Coordination Compounds Notes CBSE Class 12

Coordination Compounds Terminology

Coordination or Complex Compound

The compound which retain their identity in solid state as well as in dissolved state is called coordination compound. In these compounds. the central metal atom or ion is linked by ions or molecules with coordinate bonds. e.g., Potassium ferrocyanide, K₄[Fe(CN)₆].

Coordination Entity

Compound in which the central ion or atom (or the coordination centre) is bound to a set number of atoms, molecules, or ions is called a coordination entity.
[CoCl₃(NH₃)₃] and [Fe(CN)₆]^-4 are coordination entities.

Coordination Sphere

The central ion and the ligands attached to it are enclosed in square bracket which is known as coordination sphere. The ionisable group written outside the bracket is known as counter ions.

Central Metal Ions

The central atoms and anions are atoms and ions attached to ligands. In coordination complexes, the central atoms or ions are Lewis acids and can therefore appear as electron-pair acceptors. In [Ni(CO)₄], Ni is central metal atom. It is generally transition element or inner-transition element.

Coordination Polyhedron

The geometric shape created by the attachment of the ligands to the coordination centre is known as the coordination polyhedron. Such kinds of spatial arrangements in coordination compounds include tetrahedral and square planar shapes.
For example, [Co(NH₃)₆]⁺³ is octahedral, [Ni(CO)₄] is tetrahedral and [PtCl₄]^–2 is square planar.

Homoleptic and Heteroleptic Complexes

When the central atom in complex compound is attached to only one type of ligand, the complex compound is known as homoleptic complex.
Example: [Cu(CN)₄]^-3.
When the central atom in complex compound is attached to different types of ligands, the complex compound is known as heteroleptic complex.
Example: [Co(NH₃)₄Cl₂]⁺.

Ligands

It is a donar atom, molecule or ion that donate a pair of electron to the central atom to form complex compound. It may be negative, neutral or even positive. It is of different types such as monodentate, didentate, tridentate and polydentate etc.
Unidentate ligands: Ligands with only one donor atom, e.g. NH₃, Cl^-, F^- etc.
Bidentate ligands: Ligands with two donor atoms, e.g. ethylenediamine, C₂O₄^-2(oxalate ion) etc.
Tridentate ligands: Ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.
Hexadentate ligands: Ligands which have six donor atoms per ligand, e.g. EDTA.
Ambidentate ligands: These are the monodentate ligands which can ligate through two different sites, e.g., NO^-2, SCN^-, etc.
Chelating ligands: Di or polydentate ligands cause cyclisation around the metal atom which are known as chelate. Such ligands uses two or more donor atoms to bind a single metal ion and are known as chelating ligands. More number of chelate rings means more stable the complex is.

Oxidation Number

The charge of the complex if all the ligands are removed along with the electron pairs that are shared with the central atom, is called oxidation number of central atom.
Oxidation number of copper is +1 in [CU(CN₄)^-3 complex and is represented as Cu(I).

Coordination Number

The coordination number in coordination compounds is defined as the number of ligand (donor) atoms/ions surrounding the central metal atom in a complex ion.
For example, the coordination number of cobalt is six in [Co(NH₃)₆]⁺³ complex ion.

Werner's Theory and Limitations

Werner's Theory and It's Limitations

Valence Bond Theory: Assumptions, Merits and Demerits

Valence Bond Theory: Assumptions, Merits and Demerits

Crystal Field Theory: Assumptions and Limitations

Crystal Field Theory: Assumptions and Limitations

IUPAC Nomenclature of Inorganic Compounds

IUPAC Nomenclature of Inorganic Compounds pdf

Applications of Coordination Compounds

Coordination compounds are found in living systems and have many uses in the home, in industry and in medicines. A few examples are given below:

Extraction of metals

Cyanide ions are used for the for the extraction of gold and silver. The crushed ore is heated with an aq. cyanide solution in the presence of air to dissolve the gold by forming the soluble complex ion [Au(CN)₂]^–.

