Coordination compounds

Isomerism in Coordination complexes

Introduction

The coordination compounds which have the same molecular formula but have their ligands attached to central metal atom in different ways are called isomers. These isomers have different properties. The phenomenon that gives rise to different isomers is called isomerism.

Isommerism may be of two types viz, structural isomerism and stereo (space) isomerism.

1) Structural isomerism (Constitutional isomerism)

In this isomerism, molecule exhibit different mmolecular structures i.e, structural isomers of a molecule are different types of compounds. Structural isomers generally have different physical and chemical properties. This isomerism is of following types:

i) Polymerization isomerism

This type of isomerism is found in those complex compound whose formulae apear to be polymers of some simple complex compound. All these complex compounds have the same ratio of different metal atoms and ligands in them

This is not true isomerism. It occur between the compounds having same emperical formula but different molecular weight.

[Pt(NH₃)₂Cl₂]→ [Pt(NH₃)₄] [PtCl₄]

→ [Pt(NH₃)₃Cl] [Pt(NH₃)Cl₃]₂

ii) Ionisation isomerism

the complex compounds showing ionisation isomerism are called ionisation isomers. These isommers have the same molecular formulae and are produced when the ligands given in the coordination sphere and the anions present out side the coordination sphere are exchanged with each other. Thus both the complex compounds of each of the following pairs are ionisation isomers to each other.

i) [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅(SO₄)]Br

ii) [Co(NH₃)₄Cl₂]NO₂ and [Co(NH₃)₄Cl(NO₂)]Cl

iii) [Pt(NH₃)₄Cl₂]Br₂ and [Pt(NH₃)₄Br₂]Cl₂

iv)[Co(NH₃)₅(NO₃)]SO₄and [Co(NH₃)₅(SO₄)]NO₃

v) [Cr(NH₃)₄I₂]Br and [Cr(NH₃)₄IBr]I

vi) [Fe(NH₃)₅(CN)]SO₄ and [Fe(NH₃)₅SO₄]CN.

Ionisation isomers show different properties, e.g.,[Co(NH₃)₅Br]SO₄(I)and[Co(NH₃)₅(SO₄)]Br (II) which are ionisaton isomers to each other show different properties as mentioned below:

i) (I) is red-violet while (II) is red.

ii) When these isomers are dissolved in water, they give different ions. For example :

[Co(NH₃)₅Br]SO₄⇔[Co(NH₃)₅Br]²⁺ + SO₄^2-

^{(I) (red-violet)}

[Co(NH₃)₅(SO₄)]Br⇔[Co(NH₃)₅(SO₄)]⁺ Br^-

^{(II) (red)}

It is due to the formation of different ions in solution that these compounds show different behaviour when treated with an aqueous solution of BaCl₂ and AgNO₃.(I) gives a white ppt. of BaSO₄ with an aqueous solution ofBaCl₂ indicating that SO₄^2- ions in (I) are outside the coordination sphere. This compound does not give any ppt. with the aqueous solution of AgNO₃. On the other hand (II) does not give any ppt. with a solution of BaCl₂ but forms a yellow ppt. of AgBr with AgNO₃ solution, indicating that Br^- ions in this compound are outside the coordination sphere.

$$[Co(NH_3)_5Br]SO_4+BaCl_2\longrightarrow BaSO_4+[Co(NH_3)_5Br]Cl_2$$

$$[Co(NH_3)_5Br]SO_4+AgNO_3\longrightarrow No-reaction$$

$$[Co(NH_3)_5SO_4]Br+BaCl_2\longrightarrow No-reaction$$

$$[Co(NH_3)_5SO_4]Br+AgNO_3\longrightarrow AgBr+[Co(NH_3)_5SO_4]NO_3$$

It may be noted that the coordination number of the central atom in both the ionisation isomers is the same. Ioniisation isomers can be detected by the determination of their conductance in solution.

iii) Hydrate isomerism

This type of isoerism is similar to ionisation isoerism. Hydrate isomers are obtained when the H₂O molecules acting as ligands are replaced by the anions present outside the coordination sphere. CrCl₃.6H₂O exists in three hydrate isomers whch are [Cr(H₂O)₆]Cl₃ (I),[Cr(H₂O)₅Cl]Cl₂.H₂O (II) and [Cr(H₂O)₄Cl₂]Cl.2H₂O (III). These have different physical and chemical properties, e.g.,

(i) (I) is violet, does not lose water over conc.H₂SO₄ and all the three Cl^- iions as precipitated as AgCl with AgNO₃ solution.

$$[Cr(H_2O)_6]Cl_3+3AgNO_3\longrightarrow AgCl_2↓+[Cr(H_2O)_6](NO_3)_3$$

or $$3Cl^-+3Ag^+\longrightarrow 3AgCl↓$$

(ii) (II) is blue-green, loses only one H₂O molecule over conc.H₂SO₄and only two Cl^- ions are precipitated as AgCl with AgNO₃ solution.

$$[Cr(H_2O)_5Cl]Cl_2.H_2O+2AgNO_3\longrightarrow 2AgCl+[Cr(H_2O)_5Cl](NO_3)_2.H_2O$$

or $$2Cl^-+2Ag^+\longrightarrow 2AgCl↓$$

(iii) (III) is blue-green loses twoH₂O molecule over conc.H₂SO₄and only oneCl^- ion isprecipitated by AgNO₃ solution.

$$[Cr(H_2O)_4Cl_2]Cl.H_2O+AgNO_3\longrightarrow AgCl+[Cr(H_2O)_4Cl_2](NO_3).H_2O$$

or $$Cl^-+Ag^+\longrightarrow AgCl↓$$

The behaviour of these isomers over conc.H₂SO₄ as mentioned above indicates that the number of H₂O molecules outside the coordination sphere in (I), (II) and (III) have three, two and one Cl^- ions outside the coordination sphere.

