NCERT Grade 12-Coordination Compounds-Answers

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1.      

2.    (i) Hexaamminecobalt (III) chloride

(ii) Pentaamminechloridocobalt (III) chloride

(iii) Potassium hexacyanoferrate (III)

(iv) Potassium trioxalatoferrate (III)

(v) Potassium tetrachloridopalladate (II)

(vi) Diamminechlorido (methylamine) platinum (II) chloride

3.    (i) Both geometrical (cis-, trans-) isomers for K [Cr (H2O)2 (C2O4)2] can exist. Also, optical isomers for cis-isomer exist.

Trans-isomer is optically inactive. On the other hand, cis-isomer is optically active.

(ii) Two optical isomers [Co(en)3]Cl3 for exist.

Two optical isomers are possible for this structure.

(iii) [Co(NH3)5 (NO2) (NO3)2]

A pair of optical isomers:

It can also show linkage isomerism.

[Co(NH3)5 (NO2)] (NO3)2 and [Co(NH3)5 (ONO)] (NO3)2

It can also show ionization isomerism.

[Co(NH3)5 (NO2)] (NO3)2 and [Co(NH3)5 (NO3)] (NO3) (NO2)

(iv) Geometrical (cis-trans-) isomers of [Pt(NH3) (H2O) Cl)2] can exist.

4.    When ionization isomers are dissolved in water, they ionize to give different ions. These ions then react differently with different reagents to give different products.

5.    Ni is in the +2 oxidation state i.e., in d8 configuration.

There are 4 CN ions. Thus, it can either have a tetrahedral geometry or square planar geometry. Since CN– ion is a strong field ligand, it causes the pairing of unpaired 3d electrons.

It now undergoes dsp2 hybridization. Since all electrons are paired, it forms diamagnetic compound.

In case of [NiCl4]2-, Cl- ion is a weak field ligand. Therefore, it does not lead to the pairing of unpaired 3d electrons. Therefore, it undergoes sp3 hybridization.

Since there are 2 unpaired electrons in this case, due to weak field ligand so, it is paramagnetic in nature.

6.    Though both [NiCl4]2- and [Ni(CO)4] are tetrahedral, their magnetic characters are different. This is due to a difference in the nature of ligands. CN is a weak field ligand and it does not cause the pairing of unpaired 3d electrons. Hence, [NiCl4]2- is paramagnetic.

In Ni(CO)4, Ni is in the zero oxidation state i.e., it has a configuration of 3d8 4s2.

But CO is a strong field ligand. Therefore, it causes the pairing of unpaired 3d electrons. Also, it causes the 4s electrons to shift to the 3d orbital, thereby giving rise to sp3 hybridization. Since no unpaired electrons are present in this case, [Ni(CO)4] is diamagnetic.

7.    In both [Fe(HO)6]3+ and [Fe(CN)6]3, Fe exists in the +3 oxidation state i.e., in d5 configuration.

Since CN is a strong field ligand, it causes the pairing of unpaired electrons. Therefore, there is only one unpaired electron left in the d-orbital.

On the other hand, H2O is a weak field ligand. Therefore, it cannot cause the pairing of electrons. This means that the number of unpaired electrons is 5.

Therefore,

Thus, it is evident that [Fe(H2O)6]3+is strongly paramagnetic, while [Fe(CN)6]3 is weakly paramagnetic.

8.   

9.    [Ni(NH)4]2-

In this complex, Pt is in the +2 state. It forms a square planar structure. This means that it undergoes dsp2 hybridization. Now, the electronic configuration of Pd(+2) is 5d8.

CN2 being a strong field ligand causes the pairing of unpaired electrons. Hence, there are no unpaired electrons in [Pt(CN)4]2-.

10.  

Hence, hexaaquo manganese (II) ion has five unpaired electrons, while hexacyano ion has only one unpaired electron. So pairing takes place and low spin complexes are formed with this 1 unpaired electron.

11.  β4 = 2.1 × 1013

The overall complex dissociation equilibrium constant is the reciprocal of the overall stability constant, β4.

12.  Werner’s postulates explain the bonding in coordination compounds as follows:

(i) A metal exhibits two types of valencies namely, primary and secondary valencies. Primary valencies are satisfied by negative ions while secondary valencies are satisfied by both negative and neutral ions.