4Au(s) + 8CN^–(aq) + O₂(g) + 2H₂O(l) → 4[Au(CN)₂]^–(aq) + 4 OH^–(aq)
Zn(s) + 2[Au(CN)₂]^–(aq) → [Zn(CN)₄]^–2 (aq) + 2Au(s)

Complex formation is also useful for the purification of metals. Nickel is purified by converting the metal to the gaseous compound Ni(CO)₄ and then decomposing the latter to pure nickel.

Medicines

EDTA is a chelating agent which is used in the treatment of lead poisoning. Cis platin cis [Pt(NH₃)₂Cl₂] is used in the treatment of cancer. Sodium nitroprusside, Na2[Fe(CN)₅NO] is used to lower blood pressure during surgery.

Qualitative Analyses

Complex formation is useful for qualitative analyses.
1. Separation of Ag⁺ from Pb⁺² & Hg⁺²
Ag⁺ + 2NH₃(aq.) → [Ag(NH₃)₂]⁺(Soluble)
2. Separation of IIA and IIB groups: The cations of IIB group form soluble complex with yellow ammonium sulphide.
3. Cu⁺² ion forms complex on addition of ammonia [Cu(NH₃)₄]⁺²
4. Fe⁺² forms a blue complex with K₃Fe(CN)₆, i.e. K Fe^II[Fe^III(CN)₆].
5. Cobalt(II) gives color with HCl due to the formation of complex [CoCl₄]^–2.
6. Nickel forms a red complex [Ni(DMG)₂] with dimethylglyoxime (H₂DMG).

Isomerism

Isomers are two or more compounds that have the same chemical formula but a different arrangement of atoms. Because of the different arrangement of atoms, they differ in one or more physical or chemical properties. 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, Linkage isomerism
B. Coordination isomerism
C. Ionisation isomerism
D. Solvate isomerism

Stereoisomers have the same chemical formula and chemical bonds but they have different spatial arrangement. Structural isomers have different bonds.

Geometrical isomerism

This type of isomerism arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. Important examples of this behaviour are found with coordination numbers 4 and 6. In a square planar complex of formula [MX₂L₂] (X and L are unidentate), the two ligands X may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans isomer as shown in figure.

Other square planar complex of the type MABXL (where A, B, X, L are unidentates) shows three isomers-two cis and one trans. Such isomerism is not possible for a tetrahedral geometry but similar behaviour is possible in octahedral complexes of formula [MX₂L₄] in which the two ligands X may be oriented cis or trans to each other.

This type of isomerism also arises when didentate ligands L–L [e.g., NH₂-CH₂-CH₂-NH₂ (en)] are present in complexes of formula [MX₂ (L–L)₂].

Another type of geometrical isomerism occurs in octahedral coordination entities of the type [Ma₃b₃] like [Co(NH₃)₃ (NO₂)₃].
If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer

Optical isomerism

Optical isomers are mirror images that cannot be superimposed on one another. These are called as enantiomers. The molecules or ions that cannot be superimposed are called chiral. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarised light in a polarimeter (d rotates to the right, l to the left).

Optical isomerism is common in octahedral complexes involving didentate ligands. In a coordination entity of the type [PtCl₂(en)₂]⁺², only the cis-isomer shows optical activity

Linkage Isomerism

Linkage isomerism arises in a coordination compound containing ambidentate ligand. A simple example is provided by complexes containing the thiocyanate ligand, NCS^–, which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN. Jørgensen discovered such behaviour in the complex [Co(NH₃)₅ (NO₂)]Cl₂, which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO₂).

Coordination Isomerism

This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. An example is provided by [Co(NH₃)₆][Cr(CN)₆], in which the NH₃ ligands are bound to Co⁺³ and the CN^– ligands to Cr⁺³. In its coordination isomer [Cr(NH₃)₆][Co(CN)₆], the NH₃ ligands are bound to Cr⁺³ and the CN^– ligands to Co⁺³.

Ionization Isomerism

This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄.

Solvation Isomerism

This form of isomerism is known as hydrate isomerism in case where water is involved as a solvent. This is similar to ionisation isomerism. Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. An example is provided by the aqua complex [Cr(H₂O)₆]Cl₃ (violet) and its solvate isomer [Cr(H₂O)₅Cl]Cl₂.H₂O (grey-green).