Co-ordination Compound- Questions

Short Answer Type Questions

  1.  Arrange the following complexes in the increasing order of conductivity of their solution: [Co(NH3)3Cl3], [Co(NH3)4Cl2]Cl, [Co(NH3)6]Cl3, [Cr(NH3)5Cl]Cl2
  2. A coordination compound

    CrCl3⋅4H2O precipitates silver chloride when treated with silver nitrate. The molar conductance of its solution corresponds to a total of two ions. Write structural formula of the compound and name it.

  3. A complex of the type [M(AA)2X2]n+ is known to be optically active. What does this indicate about the structure of the complex?

    Give one example of such complex.

  4. Magnetic moment of [MnCl4]2– is 5.92 BM. Explain giving reason.
  5. On the basis of crystal field theory explain why Co(III) forms paramagnetic octahedral complex with weak field ligands whereas it forms diamagnetic octahedral complex with strong field ligands.
  6. Why are low spin tetrahedral complexes not formed?
  7. Give the electronic configuration of the following complexes on the basis of Crystal Field Splitting theory.
    [CoF6]3- , [Fe(CN)6]4- and [Cu(NH3)6]2+.
  8. Explain why [Fe(HO)6]3+ has magnetic moment value of 5.92 BM whereas
    [Fe(CN)6]3- has a value of only 1.74 BM.
  9. Arrange following complex ions in increasing order of crystal field splitting energy (ΔO):
    [Cr(Cl)6]3– , [Cr(CN)6]3–,[Cr(NH3)6]3+.
  10. Why do compounds having similar geometry have different magnetic moment?
  11. CuSO4.5H2O is blue in colour while CuSO4 is colourless. Why?
  12. Name the type of isomerism when ambidentate ligands are attached to central metal ion. Give two examples of ambidentate ligands.

Assertion and Reason Type Questions

Note : In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(i) Assertion and reason both are true, reason is correct explanation of  assertion.
(ii) Assertion and reason both are true but reason is not the correct  explanation of assertion.
(iii) Assertion is true, reason is false.
(iv) Assertion is false, reason is true.

  1. Assertion : Toxic metal ions are removed by the chelating ligands.
    Reason : Chelate complexes tend to be more stable.
  2. Assertion :[Cr(H2O)6]Cl2 and [Fe(H2O)6]Cl2 are reducing in nature.
    Reason : Unpaired electrons are present in their d-orbitals.
  3. Assertion : Linkage isomerism arises in coordination compounds containing ambidentate ligand.
    Reason : Ambidentate ligand has two different donor atoms.
  4. Assertion : Complexes of MX6 and MX5 L type (X and L are unidentate) do not show geometrical isomerism.
    Reason: Geometrical isomerism is not shown by complexes of coordination number
  5. Assertion : ([Fe(CN)6]3- ion shows magnetic moment corresponding to two unpaired electrons.
    Reason : Because it has d2sp3 type hybridisation.

Long Answer Type Questions

  1. Using crystal field theory, draw energy level diagram, write electronic configuration of the central metal atom/ion and determine the magnetic moment value in the following :
    (i) [CoF6]3– ,[Co(H2O)6]2+, [Co(CN)6]3–
    [FeF6]3– , [Fe(H2O)6]2+ , [Fe(CN)6]4–
  2. Using valence bond theory, explain the following in relation to the complexes given below:
    [Mn(CN) 6 ]3– , [Co(NH3)6]3+ , [Cr(H2O)6]3+ , [FeCl6]4–
    (i) Type of hybridisation.
    (ii) Inner or outer orbital complex.
    (iii) Magnetic behaviour.
    (iv) Spin only magnetic moment value.
  3. CoSO4 Cl.5NH3

    exists in two isomeric forms ‘A’ and ‘B’. Isomer ‘A’ reacts with AgNO 3 to give white precipitate, but does not react with BaCl2. Isomer ‘B’ gives white precipitate with BaCl2 but does not react with AgNO3 . Answer the following questions.
    (i) Identify ‘A’ and ‘B’ and write their structural formulas.
    (ii) Name the type of isomerism involved.
    (iii) Give the IUPAC name of ‘A’ and ‘B’.

  4. What is the relationship between observed colour of the complex and the wavelength of light absorbed by the complex?
  5. Why are different colours observed in octahedral and tetrahedral complexes for the same metal and same ligands?

