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CBSE Class 12th Chemistry Notes: Coordination Compounds

Oct 13, 2016 16:00 IST

    In this article, you will get the revision notes on CBSE Class 12 Chemistry: Chapter 9 - Coordination Compounds. These notes will give you a quick glance of the chapter. These notes are prepared strictly based on the latest CBSE syllabus for CBSE Class 12th Chemistry.

    The main topics covered in these quick notes are:

    •    Definition of

                    o Coordination compounds

                    o Double salt

                    o Central atom or ion

    •    Ligand and its types

    •    Chelate, Chelating ligands and Chelation

    •    Coordination number and Oxidation number

    •    Nomenclature of coordination compounds

    •    Isomerism in coordination compounds and its types

    •    Werner's theory of coordination compounds

    •    Valence bond theory (VBT)

                    o Principles involved

                    o Limitations

    •    Crystal field splitting theory (CFT)

                    o Crystal field splitting in octahedral complexes

                    o Crystal field splitting in tetrahedral complexes

                    o Stability of coordination compounds

                    o Colour in Coordination Compounds

                    o Limitations of crystal field theory

    •    Metal carbonyls

                    o Structure of some important metal carbonyls

                    o Bonding in metal carbonyls

                    o Properties of metal carbonyls

    •    Applications of coordination compounds

    Double salt

    When two salts in stoichiometric ratio are crystallised together from their saturated solution they are called double salts. They are stable in solid state but dissociate into constituent  ions when dissolved in water.
    For example: FeSO4. (NH4) 2SO4.6H2O (Mohr's salt)

    Central atom or ion

    The atom or ion to which a fixed number of ions or groups are bound in a definite geometrical arrangement around it, is called the central atom or ion.
    For example: In K4[Fe(CN)6], Fe2 is the central metal ion.


    The atoms or groups which are attached directly to central atoms are called ligands. Ligands are Lewis bases which donates electron pair and forms coordinate bonds with the metal atom.

    For example: H2O, CO, NO2, etc.

    A ligand may be neutral, positively or negatively charged.

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    Types of ligands

    On the basis of charge present, ligands may be categorized into the following three groups:

    Anionic ligands: CN (cyanide), NO2 (nitrito-N), NO3 (nitrito), X (halido), etc.

    Cationic ligands: NO2+ (nitrosonium), NO+ (nitronium), N2F5+ (Hydrazenium), etc.

    Neutral ligands: NH3 (ammine), H2O (aqua), CO (carbonyl), etc.

    On the basis of number of coordinating atoms present ligands may be classified as follows:

    Monodentate: It is the ligand which has only one coordinating atom.

    For example: X (halido), NH3 (ammine), H2O (aqua), CN (Cynaido)  .

    Diddentate: It is the ligand which has two coordinating sites available.

    For example: H2NCH2CH2NH2 (ethane-1, 2-diamine) and C2O42‒ (oxalate).

    Polydentate: Ligand containing several donor sites are called polydentate.

    For example: N(CH2CH2NH2)3 (Nitrilotriethylamine, a tetradentate ligand )

    EDTA (Ethylene Diamine tetraetato ion): It is a hexadentate ligand.

    Ambidentate ligand: This is the ligand which can ligate through two different atoms present in it.

    For examples: NO2− ion can coordinate either through nitrogen or through oxygen to a central metal atom/ion.

    Chelate, Chelating ligands and Chelation

    Di- and ploydentqte ligands results in the formation of cyclic structure around the central metal atom. Such cyclic metal complex is called chelate and the ligands which gives chelates are called chelating ligands, and the process is known as chelation.

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    Denticity is defined as the number of coordinating atoms present per ligand.

    Coordination number

    It is defined as the number of coordinate bonds formed by central metal atom, with the ligands. For example: In K4[Fe(CN)6], Fe has coordination number 6.

    Oxidation number

    The oxidation number of the central atom is defined as the charge left on a given atom when all other atom in a complex are removed as ions. The oxidation number is indicated by Roman numerals.

    Nomenclature of coordination compounds

    Step to name a complex compound are:

    Cations are always named before the anions.

    The ligands are then listed in alphabetical order.

    In case of polydentate ligands, ligands are named alphabetically using a prefix di, tri, tetra, penta etc, to indicate the number of ligands of that type present.