(In modern terminology, the primary valency corresponds to the oxidation number of the metal ion, whereas the secondary valency refers to the coordination number of the metal ion.

(ii) A metal ion has a definite number of secondary valencies around the central atom. Also, these valencies project in a specific direction in the space assigned to the definite geometry of the coordination compound.

(iii) Primary valencies are usually ionizable, while secondary valencies are non-ionizable.

13.  

Both the compounds i.e., FeSO4.(NH4)2 SO4.6H2O and [Cu(NH3)4]SO4.5H2O fall under the category of addition compounds with only one major difference i.e., the former is an example of a double salt, while the latter is a coordination compound.

A double salt is an addition compound that is stable in the solid state but that which breaks up into its constituent ions in the dissolved state. These compounds exhibit individual properties of their constituents. For e.g. FeSO4.(NH4)2 SO4.6H2O breaks into Fe2+, NH4+ and SO42- ions. Hence, it gives a positive test for Fe2+ ions.

A coordination compound is an addition compound which retains its identity in the solid as well as in the dissolved state. However, the individual properties of the constituents are lost. This happens because [Cu(NH3)4]SO4.5H2O does not show the test for Cu2+. The ions present in the solution of [Cu(NH3)4]SO4.5H2O are [Cu(NH3)4]2+ and SO42-.

14.  (i) Coordination entity:

A coordination entity is an electrically charged radical or species carrying a positive or negative charge. In a coordination entity, the central atom or ion is surrounded by a suitable number of neutral molecules or negative ions (called ligands). For example:

(ii) Ligands

The neutral molecules or negatively charged ions that surround the metal atom in a coordination entity or a coordinal complex are known as ligands. For example, Ligands are usually polar in nature and possess at least one unshared pair of valence electrons.

(iii) Coordination number:

The total number of ligands (either neutral molecules or negative ions) that get attached to the central metal atom in the coordination sphere is called the coordination number of the central metal atom. It is also referred to as its ligancy.

For example:

(a) In the complex, K2[PtCl6], there as six chloride ions attached to Pt in the coordinate sphere. Therefore, the coordination number of Pt is 6.

(b) Similarly, in the complex [Ni(NH3)4]Cl2, the coordination number of the central atom (Ni) is 4.

(vi) Coordination polyhedron:

Coordination polyhedrons about the central atom can be defined as the spatial arrangement of the ligands that are directly attached to the central metal ion in the coordination sphere. For example:

(v) Homoleptic complexes:

These are those complexes in which the metal ion is bound to only one kind of a donor group. For eg: [CO(NH3)6]3+, [PtCl4]2- etc.

(vi) Heteroleptic complexes:

Heteroleptic complexes are those complexes where the central metal ion is bound to more than one type of a donor group.

For e.g.: [CO(NH3)4Cl2]+, [Co(NH3)5Cl]2+

15.  A ligand may contain one or more unshared pairs of electrons which are called the donor sites of ligands. Now, depending on the number of these donor sites, ligands can be classified as follows:

(a) Unidentate ligands: Ligands with only one donor sites are called unidentate ligands. For e.g., 

(b) Didentate ligands: Ligands that have two donor sites are called didentate ligands. For e.g.,

(a) Ethane-1, 2-diamine

(b) Oxalate ion

(c) Ambidentate ligands:

Ligands that can attach themselves to the central metal atom through two different atoms are called ambidentate ligands. For example:

16.  (i) [CI(H2O)(CN)(en)2]2+

Let the oxidation number of Co be x.

The charge on the complex is +2.

Let the oxidation number of Co be x.

The charge on the complex is -2.

17.  

18.  (i) Hexaamminecobalt(III) chloride

(ii) Diamminechlorido(methylamine) platinum(II) chloride

(iii) Hexaquatitanium(III) ion

(iv) Tetraamminichloridonitrito-N-Cobalt(III) chloride

(v) Hexaquamanganese(II) ion

(vi) Tetrachloridonickelate(II) ion

(vii) Hexaamminenickel(II) chloride

(viii) Tris(ethane-1, 2-diammine) cobalt(III) ion

(ix) Tetracarbonylnickel(0)

19.  

(a) Geometric isomerism:

This type of isomerism is common in heteroleptic complexes. It arises due to the different possible geometric arrangements of the ligands. For example:

(b) Optical isomerism:

This type of isomerism arises in chiral molecules. Isomers are mirror images of each other and are non-superimposable.