Co-ordination Compounds-III

Isomerism in Coordination Compounds-
Coordination compounds exhibit the following types of isomerism:
1.Structural Isomerism-In this isomerism, isomers have different bonding pattern. Different types of structural isomers are
(i)Linkage isomerism– This type of isomerism is shown by the coordination compounds having ambidentate ligands. e.g.,[Co(NH3)5(NO2)]Cl and [Co(NH3)5(ONO)]Cl or
pentaammine nitrito- N Cobalt (III) chloride and pentaammine nitrito-O’Cobalt (III) chloride.
(ii)Coordination isomerism– This type of isomerism arises from the interchange of ligands between cationic and anionic complexes of different metal ions present in a
complex, e.g.,[Cr(NH3)6) [CO(CN)6] and [CO(NH3)6] [Cr (CN)6]
(iii) Ionisation isomerism– This isomerism arise due to exchange of ionisable anion with anionic ligand.
(iv) Solvate isomerism-This is also known as hydrate isomerism. In this isomerism, water is taken as solvent. It has different number of water molecules in the coordination
sphere and outside it. e.g..[Co(H2O)6]CI3, [Co(H2O) 4C12]Cl·2H2O, [Co(H2O)3Cl3].3H2O
2. Stereoisomerism– Stereoisomers have the same chemical formula and chemical bonds but they have different spatial arrangement. These are of two types :
(i)Geometrical isomerism-Geometrical isomers are of two types i.e., cis and trans isomers. This isomensm is common in complexes with coordination number 4 and 6.
Geometrical isomerism in complexes with coordination number 4
(i)Tetrahedral complexes do not show geometrical isomerism.
(ii) Square planar complexes of formula [MX2L2] (X and L are unidentate) show geometrical isomerism. The two X ligands may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans isomer,
(iii) Square planar complex of the type [MABXL] (where A, B, X, L, are unidentate ligands) shows three isomers, two cis and one trans.e.g., [Pt(NH3) (Br)(Cl)(Py)].
Geometrical isomerism in complexes with coordination number 6.

Octahedral complexes of formula [MX2L4], in which the two X ligands may be oriented cis or trans to each other, e.g., [Co(NH3)4Cl2)]+. Octahedral complexes of formula [MX2A2], where X are unidentate ligands and A are bidentate ligand.form cis and
trans isomers, e.g., [CoC12(en)2]’.
In octahedral complexes of formula [MA3X3], if three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face. it is known as facial (fae)
isomer, when the positions are around the meridian of the octahedron, it is known as meridional (mer) isomer. e.g., [Co(NH3)3(NO2)3]
(ii) Optical isomerism– These are the complexes which have chiral structures. It arises when mirror images cannot be superimposed on one another. These mirror images are
called enantiomers. The two forms are called dextro (d) and laevo (l) forms.

Tetrahedral complexes with formula [M(AB)2] show optical isomers and octahedral complexes (cis form) exhibit optical isomerism.
Bonding in Coordination Compounds-
Werner’s Theory– Metals exhibit two types of valencies in the formation of complexes.These are primary valencies and secondary valencies.
1. Primary valencies correspond to oxidation number (ON) of the metal and are satisfied by anions. These are ionisable and non-directional.
2. Secondary valencies correspond to coordination number (CN) of the metal atom and are satisfied by ligands. These are non-ionisable and directional. Hence, geometry is
decided by these valencies.
Valence Bond Theory(VBT)-
This theory was proposed by L.Pauling in 1930 s. According to this theory, when a complex is formed, the metal ion/atom provides empty orbitals to the surrounding
ligands. Coordination number shows the number of such empty orbitals, i.e., number of empty orbitals is equal to the coordination number. These empty orbitals hybridised
before participation in bonding and the nature of hybridisation depends on the nature of metal and on the nature of approaching ligand.
Inner orbital complexes or outer orbital complexes.
When outer d-orbital are used in bonding, the complexes are called outer orbital complexes. They are formed due to weak field ligands or high spin ligands and hybridisation is sp3d2. They have octahedral shape.
When d- orbitals of (n – 1) shell are used, these are known as inner orbital complex, they are formed due to strong field ligands or low spin ligands and hybridisation is d2sp3.
They are also octahedral shape.
1. 6 – ligands(unidentate), octahedral entity.
(i) Inner orbital complex- [Co(NH3)6]3+ All electrons are paired, therefore complex will be diamagnetic in nature.
(ii) Outer orbital complex, [CoF6]3- Complex has unpaired electrons, therefore, it will be paramagnetic in nature.
2. 4-ligands(unidentate) tetrahedral entity
(i)Inner orbital complex, [Ni(CN)4]2- All electrons are paired so complex will be diamagnetic in nature.
(ii) Outer orbital complex,[CoCI4]– Since, complex has unpaired electrons. so it will be paramagnetic in nature.
Limitations of VBT-
This theory could not explain the quantisation of the magnetic data, existence of inner orbital and outer orbital complex, change of magnetic moment with temperature and
colour of complexes.
Crystal Field Theory (CFT)-
This theory was proposed by H.Bethe and van Vleck. Orgel. in 1952, applied this theory to coordination compounds. In this theory, ligands are treated as point charges in case of
anions and dipoles in case of neutral molecules.
The five d-orbitals are classified as
(i) Three d-orbitals i.e., dxy, dyz and dzx are oriented in between the coordinate axes and are called t2g – orbitals.
(ii) The other two d-orbitals, i.e., dx2– y2 and dz2 oriented along the x – y % axes are called eg – orbitals.
Due to approach of ligands, the five degenerate d-orbitals split. Splitting of d-orbitals depends on the nature of the crystal field.[The energy difference between t2g and eg
level is designated by Δ and is called crystal field splitting energy.] By using spectroscopic data for a number of coordination compounds, having the same metal ions but
different ligand, the crystal field splitting for each ligand has been calculated. A series in which ligand are arranged in order of increasing magnitude of crystal field splitting, is
called spectrochemical series.
Limitations of CFT
1. It does not consider the formation of 7t bonding in complexes.