    Then the name the metal atom is written followed by its oxidation state in Roman numerals.

    Finallythe anion is named.

    For example: [Cu(NH3)4]SO4  Tetra ammine copper (II) sulphate

    If the ligands itself include di, tri etc. then we use bis – (for two), tris (for three) as prefix. For example:

    [PtCl2(en)2]2+  Dichloridobis (1, 2 – ethanediamine) platinum (IV) ion.

    [Co(en)3]3+ Tris(ethanediamine)cobalt (III) ion.

    If the complex ion is an anion, the name of the metal ends with the suffix – ate.

    For example: [Cr(C2O4)3]3‒ Trioxalatochromate (III) ion.

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    Isomerism in coordination compounds

    Isomers are two or more compounds that have the same chemical formula but a different arrangement of atoms.

    Types of Isomerism

    There are two types of isomerism:

    (a) Stereoisomerism

    (i) Geometrical isomerism (ii) Optical isomerism

    (b) Structural isomerism

    (i) Linkage isomerism       (ii) Coordination isomerism

    (iii) Ionisation isomerism  (iv) Solvate isomerism Structural isomerism


    It arises due to different arrangement of atoms or groups in space. Two different types of stereoisomerism are described as follows:

    (i) Geometrical isomerisms:

    It arises in heteroleptic complexes due to different possible geometrical arrangements of ligands.  In square planar complex of formula [MX2L2] (where 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.

    For example: Geometrical isomers of Pt(NH3)2Cl2) are shown below:

    Similarly in octahedral complexes of formula [MX2L4], two ligands X may be oriented cis or trans to each other.

    For example: Geometrical isomers of [Co(NH3)4Cl2] are shown below:

    (ii) Optical isomerism:

    It arises due to the presence of non-super imposable mirror images. The non-superimposable mirror images are called enantiomers.

    The enantiomers reacts differently with the plane polarised light.

    The enantiomer which rotate the plane polarised light in a clockwise direction is called dextrorotatry (d) or (+) and the enantiomer which rotate the plane polarised light in anticlockwise direction is called lavorotatory (l) or (‒).

    For example: Optical isomers of [Co(en)3 ] 3+ are shown below:

    Structural isomerism

    It arises due to the difference in structures of coordination compounds. Four different types of structural isomerism are described as follows:

    Ionisation isomerism: These are the isomers having the same molecular formula but gives different ions in solution.

    For example: [Cr(NH3)Br]SO4 and [Cr(NH3)(SO4)]Br are ionization isomers.

    Linkage isomerism: Isomers having the same molecular formula but different linking atoms are named as linkage isomers. This arises due to the presence of ambident ligands.

    For example:[CO(NH3)5(NO2)]2+ and [CO(NH3)5 (ONO)]2+ are linkage isomers.

    Coordination isomerism: This type of isomerism is possible when cation and anion both are complex and isomerism arises due to complete exchange of coordination sphere.

    For example: [Pt (NH3)4] [Ni (CN)4] and [Ni (NH3)4] [Pt (CN)4] are coordination isomers.

    Hydration isomerism: Isomers having the same molecular formula but different water molecules of hydration are known as hydration isomers of each other.

    For example: [Cr (H2O)5Cl]Cl2.H2O and [Cr(H2O)4Cl2]Cl.2H2O are hydration isomers.

    Werner's theory of coordination compounds

    According to Werner's theory of coordination compounds, there are two types of valencies in coordination compounds:

      o   Primary valencies: These are ionizable valencies, satisfied by anions and determines the charge on the complex ions.

      o   Secondary valencies: These are non-ionisable valencies, satisfied by ligands and determines the coordination number of the metal atom.

    Valence Bond Theory

    Valence bond theory states that the metal atom or ion under the influence of ligands can use its (n-1)d, ns, np or ns, np, nd orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.

    Following table shows the combination of different number of orbitals to give different types of hybridization:

    Thus the basic principles involved in the valence bond theory are:

    •    Hybridisation of orbitals

    •    Bonding between the ligands and the metal atoms/ion

    •    Relation between the observed magnetic moment and the bond type

    Limitation of the VB theory

    •    It does not tell anything about the spectral properties of the complexes.

    •    It does not give quantitative interpretation of magnetic data.

    •    It does not distinguish between strong and weak ligands.

    •    It does not explain the colour exhibited by coordination compounds.

