NCERT Solutions for class 12th Chemistry Chapter 9 Coordination Compounds


Question 1. Write the formulas for the following coordination compounds:

Question 2. Write the IUPAC names of the following coordination compounds:

Question 3. Indicate the types of isomerism exhibited by the following complexes and draw the structures for these isomers:

Question 4. Give evidence that [Co(NH3)5Cl]SO4 and [Co(NH3)5SO4]Cl are ionisation isomers.

Question 5. Explain on the basis of valence bond theory that [Ni(CN)4]2 ion with square planar structure is diamagnetic and the [NiC14]2 ion with tetrahedral geometry is paramagnetic.

Question 6. [NiCl4]2- is paramagnetic while [Ni(CO)4] is diamagnetic though both are tetrahedral. Why?

Question 7. [Fe(H2O)6]3+ is strongly paramagnetic whereas [Fe(CN)6]3 is weakly paramagnetic. Explain.

Question 8. Explain [Co(NH3)6]3+ is an inner orbital complex whereas [Ni(NH3)6]2+ is an outer orbital complex.

Question 9. Predict the number of unpaired electrons in the square planar [Pt(CN)4]2- ion.

Question 10. The hexa aqua manganese (II) ion contains five unpaired electrons, while the hexa cyano ion contains only one unpaired electron. Explain using Crystal Field Theory.

Question 11. Calculation the overall complex dissociation equilibrium constant for the Cu(NH3)42+ ion, given that Beta4, for this complex is 2.1 X 1013.


Question 1. Explain the bonding in coordination compounds in terms of Werner’s postulates.

Sol. The main postulates of Werner’s theory of coordination compounds are as follows:

(i) Metals possess two types of valencies called;

  • (a) primary valency which are ionisable
  • (b) secondary valency which are non-ionisable

(ii) Primary valency is satisfied by the negative ions and metals exhibits primary valency in the formation of its simple salts.

(iii) Secondary valencies are satisfied by neutral ligand or negative ligand and are those which metal exercises in the formation of its complex ions. In modern theory, it is now referred as coordination number of central metal atom or ion.

(iv) Every cation has a fixed number of secondary valencies which are directed in space about central metal ion in certain fixed directions, e.g., in CoCl36NH3, valencies between Co and Cl are primary valencies and valencies between Co and NH3 are secondary valencies.

In COCl36NH3, six ammonia molecules linked to CO by secondary valencies are directed to six corners of a regular octahedron and thus account for structure of CoCl36NH3 as follows:

Question 2. FeSO4 solution mixed with (NH4)2SO4 solution in 1 : 1 molar ratio gives the test of Fe2+ ion but CuSO4 solution mixed with aqueous ammonia in 1 : 4 molar ratio does not give the test of Cu2+ ion. Explain why.

Sol. FeSO4 solution mixed with (NH)2SO4 solution in 1:1 molar ratio forms a double salt, FeSO4.(NH4)2SO4.6Hp (Mohr’s salt) which ionises in the solution to give Fe2+ ions. Hence, it gives the tests of Fe2+ ions.
CuSO4 solution mixed with aqueous ammonia in 1 : 4 molar ratio forms a complex salt, with the formula [Cu(NH3)4]SO4. The complex ion [Cu(NH3)4]2+ does not ionise to give Cu2+ ions. Hence, it does not give the tests of Cu2+ ion.

Note: Double salts give the test of all the constituent ions whereas complexes do not.

Question 3. Explain with two examples each of the following: coordination entity, ligand, coordination number, coordination polyhedron, homoleptic and heteroleptic.

Sol. Coordination entity: It constitutes of a central atom/ion bonded to fixed number of ions or molecules by coordinate bonds e.g. [CoCl3 (NH3)3], [Ni (CO)4] etc.

Ligand : The ions/molecules bound to central atom/ion in coordination entity are called ligands. Ligands in above examples are Cl, NH3, CO.

Coordination number : This is the number of bond formed by central atom/ion with ligands. e.g. C.N. in [Ni(H2O)4en]2+ is 6 and in [Ni(CO)4] is 4.

Coordination polyhedron : Spatial arrangement of ligands defining the shape of complex. Co and Ni polyhedron are octahedral and tetrahedral in [CoCl3 (NH3)3] and [Ni(CO)4] respectively.

Homoleptic: Metal is bound to only one kind of ligands e.g. [Ni(CO)4] and [Fe(CN)l3- are homoleptic complexes.