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

3. It gives no account of the partly covalent nature of metal-metal bonds.
Ligand Field or Molecular Orbital Theory-
This theory was put forward by Hund and Mulliken. According to this theory, all the atomic orbitals of the atom participating in molecule formation get mixed to give rise an
equivalent number of new orbitals, called the molecular orbitals. The electrons are now under the influence of all the nuclei.
Stability of Coordination Compounds
The stability of complex in solution refers to the degree of association between the two species involved in the state of equilibrium. It is expressed as stability constant (K).The
factors on which stability of the complex depends :
(i)Charge on the central metal atom. As the magnitude of charge on metal atom increases, stability of the complex increases.
(ii)Nature of metal ion. The stability order is 3d < 4d <5d series.

(iii)Basic nature of ligands. Strong field ligands form stable complex.
The instability constant or the dissociation constant of
compounds is defined as the reciprocal of the formation or
stability Constant.
Importance and Applications of Coordination Compounds –
1. They are used in many qualitative and quantitative analysis.

2. Hardness of water is estimated by simple titration with Na2 EDTA.

3. Purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds.

4. They have great importance in biological systems.

5. They are used as catalyst for many industrial processes.

6. In medicinal chemistry, there is a growing interest of chelating therapy.

Organometallic Compounds- They contain one or more metal-carbon bond in their molecules. They are of the following types:
1.Sigma (σ) bonded compounds- Metal-carbon bond is sigma bond, e.g.,(C2H5)4 Pb, Zn(C2H5)2 R – Mg – X, etc.
2.Pi(π) bonded compounds- In which molecules/ions containing π bonds act as a ligand. e.g., Ferrocene, Dibenzene chromium and Zeise’s salt. Zeise’s salts is K [PtCI3(η2 – C2H4)] In which ethylene acts as a ligand which do not have a lone pair oi electron.In ferrocene, Fe (η5 – C5H5)2 represents the number of carbon atoms with which metal ion is directly attached.
3.σ and π bonded compounds- Metal carbonyls are their examples. Metal-carbon bond of metal carbonyls have both σ and π – bond character. They have CO molecule as
Wilkinson’s catalyst (Rh(PPh3)3CI] is used as homogeneous catalyst in the hydrogenation of alkenes. Zeigler-Natta catalyst [Ti CI4 + (C2H5>3Al] acts as heterogeneous catalyst in the polymerisation of ethylene.


