the centre and six numbered/lettered vertex positions: the left drawing numbers the six ligand positions 1-6 (1 at the top apex, 6 at the bottom apex, 2/3/4/5 around the equatorial plane); the right drawing shows the same octahedron with axes labelled X/X', Y/Y', Z/Z' and ligand positions marked L. This numbering scheme is the reference used to state which pairs of positions count as 'cis' and which count as 'trans': positions (1,2),(1,3),(1,4),(1,5),(2,3),(2,5),(2,6),(3,4),(3,6),(4,5),(4,6),(5,6) are all identical/cis pairs, while (1,6),(2,4),(3,5) are the identical/ …
What this figure shows. Two octahedral cobalt(III) complexes each with three CN and three Cl ligands, drawn as two triangular-faced pyramids joined base to base with Co³⁺ at the centre. Figure 5.6(a), the facial (fac) isomer, has all three CN ligands occupying the corners of one triangular face of the octahedron and all three Cl ligands occupying the corners of the opposite triangular face. Figure 5.6(b), the meridional (mer) isomer, instead has the three CN ligands arranged around an imaginary semicircular meridian running from one apex of the octahedron to the opposite apex, with the three Cl ligands fillin …
Worked out. A self-check box continuing the CrCl₃.6H₂O dehydration problem (compounds A, B, C above) and then, as question 6, asking the student to state the possible type(s) of isomerism and draw the isomers for three named complexes: (i) [Co(en)₃][Cr(CN)₆] -- a coordination-isomerism pair (Section 5.5.1.2), since both ions here are themselves complex; (ii) [Co(NH₃)₅(NO₂)]²⁺ -- a linkage-isomerism pair via the ambidentate nitrite ligand (Section 5.5.1.1); and (iii) [Pt(NH₃)₃(NO₂)]Cl -- an ionisation-isomerism pair via the exchangeable Cl⁻ counter ion and coordinated …