Isomerism in Coordination Compounds

Lecture 6. CHEM1902 Coordination Chemistry

Two or more different compounds having the same formula are called isomers. Two principal types of isomerism are known among coordination compounds. Each of which can be further subdivided.

1. Stereoisomerism.

a) Geometrical isomerism
b) Optical isomerism

2. Structural Isomerism.

a) Coordination isomerism
b) Ionisation isomerism
c) Hydrate isomerism
d) Linkage isomerism

1. Stereoisomers

Stereoisomers have the same atoms, same sets of bonds, but differ in the relative orientation of these bonds.

Ignoring special cases involving esoteric ligands, then:

Geometric isomers are possible for both square planar and octahedral complexes, but not tetrahedral.

Optical isomers are possible for both tetrahedral and octahedral complexes, but not square planar.

The earliest examples of stereoisomerism involve complexes of Co(III). In 1889, Jorgensen observed purple and green salts of [CoCl₂(en)₂]+, which Werner later correctly identified as the cis- and trans- geometric isomers. In 1911, the first resolution of optical isomers was reported by Werner and King for the complexes cis-[CoX(NH₃)(en)₂]²⁺, where X=Cl- or Br-.

Geometric Isomers

The number of geometric isomers expected for common stereochemistries are as follows:

Square Planar:

Compound type       No. of isomers
Ma₂b₂                      2 (cis- and trans-)
Mabcd                     3 (use cis- and trans- relations)

here a, b, c, and d refer to monodentate ligands.

A number of examples of these types have been isolated and characterised and they show very different chemical and biological properties. Thus for example, cis-PtCl₂(NH₃)₂ is an anti-cancer agent (cisplatin) whereas the trans- isomer is inactive against cancer (it is toxic), and so not useful in Chemotherapy.

cis- and trans- isomers of [PtCl₂(NH₃)₂]

cis- and trans- refer to the position of 2 groups relative to each other. In the cis- isomer they are "next to each other" i.e. at 90 degrees in relation to the central metal ion, whereas in the trans- isomer they are "opposite each other", i.e. at 180 degrees relative to the central metal ion.

           a
           |   
           |
           M ----b         a----M----b
             cis-            trans-

3 geometric isomers of a square planar complex [PtBrClNH₃(pyr)].

The first report of the three geometric isomers being isolated and characterised for complexes of the type [Mabcd] was by Il'ya Chernyaev in 1928. The example above was reported by Anna Gel'man in 1948.

Question. Does cis-amminebromo-cis-chloropyridineplatinum(II) uniquely identify isomer (ii) above??

Octahedral:

Compound type          No. of isomers
Ma₄b₂                         2 (cis- and trans-)
Ma₃b₃                         2 (fac- and mer-)
MAA₂b₂                     3 (2*cis- and 1 trans-)

here a, and b, represent monodentate ligands and AA is a bidentate ligand.

In the second example, new labels are introduced to reflect the relative positions of the ligands around the octahedral structure. Thus; placing the 3 groups on one face of the octahedral gives rise to the facial isomer and placing the 3 groups around the centre gives rise to the meridional isomer.

fac- and mer- isomers of [RhCl₃(pyr)₃].

[Mabcdef] is expected to give 15 geometric isomers. In the case of [PtBrClI(NO₂)(NH₃)(pyr)], several of these were isolated and characterised by Anna Gel'man and reported in 1956. Optical isomers are possible for each of these 15 forms, making a total of 30 isomers.

The cis- isomer of MAA₂b₂ may also exhibit optical isomerism although we will concentrate largely on optical isomers of the type M(AA)₃ (see below).

Optical Isomers

Optical isomers are related as non-superimposable mirror images and differ in the direction with which they rotate plane-polarised light. These isomers are referred to as enantiomers or enantiomorphs of each other and their non-superimposable structures are described as being asymmetric.

Various methods have been used to denote the absolute configuration of optical isomers such as R or S, Λ or Δ or C and A. The IUPAC rules suggest that for general octahedral complexes C/A scheme is convenient to use and that for bis and tris bidentate complexes the absolute configuration be designated Lambda Λ (left-handed) and Delta Δ (right-handed).

Priorities are assigned for mononuclear coordination systems based on the standard sequence rules developed for enantiomeric carbon compounds by Cahn, Ingold and Prelog (CIP rules). These rules use the coordinating atom to arrange the ligands into a priority order such that the highest atomic number gives the highest priority number (smallest CIP number). For example the hypothetical complex [Co Cl Br I NH₃ NO₂ SCN]^2- would assign the I- as 1, Br as 2, Cl as 3, SCN as 4, NO₂ as 5 and NH₃ as 6.

Here is one isomer where the I and Cl, and Br and NO₂ were found to be trans- to each other.

The reference axis for an octahedral centre is that axis containing the ligating atom of CIP priority 1 and the trans ligating atom of lowest possible priority (highest numerical value). The atoms in the coordination plane perpendicular to the reference axis are viewed from the ligand having that highest priority (CIP priority 1) and the clockwise and anticlockwise sequences of priority numbers are compared. The structure is assigned the symbol C or A, according to whether the clockwise (C) or anticlockwise (A) sequence is lower at the first point of difference. In the example shown above this would be C.