Other examples of hydrate isomers are given below:

i) [Co(en)₂(H₂O)Cl]Cl₂and[Co(en)₂Cl₂]Cl.H₂O

ii) [Co(NH₃)₄(H₂O)Cl]Cl₂ and[Co(NH₃)₄Cl₂]Cl.H₂O

iii) [Cr(py)₂(H₂O)₂Cl₂]Cl and [Cr(py)₂(H₂O)Cl₃].H₂O

iv) [Cr(en)₂(H₂O)]Br₃ and [Cr(en)₂(H₂O)Br]Br₂.H₂O

iv) Linkage isomerism

Some ligand can bind in more than one way to the central metal atom. Linkage isomerism arises due to different tupe of attachment with the same ligand to the metal atom.

Example:

In the complex[Co(NH₃)₅NO₂]Cl₂, NO₂ can have two types of linkage with Co atom i.e,

[Co(NH₃)₅NO₂]Cl₂ → N - attachment with Co

^{nitro complex}

and

[Co(NH₃)₅ONO]Cl₂ → O - attachment with Co

^{nitrito complex}

These two complexes are linkage isomers.

Similarly, -SCN also gives linkage isomers.

[Zn(SCN)₄]^2-→ S - attachment with Zn

^{thiocyanato-S-complex}

[Zn(NCS)₄]^2-→ N - attachment with Zn

^{thiocyanato-N-complex}

v) Coordination isomerism

This type of isomerism is shown by those complex compounds which are composed of complex cation and complex anion. Coordination isomers are obtained when some or all ligands of both the coordination sheres are interchanged wth each other. Thus both the complex compounds of each of the following pairs are coordination isomers to each other. In these pairs the central metallic atom in the two coordination spheres may be the same or different.

i) [Cr³⁺(NH₃)₆]³⁺[Cr³⁺(CN)₆]^3- and [Cr³⁺(NH₃)₄(CN)₂]⁺[Cr³⁺(CN)₄(NH₃)₂]^-

ii) [Pt²⁺(NH₃)₄]²⁺ [Pt²⁺Cl₄]^2- and [Pt²⁺(NH₃)₃Cl]⁺[Pt²⁺Cl₃(NH₃)]^-

iii) [Co³⁺(NH₃)₆]³⁺[Cr³⁺(CN)₆]^3- and[Co³⁺(CN)₆]^3-[Cr(NH₃)₆]³⁺

iv) [Cu²⁺(NH₃)₄]²⁺[Pt²⁺Cl₄]^2- and[Cu²⁺Cl₄]^2-[Pt²⁺(NH₃)₄]²⁺

v) [Cr³⁺(NH₃)₆]³⁺[Cr³⁺(SCN)₆]^3- and [Cr³⁺(NH₃)₄(SCN)₂]⁺[Cr³⁺(SCN)₄(NH₃)₂]^-

Vi) Coordination position isomerism

This type of isomerism is shown by those complex compounds which contain bridging ligands and arises when the non-bonding ligands are differently placed round the central metal atom. Thus (I) and (II) are coordination position isomers to each other, since NH₃ molecules and Cl^- ions (non-bridging ligans) are differently placed round the two Co³⁺ions.

[(NH₃)₄Co³⁺(OH)₂Co³⁺(NH₃)₂Cl₂]²⁺and[Cl(NH₃)₃Co³⁺(OH)₂Co³⁺(NH₃)₃Cl]²⁺are coordination position isomers.

[(NH₃)₄Co³⁺NH₂O₂Co⁴⁺(NH₃)₂Cl₂]²⁺and[(NH₃)₃ClCo³⁺NH₂O₂Co⁴⁺Cl(NH₃)₃]²⁺are coordination position isomers.

vii) Ligand isomerism

There are certain ligands which exist as isomers, e.g., diamine derivatives of propane exist in two isomeric forms which are called 1, 2-diamino propane or propylene diamine (pn) and 1, 3-diamino propane or trimethylene diamine (tn).

This isomerism arises when different isomeric forms of a ligand are bonded to central metal atom.

Examlple: [Co(pn)₂Cl₂]⁺ and [Co(tn)₂Cl₂]⁺ are ligand isomers.

viii) Conformation isomerism

In this isomerism two isomers have different geometries but are otherwiseidentical. For example [Ni²⁺(P.Et.Ph₂)₂Br₂] complex (four coordinated complex) gives two conformation isomers. One of these is green and paramagnetic while the other is brown and diamagnetic. The green form is tetrahedral while the other form is square planar. Thus these have different geometries but the coordination number of Ni²⁺ion in both the isomerism is the same (=4).

Reference.

F.A.Cotton and Wilkinson G.Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D.Concise Inorganic Chemistry.fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary.A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.