Heteroleptic : Metal is bound to more than one kind of ligands e.g.[CoCl3(NH3)3] and [Ni(H2O)4en]2+ are heteroleptic complexes.

Question 4. What is meant by unidentate, didentate and ambidentate ligands? Give two examples for each.
Denticity refers to the number of donor groups in a single ligand that bind to a central atom in a coordination complex.

Sol. A molecule or an ion which has only one donor atom to form one coordinate bond with the central metal atom is called unidentate ligand, e.g., Cl and ­ A molecule or ion which contains two donor atoms and hence forms two coordinate bonds with the central metal atom is called a didentate ligand, e.g.,

A molecule or an ion which contains two donor atoms but only one of them forms a coordinate bond at a time with the central metal atom is called ambidentate ligand, e.g., :CN or NC and NO2 or ONO

Question 5. Specify the oxidation numbers of the metals in the following coordination entities:

Question 6. Using IUPAC norms write the formulas for the following:
(i) Tetrahydroxozincate (II)
(ii) Potassium tetrachloridopalladate (II)
(iii) Diamminedichlorido platinum (II)
(iv) Potassium tetracyanonickelate (II)
(v) Pentaamminenitrito-O-cobalt(III)
(vi) Hexaamminecobalt (III) sulphate
(vii) Potassium tri(oxalato) chromate (III)
(viii) Hexaammineplatinum (IV)
(ix) Tetrabromidocuprate(II)
(x) Pentaamminenitrito-N-cobalt (III)

Note: When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parenthesis.

Question 7. Using IUPAC norms write the systematic names of the following:

Sol. (i) Hexaamminecobalt (III) chloride.
(ii) Diamminechloridomethylamine platinum (II) chloride.
(iii) Hexaaquatitanium (III) ion.
(iv) Tetraamminechloridonitrito-N-cobalt(IV) chloride.
(v) Hexaaquamanganese (II) ion.
(vi) Tetrachloridonickelate(II) ion.
(vii) Hexaamminenickel (II) chloride.
(viii) Tris(ethane-1,2-diamine)cobalt (III) ion.
(ix) Tetracarbonylnickel(0).

Note: (i) Cation is named first followed by the name of anion.
(ii) Ligands are named in alphabetical order before the name of central metal atom/ ion. When there is a numerical prefix in the name of a ligand, then the terms, bis, tris, tetrakis are used.
(iii) If the complex ion is an anion, the name of the metal ends with the suffix-ate.
(iv) Oxidation state of metal is written in Roman numeral in parenthesis.

Question 8. List various types of isomerism possible for coordination compounds, giving an example of each.

Sol. Coordination compounds exhibit stereo isomerism and structural isomerism.
Two types of stereoisomerism and their examples are as follows:

Question 9. How many geometrical isomers are possible in the following coordination entities?

Question 10. Draw the structures of optical isomers of

Question 11. Draw all the isomers (geometrical and optical) of

Question 12. Write all the geometrical isomers of [Pt(NH3)(Br)(Cl) (Py)] and how many of these will exhibit optical isomerism?

Question 13. Aqueous copper sulphate solution (blue in colour) gives:

(i) a green precipitate with aqueous potassium fluoride and
(ii) a bright green solution with aqueous potassium chloride. Explain these experimental results.

Note: The crystal field theory attributes the colour of the coordination compounds to d-d transition of the electron. In the absence of ligand, the substance is colourless.

Question 14. What is the coordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate? Why is it that no precipitate of copper sulphide is obtained when H2S (g) is passed through this solution?

Question 15. Discuss the nature of bonding in the following coordination entities on the basis of valence bond theory:

Question 16. Draw figure to show the splitting of d-orbitals in an octahedral crystal field.

Question 17. What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.

Question 18. What is crystal field splitting energy? How does the magnitude of Del0 decide the actual configuration of d-orbitals in a coordination entity?

Question 19. [Cr(NH3)6]3+ is paramagnetic while [Ni(CN)4]2- is diamagnetic. Explain why?

Question 20. A solution of [Ni(H20)6]2+ is green but a solution of [Ni(CN)4]2- is colourless. Explain.

Sol. In [Ni(H20)6]2+, Ni is in + 2 oxidation state and having 3d8 electronic configuration, in which there are two unpaired electrons which do not pair up in the presence of the weak H2O ligand. Hence, it is coloured. The d d transition absorbs red light and the complementary light emitted is green. In [Ni(CN)4]2-, Ni is also in + 2 oxidation state and having 3d8 electronic configuration. But in presence of strong ligand CN-, the two unpaired electrons in the 3d orbitals pair up. Thus, there is no unpaired electron present. Hence, it is colourless.