The two optical isomers of [Co(en)₃]³⁺ have identical chemical properties and just denoting their absolute configuration does NOT give any information regarding the direction in which they rotate plane-polarised light. This can ONLY be determined from measurement and then the isomers are further distinguished by using the prefixes (-) and (+) depending on whether they rotate left or right.

left-handed Λ isomer

and right-handed Δ isomer

To add to the confusion, when measured at the sodium D line (589nm), the tris(1,2-diaminoethane)M(III) complexes (M= Rh(III) and Co(III)) with IDENTICAL absolute configuration, rotate plane polarised light in OPPOSITE directions!
The left-handed (Λ)-[Co(en)₃]³⁺ isomer gives a rotation to the right and therefore corresponds to the (+) isomer.

Since the successful resolution of an entirely inorganic ion (containing no C atoms) (hexol) only a handful of truly inorganic complexes have been isolated as their optical isomers e.g. (NH₄)₂Pt(S₅)₃.2H₂O.

For tetrahedral complexes, R and S would be used in a similar method to tetrahedral Carbon species and although it is predicted that tetrahedral complexes with 4 different ligands should be able to give rise to optical isomers, in general they are too labile and can not be isolated.

2. Structural Isomers

There are several types of this isomerism frequently encountered in coordination chemistry and the following represents some of them.

a) Coordination isomerism: where compounds containing complex anionic and cationic parts can be thought of as occurring by interchange of some ligands from the cationic part to the anionic part.

one isomer [Co(NH₃)₆] [Cr(C₂O₄)₃]
another isomer [Co(C₂O₄)₃] [Cr(NH₃)₆]

b) Ionisation isomers: where the isomers can be thought of as occurring because of the formation of different ions in solution.

one isomer [PtBr(NH₃)₃]NO₂ -> NO₂- anions in solution
another isomer [Pt(NO₂)(NH₃)₃]Br -> Br- anions in solution

Notice that both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not. A very similar type of isomerism results from replacement of a coordinated group by a solvent molecule (Solvate Isomerism). In the case of water, this is called Hydrate isomerism.

c) Hydrate isomerism: the best known example of this occurs for chromium chloride "CrCl₃.6H₂O" which may contain 4, 5, or 6 coordinated water molecules.

[CrCl₂(H₂O)₄]Cl.2H₂O bright-green
[CrCl(H₂O)₅]Cl₂.H₂O grey-green
[Cr(H₂O)₆]Cl₃ violet

These isomers have very different chemical properties and on reaction with AgNO₃ to test for Cl- ions, would find 1, 2, and 3 Cl- ions in solution respectively.

d) Linkage isomerism occurs with ambidentate ligands. These ligands are capable of coordinating in more than one way. The best known cases involve the monodentate ligands SCN- / NCS- and NO₂^- / ONO^-.

For example:

[Co(ONO)(NH₃)₅]Cl the nitrito isomer -O attached
[Co(NO₂)(NH₃)₅]Cl the nitro isomer - N attached.

Inorganic Nomenclature

As part of this course, you are required to make yourselves familiar with the rules related to Inorganic Nomenclature.

Uses of Coordination Compounds

A brief survey of some of the uses of coordination compounds includes:

l. Dyes and Pigments: Coordination compounds have been used from the earliest times as dyes and pigments, for example madder dye which is red, was used by the ancient Greeks and others. It is a complex of Hydroxyanthraquinone. A more modern example is the pigment copper phthalocyanine, which is blue.

2. Analytical Chemistry: You have already encountered many such uses during the laboratory course.

(a) Colour Tests: Since many complexes are highly coloured they can be used as colourimetric reagents e.g. formation of red 2,2'-bipyridyl and l,l0-phenanthroline complexes as a test for Fe(II)

(b) Gravimetric Analysis: Here chelating ligands are often used to form insoluble complexes e.g. Ni(DMG)₂ and Al(oxine)₃ (see laboratory manual).

(c) Complexometric Titrations and Masking Agents: An example of this is the use of EDTA in the volumetric determination of a wide variety of metal ions in solution, e.g. Zn²⁺, Pb²⁺, Ca²⁺,Co²⁺, Ni²⁺, Cu²⁺, etc. By careful adjustment of the pH and using suitable indicators, mixtures of metals can be analysed, e.g. Bi³⁺ in the presence of Pb²⁺ (see laboratory manual). Alternatively, EDTA may be used as a masking agent to remove a metal ion which would interfere with the analysis of a second metal ion present.

3. Sequestering Agents: Related to their use as masking agents is the use of ligands for "sequestering" i.e. for the effective removal of objectionable ions from solution in industrial processing, e.g. EDTA is used to "soften" water. The addition of EDTA to water is used in boilers etc., to prevent "scaling" or build up of insoluble calcium salts.

4. Extraction of Metals: Sometimes certain metals can be leached from their ores by formation of stable complexes e.g. Ag and Au as complexes of cyanide ion.

5. Bio-Inorganic Chemistry: Naturally occurring complexes include haemoglobin, chlorophyll, vitamin B₁₂ etc.
EDTA and other complexing agents have been used to speed the elimination of harmful radioactive and other toxic elements from the body. (e.g. Pb²⁺). In these cases soluble metal chelate complexes are formed.

6. Chemo-therapy: an example here is the use of cis-PtCl₂(NH₃)₂ as an anti-tumour drug.

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