Coordination Compounds Class 12 Notes Chemistry Chapter 9

Introduction

In this chapter, you will be able to learn about the terms : Coordination entity, central atom/ion, ligand, coordination number, types of complexes etc. You will also learn about IUPAC nomenclature of coordination compounds, theories of bonding in terms of Werner’s theory, valence bond theory and crystal field theory.

Definitions of Some Important Terms

1. Addition Compound

Compounds made up of two or more stable compounds by crystallization in a fixed stoichiometric ratio are called addition compound. For example

KCl + MgCl₂ + H₂O ⟶ KCl.MgCl₂.H₂O

Addition compounds are of two types :

(i) Those which lose their identity in solution (double salt)

FeSO₄.(NH₄)₂SO₄.H₂O ⟶ FeSO₄ + (NH₄)₂SO₄ + H₂O

(ii) Those which retain their identity in solution (coordination or complex compounds)

Fe(CN)₂ + 4KCN ⟶ K₄[Fe(CN)₆] (Potassium ferrocyanide)

2. Coordination Entity

It constitutes a central metal atom or ion bonded to a fixed number of ions or molecules with coordinate bonds. For example, in coordination sphere (entity) of [CoCl₃(NH₃)₃], cobalt ion (Co³⁺) is surrounded by three ammonia molecules and three chloride ions.

3. Central Atom/Ion

In coordination entity, the atom/ion to which a fixed number of ions/groups are attached in a definite geometrical arrangement around it, is called central atom/ion. For example, in K₂[PtCl₆], Pt⁺⁴ is central metal ion. Central metal atom or ion accepts lone pairs from the ligands hence it acts as Lewis acid.

4. Ligands

The donor atoms, molecules or anions which donate a pair of electrons to the metal atom/ion are called ligands. Hence ligands are Lewis bases. For example, in [Ni(NH₃)₆]Cl₂, NH₃ is ligand (Lewis base).

5. Coordination Number

It may be defined as number of coordinate bonds formed with central atom/ion by the ligands. For example in coordination entity [Ag(CN)₂]^–,[Cu(NH₃)₄]²⁺ and [Cr(H₂O)₆]³⁺, the coordination number of Ag, Cu and Cr are 2, 4 and 6 respectively.

6. Coordination Polyhedron

The spatial arrangement of ligands around central metal atom/ion is called coordination polyhedron. The most common coordination polyhedra are

Coordination Compounds Chemistry Class 12 Notes

7. Oxidation Number

Oxidation number of the central metal atom/ion in a complex is the charge present on it if all the ligands are removed along with the electron pairs that are shared with central atom. It is represented by Roman numerals in parenthesis after the name of central atom. For example, oxidation number of Co, Fe and Ni in [Co(NH₃)₆]³⁺, [Fe(CN)₆]^4– and [Ni(CO)₄] is +3, +2 and 0, and written as Co(III), Fe(II) and Ni(0) respectively.

8. Homoleptic and Heteroleptic Complexes

Complexes which have only one type of ligands are homoleptic e.g., [Co(NH₃)₆]³⁺ and have more than onetype of ligands are heteroleptic e.g., [Co(NH₃)₄Cl₂]⁺ complexes.

Classification of Ligands

1. On the Basis of Charge on Ligands

(i) Anionic ligands (negatively charged ligands) e.g., F^–, Cl^–, CN^–, S^2–, SO₄^2– etc.

(ii) Neutral ligands (uncharged and electron pair donor) e.g., CO, NH₃, H₂O etc.

(iii) Cationic ligands (positively charged ligands) e.g., NO⁺, NH₂ – NH₃⁺ etc.

2. On the Basis of Denticity

(i) Monodentate: Ligands having single donor atom, e.g., Cl^-, H₂O, NH₃, NH₂ – NH₃⁺, etc. (also called unidentate ligands).

(ii) Bidentate: Ligands can bind through two donor atoms (used simultaneously), e.g.,

(iii) Polydentate: Single ligands having several donor atoms, e.g., EDTA^4- ion (hexadentate)

It can bind through two N and four O atoms to a central atom.

(iv) Ambidentate: Ligands which can bind through two different atoms (only one at a time) to form coordinate bond, e.g., NO₂^-, SCN^- etc.

(v) Flexidentate: Ligands having variable denticity, which depends upon nature of metal ion. For e.g., EDTA having denticity 4 or 6.

3. On the Basis of Type of Donation of Lone Pair

(i) σ-donor: Ligands that donate lone pair and make σ-bond to central atom/ion, e.g., H₂O, NH₃ etc.

(ii) σ-donor π-acceptor: Ligands that donate lone pair to central atom/ion by making σ-bond and accept appreciable amount of electron density from metal atom/ion into its vacant π or π* orbital, e.g., CO, NO etc. (these are also called π-acid ligands).

(iii) π-donor π-acceptor: Ligands that donate and accept π-electrons through π⁺-bonds with central atom/ion, e.g., HC ☰ CH, C₂H₄, C₆H₆ etc.