    •    It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds.

    Magnetic properties of coordination compounds

    A coordination compound is paramagnetic in nature if it has unpaired electrons and diamagnetic if all the electrons in the coordination compound are paired.

    Crystal Field Theory

    In crystal field theory (CFT), ligands are considered as point charges and the interaction between the ligands and the metal ion is purely electrostatic in nature. The five d-orbitals in an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. The degeneracy is lost in the presence of the ligand field.

    The five d-orbitals are classified as:

    Three d-orbitals, dxy, dyz and dzx that are oriented in between the coordinate axes and are called t2g -orbitals.

    Two d-orbitals, dx2y2 and dz2 that are oriented along the x - y axes and are called eg - orbitals.

    Factors affecting the splitting of d-orbitals

    •    Nature of the ligand

    •    Nature of the metal ions

    •    Geometry of complex whether it is octahedral or tetrahedral

    •    Oxidation state of the metal ion

    Crystal field splitting in octahedral complexes

    Here energy of eg set of orbitals > energy of t2g set of orbitals.

    Ligands for which energy separation, Δo < P (the pairing energy, i.e., energy required for

    electron pairing in a single orbital) form a high spin complex.

    Ligands for which energy separation, Δo > P, form low spin complex.

    Crystal field splitting in tetrahedral complexes

    Here energy of t2g set of orbitals > Energy of eg set of orbitals.

    In such complexes d-orbital splitting is inverted and is smaller as compared to the octahedral field splitting.
    No pairing of electrons is possible due to the lowest splitting energies which leads to high spin complexes.

    Stability of coordination compounds

    The stability of the coordination compound depends on

    Nature of the ligand

    Chelating ligands form strong and more stable complexes than the monodentate ligands. The π-  bond ligands forms more stable complexes than the σ- bonded complex.

    Nature of the metal atom/ion

    Small, highly charged metal ions form more stable complexes than large size, lowly charged metal ion.

    Colour in Coordination Compounds

    The crystal field theory attributes the colour of the coordination compounds to d-d transition of the electron, i.e., the transiton of electron from t2g level to the higher eg level which accompanies the absorption of light in visible spectrum.
    In the absence of ligands, crystal field splitting does not occur and hence the substance is colourless.

    Limitations of crystal field theory

    It does not take into account the partly covalent character of bonding between the ligand and the central atom.

    It is also unable to explain the relative strengths of ligands e.g., it does not explain why H2O is stronger ligand than OH.

    Metal carbonyls

    The homoleptic complexes in which carbon monoxide (CO) acts as the ligand are called metal carbonyls.
    For example: Ni(CO)4

    Structure of some important metal carbonyls are:

    Bonding in metal carbonyls

    •   The metal-carbon bond in metal carbonyls possess both s and p character. CO as a ligand binds itself to metal atoms through the carbon atom to form the metal-carbon (M-C) bond. It is a weak donor.

    •   The M–C σ bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal.

    •   The M–C π bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the vacant antibonding π* orbital of carbon monoxide. This characteristic property of back bonding which stabilises the metalligand interaction is termed as synergic effect.

    Properties of metal carbonyls

    •   They are generally solids at room temperature and pressure except Ni(CO)4 and Fe (CO)5.

    •   Mononuclear carbonyls are volatile and toxic.

    •   Mononuclear carbonyls are either colourless or light coloured

    Applications of coordination compounds

    •   EDTA is used in the estimation of Ca2+ and Mg2+ in hardwater.  The Ca2+ and Mg2+ ions form stable complexes with EDTA.

    •   Metals can be purified  by the formation and subsequent decomposition of their coordination compounds. For example, impure nickel is converted to [Ni(CO)4], which is decomposed to yield pure nickel.

    •    In analytical chemistry [Ni(DMG)2]2+ complex is used in the detection of Ni in chocolates.

    •    In medicine, cisplatin, a cis isomer of [Pt(Cl)2(NH3)2] is used in the treatment of cancer.

    •     Solutions of the complexes like [Ag(CN)2] and [Au(CN)2]  can be used for the smooth and even electroplating of metals by gold or silver.

    •     Chlorophyll, a pigment responsible for photosynthesis, is a coordination compound of magnesium. Also haemoglobin, the red pigment of blood which acts as oxygen carrier is a coordination compound of iron.

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