Co-ordination Compounds- II

Types of Complexes-
1. Homoleptic complexes-
Complexes in which the metal atom or ion is linked to only one kind of donor atoms, are called homoleptic complexes e.g., [Co(NH3)6]3(+ve ions).
2. Heteroleptic complexes-
Complexes in which the metal atom or ion is linked to more than one kind of donor atoms are called heteroleptic complexes e.g., [Co(NH3)4CI2](1 +ve ion).
3. Labile and Inert complexes-
Complexes in which the ligand substitution is fast are known as labile complexes and in which ligand substitution is slow, are known as inert complexes.
Effective Atomic Number (EAN) – 
This concept was proposed by Sidgwick. In a complex, the EAN of metal atom is equal to the total number of electrons present in it.
EAN = Z – ON of metal + 2 * CN
(where, Z = atomic number of metal atom
ON = oxidation number of metal
and CN = coordination number of complex).
An ion with central metal atom having EAN equal to next inert gas will be more stable.
IUPAC Naming of Complex Compounds –
Naming is based on set of rules given by IUPAC.
1. Name of the compound is written in two parts (i) name of cation, and (ii) name of anion.
2. The cation is named first in both positively and negatively charged coordination complexes.
3. The dissimilar ligands are named in au alphabetical order before the name of central metal atom or ion.
4. For more then one similar ligands. the prefixes di, tri, tetra, etc are added before its name. If the di, tri, etc already appear in the complex then bis, tris, tetrakis are used.
5. If the complex part is anion, the name of the central metal ends with suffix ‘ate’.
6. Names of the anionic ligands end in ‘0’, names of positive ligands end with ‘ium’ and names of neutral ligands remains as such. But exception are there as we use aqua for H2O, ammine for NH3, carbonyl for CO and nitrosyl for NO.
7. Oxidation state for the metal in cation, anion or neutral coordination compounds is indicated by Roman numeral in parentheses.
8. The name of the complex part is written as one word.
9. If the complex ion is a cation, the metal is named same as the element.
10. The neutral complex molecule is named similar to that of the complex cation.
Some examples are
(i) [Cr(NH3)3(H2O)3]Cl3
triamminetrichlorochromium (III) chloride
(ii) [Co(H2CH2CH2H2)3]2(SO4)3
tris (ethane-l,2-diamine) cobalt (III) sulphate
(iii) [Ag(NH3)2] [Ag(CN)2]
diamminesilver (I) dicyanoargentate(I)
(iv) K4 [Fe(CN)6]
potassium hexacyanoferrate


Co-ordination Compounds- I

Coordination compounds are those addition molecular compounds which retain their identity in solid state as well as in dissolved state.
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].

Double Salts-
These are the addition molecular compounds which are stable in solid state but dissociate into constituent ions in the solution. e.g., Mohr’S salt, [FeSO4.(NH4)2So4.6H2O] get dissociate into Fe(2 +ve ions), NH4(1 +ve ion) and SO4(2 -ve ions).

Terms Related to Coordination Compounds-

1. Complex ion or Coordination Entity-
It is an electrically charged species in which central metal atom or ion is surrounded by number of ions or neutral molecules.
(i) Cationic complex entity– It is the complex ion which carries positive charge. e.g., [Pt(NH3)4]2+
(ii) Anionic complex -It is the complex ion which carries negative charge. e.g., [Fe(CN)6].

2. Central Atom or Ion-
The atom or ion to which a fixed number of ions or groups are bound is .ned central atom or ion. It is also referred as Lewis acid. e.g., in (NiCI2(H2O)4]. Ni is central metal atom. It is generally transition element or inner-transition element.

3. Ligands –
Ligands is electron donating species (ions or molecules) bound to the Central atom in the coordination entity.

These may be charged or neutral. LIgands are of the following types :
(i) Unidentate-  It is a ligand, which has one donor site, i.e., the ligand bound to a metal ion through a single donor site. e.g., H2O, NH3, etc.
(ii) Didentate– It is the ligand. which have two donor sites.
(iii) Polydentate- It is the ligand, which have several donor sites. e.g., [EDTA]4(-ve ions) is hexadentate ligand.
(iv) Ambidentate ligands- These are the monodentate ligands which can ligate through two different sites, e.g., NO(2 -ve ions), SCN(-ve ion), etc.
(v) Chelating ligands- Di or polydentate ligands cause cyclisation around the metal atom which are known as chelate IS , Such ligands uses two or more donor atoms to bind a single metal ion and are known as chelating ligands.
More the number of chelate rings, more is the stability of complex. The stabilisation of coordination compounds due to chelation is known as chelate effect. π – acid ligands are those ligands which can form π – bond and n-bond by accepting an appreciable amount of 1t electron density from metal atom; to empty π or π – orbitals.

4. Coordination Number-
It is defined as the number of coordinate bonds formed by central metal atom, with the ligands.e.g., in [PtCI6]2(-ve ions), Pt has coordination number 6. In case of monodentate ligands, Coordination number = number of ligands. In polydentate ligands, Coordination number = number of ligands * denticity.

5. 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.

6. Coordination Polyhedron-
The spatial arrangement of the ligands which are directly attached to the central atom or ion, is called coordination polyhedron around the central atom or ion.

7.Oxidation Number of Central Atom-
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.e.g., [CU(CN4)3(-ve ions), oxidation number of copper is +1, and represented as Cu(I).