4. List of Ligand

IUPAC Nomenclature of Coordination Compounds

Coordination compounds are formulated and named according to the system set up by IUPAC (International Union of Pure and Applied Chemistry). It is important for writing systematic names and formulas, particularly when dealing with isomers.

1. Rules for Writing Formulae of Mononuclear Coordination Entities

It is very convenient to get information about constitution of a compound if we know the formula, mononuclear entities contain a single central atom. To write the formula, following rules are applied:

The central atom is listed first.
Ligands are then listed in alphabetical order and their placement does not depend on their charge.
In case of abbreviated ligands (polydentate) the first letter of abbreviation is used to determine the position of ligand in alphabetical order.
Entire coordination entity is enclosed in square brackets, whether charged or not. Polyatomic ligands are enclosed in paranthesis, and also their abbreviations.
No space is left in between names of ligands and central atom/ion.
When the formula of a charged coordination entity is to be written without counter ion, the charge is indicated outside square bracket as a right superscript, with the number before sign. For e.g., [Co(CN)₆]^3–, [Cr(H₂O)₆]³⁺, etc.
Charge on cation and anion is counter balanced.

2. Rules for Naming of Mononuclear Coordination Compounds

Following rules are applied for naming of coordination compounds:

Positive part of complex compounds will be named first, followed by negative part.
Ligands are named first in alphabetical order, followed by central atom (reverse in case of writing formula).
Prefixes mono, di, tri, tetra etc. are used to indicate number of ligands. Prefixes bis, tris, tetrakis are used for complex ligands (including a numerical prefix). For example, [NiCl₂(en)₂]SO₄ is named as dichloridobis (ethylene diamine) nickel (II) sulphate.
Name of anionic ligands end with ‘o’, cationic ligands end with ‘ium’. For neutral ligands regular names are used except ‘aqua’ for H₂O, ‘ammine’ for NH₃, ‘nitrosyl’ for NO, ‘carbonyl’ for CO. These are placed within ( ).
Oxidation state of central atom/ion is indicated in roman numerals in brackett after the name of metal.
When coordination entity has negative charge, then name of central metal ends with ‘ate’, otherwise not. For e.g., in [Fe(CN)₆]^4-, ferrate is used for Fe.

The following examples will make the rules more clear:

(a) K₄[Fe(CN)₆] – Potassium hexacyanoferrate (II)

(b) [Ni(NH₃)₆]Cl₂ – Hexaamminenickel (II) chloride

(d) [NiCl₂(PPh₃)₂] – Dichloridobis (triphenylphosphine) nickel (II)

(e) [Mn(H₂O)₆]²⁺ – Hexaaquamanganese (II) ion

(f) K₂[Ni(EDTA)] – Potassium ethylenediamminetetraacetatonickelate (II)

(g) [Pt(NH₃)₄][PtCl₄] – Tetraammine platinum (II) tetrachloridoplatinate (II)

Werner’s Theory of Coordination Compounds

To explain these properties, Werner proposed following postulates:

Each metal ion possesses two types of valencies:
(a) Primary valency (principle or ionizable)
(b) Secondary valency (subsidiary or non-ionizable)
Primary valency: These are normally ionizable and are satisfied by anions only.
Secondary valency: These are non-ionizable, and are satisfied by ions or neutral electron pair donor molecules (i.e., ligands). It represents coordination number of central metal atom/ion.
Primary valencies are non-directional while secondary valencies are directional.
Geometry of complex is decided by secondary valency. Thus, [Co(NH₃)₆]³⁺, [CoCl(NH₃)₅]²⁺ and [CoCl₂(NH₃)₄]⁺ are octahedral entities, while [Ni(CO)₄] and [PtCl₄]^2- are tetrahedral and square planar respectively.

Effective Atomic Number

Transition metals form coordination compounds very readily because they have vacant 'd' orbitals which can accommodate electron pairs donated by ligands. Metal ion in the complex tends to attain nearest stable inert gas configuration by gaining electrons from ligands. Generally, this is krypton (Z = 36).

Effective atomic number (EAN) of metal in a complex is given by:

EAN = Z–(O.N.) + 2(C.N.) or number of lone pairs donated to central atom.

where Z = atomic number

O.N. = oxidation number

C.N. = coordination number

Bonding in Coordination Compounds

The bonding features in coordination compounds was first described by Werner’s theory. A lot of theories were introduced to explain the nature of bonding in coordination compounds, like Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT).

(1) Valence Bond Theory (VBT)

According to this theory, the metal atom or ion can use (n–1)d, ns, np, nd orbitals for hybridisation under the influence of ligands to yield a set of equivalent orbitals of definite geometry such as square planar, tetrahedral, octahedral and so on. These hybrid orbitals and ligand orbitals are allowed to overlap with each other that latter can donate electron pair for bonding.

For (n–1)d orbitals used in hybridisation, there is formation of inner orbital complex, and for np or nd orbitals the outer orbital complex formation takes place. Generally inner orbital complexes are low spin complexes with lesser number of unpaired electrons while outer orbital complexes are generally high spin complexes with more numbers of unpaired electrons.