Question 21. [Fe(CN)6]4- and [Fe(H20)6]2+are of different colours in dilute solutions. Why?

Sol. In both the complexes, Fe is in + 2 oxidation state with d6 configuration. This means that it has four unpaired electrons. Both CN ion and H20 molecules act as ligands occupy different relative positions in the spectrochemical series but they differ in crystal field splitting energy (L). Quite obviously, they absorb radiations corresponding to different wavelengths and frequencies from the visible region of light (VIBGYOR), and thus the transmitted colours are also different. This means that the complexes have different colours in solutions.

Question 22. Discuss the nature of bonding in metal carbonyls.

Sol. In metal carbonyl, the metal carbon bond (M – C) possess both the a and pi -bond character. The bond are formed by overlap of atomic orbital of metal with that of C-atom of carbon monoxide in the following sequence:

(i) a-bond is first formed between metal and carbon when a vacant d-orbital of metal atom overlaps with an orbital containing lone pair of electrons on C-atom of carbon monoxide(: C = 0:)

(ii) In addition to a-bond in metal carbonyl, the electrons from filled d-orbitals of a transition metal atom/ion are back donated into anti bonding pi­ orbitals of carbon monoxide. This stabilises the metal ligand bonding. The above two concepts are shown in following figure:

Question 23. Give the oxidation state, d-orbital occupation and coordination number of the central metal ion in the following complexes:

Question 24. Write down the IUPAC name for each of the following complexes and indicate the oxidation state, electronic configuration and coordination number. Also give stereochemistry and magnetic moment of the complex:

Question 25. What is meant by stability of a coordination compound in solution? State the factors which govern stability of complexes.

Greater the stability constant, stronger is the metal-ligand bond. The stability of complex will depend on:

  1. Nature of metal
  2. Oxidation state of metal
  3. Nature of ligand e.g. chelating ligand form more stable complexes
  4. Greater the basic strength of the ligand, more will be the stability.

Note: instability constant may be defined as equilibrium constant for reverse reaction.

Question 26. What is meant by the chelate effect? Give an example.

Sol. When a didentate or a polydentate ligand contains donor atoms positioned in such a way that when they coordinate with the central metal ion, a ring is formed, the effect is called chelate effect. For e.g.

Note: Complexes containing chelate rings are more stable than complexes without chelate rings.

Question 27. Discuss briefly giving an example in each case the role of coordination compounds in:

  1. biological systems
  2. medicinal chemistry and
  3. analytical chemistry
  4. extraction/metallurgy of metals.

Sol. (i) In Biological System
The chlorophyll is a vital component of photosynthesis, which is how plants get their energy. It contains the porphyrin ring but the metal ion there is Mg2+ rather than Fe2+.

(ii) In Medicinal Chemistry
Certain coordination compounds of platinum are used for inhibiting the growth of tumors e.g. cis-platin.

(iii) In Analytical Chemistry
There are numerous reactions in quantitative analysis where complexes are formed. To mention a few are –

  1. Estimation of nickel and other metals as dimethylglyoxime complex.
  2. The most common application is the estimation of several cations (Mg2+, Ca2+, Zn2+, Ni2+ and AJ3+) using EDTA as titrant in presence of suitable
  3. indicator, e.g., murexide solution.
  4. EDTA is used in the estimation of Ca2+ and Mg2+ ions in water (total hardness of water).
  5. EDTA is used in softening of hard water.

(iv) In Extraction Metallurgy of Metals

Extraction of metals like silver and gold is carried out by forming their soluble cyanide complex for e.g.

Question 28. How many ions are produced from the complex Co(NH3)6Cl2 in solution?
(i) 6 (ii) 4 (iii) 3 (iv) 2

Question 29. Amongst the following ions? Which one has the highest magnetic moment value:

Question 30. The oxidation number of cobalt in K[Co(CO)4] is
(i) + 1 (ii) + 3
(iii) – 1 (iv) – 3

Question 31. Amongst the following, the most stable complex is:

Sol. In each of the given complex, Fe is in + 3 oxidation state. As C2042- is didentate chelating ligand, it forms chelate rings and hence (iii) out of complexes given
above is the most stable complex.

Question 32. What will be the correct order for the wavelengths of absorption in the visible region for the following:

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