(2) Limitations of Valence Bond Theory

The following limitations of VBT are :

Does not give quantitative interpretation of magnetic data.
Does not explain colour of coordination compounds.
Does not give quantitative interpretation of kinetic or thermodynamic stabilities of coordination compounds.
Cannot determine exact predictions regarding tetrahedral and square planar complexes in C.N. = 4
Does not differentiate weak and strong ligands.

(3) Crystal Field Theory (CFT)

Anionic ligands are treated as negative point charges and neutral ligands as point dipoles.
Interaction between metal atom and ligand is purely electrostatic. There is no intermixing of atomic orbitals or no insertion of electron in metal orbitals.
If the field produced by surrounding ligand is symmetrical then d-orbitals of metal is degenerate. But in most of complexes, degeneracy is lost because field produced by ligand is not symmetrical and all dorbitals are not equally affected by ligand field.
Loss of degeneracy leads to splitting of d-orbitals and in their energies, called crystal field splitting (CFS).

(i) Crystal field splitting in octahedral coordination entities Six ligands are surrounded to metal atom/ion. Since eg orbitals are more directional towards orbitals, they experience greater repulsion and their energy is higher. t_2g orbitals lie away from ligands, hence they have lesser repulsion and lower energy.

(ii) Crystal field splitting in tetrahedral coordination entities Coordination number and number of ligands = 4. t_2g set have higher energy than eg set as t_2g orbitals are more closer to the direction of approach of ligands.

(4) Limitations of CFT

Assuming ligands as point charges, it follows that anionic ligands should exert the greatest splitting effect. The anionic ligands actually found at low end of spectrochemical series.
It does not explain the covalent character of bonding between ligand and central atom.

Importance and Application of Coordination Compounds

Detection and estimation of metal ions in qualitative and quantitative analysis. For example, EDTA, DMG, α-nitroso-β-naphthol, cupron, etc. give colour reactions.
Hardness of water is estimated by titration of Ca²⁺ and Mg²⁺ with Na2EDTA, and their estimation can be done due to difference in their stability constants.
Extraction of metals like silver and gold make use of complex compounds. For e.g., gold forms[Au(CN)₂]^- in aqueous solution and can be separated in metallic form by addition of zinc.
Purification of metals through formation and subsequent decomposition of coordination compounds. For example Ni (impure) is converted to [Ni(CO)₄], decomposed to pure nickel.
In biological systems, pigments responsible for photosynthesis, chlorophyll is a complex of Magnesium. Haemoglobin, the red blood pigment of blood (oxygen carrier) is complex of iron. Vitamin B₁₂, cyanocobalmine, the anti-pernicious anaemia factor, is complex of cobalt.
Complexes, like rhodium complex, [(Ph₃P)₃RhCl], a Wilkinson’s catalyst, is used for hydrogenation of alkenes.
Articles can be electroplated with silver and gold much more smoothly and evenly from solution of complexes, [Ag(CN)₂]^- and [Au(CN)₂]^- than from a solution of simple metal ions.
The developed film in black and white photography is fixed by washing with hyposolution which dissolves the undecomposed AgBr to form complex, [Ag(S₂O₃)2]^3-.
Chelate therapy is used in medicinal industry. For example, EDTA is used in treatment of lead poisoning. Growth of tumours can be inhibited by some complexes of platinum, e.g., cis-platin [PtCl₂(NH₃)₂] andrelated compounds.

Summary

Coordination compounds: Compounds formed due to combination of two or more simple stable salts, which retain their identity in solution (dissolved state) as well as solid.
Ligands: The donor atoms, molecules or anions which donate a pair of electrons to metal atom/ion.
Coordination entity: It constitutes a central metal atom/ion bonded to a fixed number of ligands with coordinate bonds.
Primary valency: Normally, it is ionizable valency and satisfied by anions only.
Secondary valency: Non-ionizable valency, satisfied by ligands only.
Structural isomerism: It is shown by compounds that have different ligands within coordination entity.
Stereoisomerism: It is due to different spatial arrangement of ligands around metal.
Valence Bond Theory (VBT): According to this theory, the metal atom/ion can use (n–1)d, ns, np, nd orbitals for hybridisation, under the influence of ligands, to yield a set of equivalent orbitals, in definite geometry.
Crystal Field Theory (CFT): CFT is an electrostatic model which describes electronic structure of metal ion in ionic crystals.
CFSE (Δ): Difference between energies of e_g orbitals and t_2g orbitals is called crystal field stabilization energy.
Spectrochemical series: Series in which ligands are arranged in order of increasing field strength.
Metal carbonyls: Complexes formes by most of transition metals with carbon monoxide through both σ- and π-bonds, to form homoleptic carbonyls.
Synergic bonding: Bonding between metal and ligand due to donation of electron pair from ligand to metal atom/ion and acceptence of electron pair from d-orbital of metal to ligand.