(c) Linkage isomerism: This type of isomerism is found in complexes that contain ambidentate ligands. For example:

[Co(NH3)5(NO2)]Cl2 and [Co(NH3)5(ONO)Cl2]

Yellow form Red form

(d) Coordination isomerism:

This type of isomerism arises when the ligands are interchanged between cationic and anionic entities of differnet metal ions present in the complex.

[CO(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]

(e) Ionization isomerism:

This type of isomerism arises when a counter ion replaces a ligand within the coordination sphere. Thus, complexes that have the same composition, but furnish different ions when dissolved in water are called ionization isomers. For e.g., [Co(NH3)5SO4]Br and [Co(NH3)5Br ]SO4.

(f) Solvate isomerism:

Solvate isomers differ by whether or not the solvent molecule is directly bonded to the metal ion or merely present as a free solvent molecule in the crystal lattice.

Violet Blue-green Dark green

20.  (i) For [Cr(C2O4)3]3-, no geometric is possible as it is a bidentate ligand.

Two geometrical isomers are possible.

21.  

22.  

In total, three isomers are possible.

Trans-isomers are optically inactive.

Cis-isomers are optically active.

23.  

From the above isomers, none will exhibit optical isomers. Tetrahedral complexes rarely show optical isomerization. They do so only in the presence of unsymmetrical chelating agents.

24.  Aqueous CuSO4 exists as [Cu(H2O4)SO4]. It is blue in colour due to the presence of [Cu(H2O4)]2+ ions.

(i) When KF is added:

(ii) When KCl is added:

In both these cases, the weak field ligand water is replaced by the F and Cl ions.

25.  

Thus, the coordination entity formed in the process is K2[Cu(CN)4]. K2[Cu(CN)4]. is a very stable complex, which does not ionize to give Cu2+ ions when added to water. Hence, Cu2+ ions are not precipitated when H2S(aq) is passed through the solution.

26.  (i) [Fe(CN)6]4- In the above coordination complex, iron exists in the +II oxidation state.

Fe2+ : Electronic configuration is 3d6

Orbitals of Fe2+ ion:

As CN is a strong field ligand, it causes the pairing of the unpaired 3d electrons.

Since there are six ligands around the central metal ion, the most feasible hybridization is d2 sp3.

d2 sp3 hybridized orbitals of Fe2+ are:

6 electron pairs from CN ions occupy the six hybrid d3 sp3 orbitals.

Then,

Hence, the geometry of the complex is octahedral and the complex is diamagnetic (as there are no unpaired electrons).

(ii) [FeF6]3-

In this complex, the oxidation state of Fe is +3.

Orbitals of Fe3+ ion:

There are 6 F- ions. Thus, it will undergo d2 sp3 or sp3 d2 hybridization. As F is a weak field ligand, it does not cause the pairing of the electrons in the 3d orbital. Hence, the most feasible hybridization is sp3 d2.

sp3 dhybridized orbitals of Fe are:

Hence, the geometry of the complex is found to be octahedral.

(iii) [Co(C2O4)2]3-

Cobalt exists in the +3 oxidation state in the given complex.

Orbitals of Co3+ ion:

Oxalate is a weak field ligand. Therefore, it cannot cause the pairing of the 3d orbital electrons. As there are 6 ligands, hybridization has to be either sp3 dor dsp3 hybridization.

sp3 dhybridization of CO3+:

The 6 electron pairs from the 3 oxalate ions (oxalate anion is a bidentate ligand) occupy these sp3 d orbitals.

Hence, the geometry of the complex is found to be octahedral.

(iv) [CoF6]3-

Cobalt exists in the +3 oxidation state.

Orbitals of Co3+ ion:

Again, fluoride ion is a weak field ligand. It cannot cause the pairing of the 3d electrons. As a result, the Co3+ ion will undergo sp3 dhybridization.

sp3 dhybridized orbitals of Co3+ ion are:

Hence, the geometry of the complex is octahedral and paramagnetic.

27.  

The splitting of the d orbitals in an octahedral field takes palce in such a way that  experience a rise in energy and form the eg level, while dxy, dyz and dzx experience a fall in energy and form the t2g level.

28.  A spectrochemical series is the arrangement of common ligands in the increasing order of their field strength and crystal-field splitting energy (CFSE) values. The ligands present on the R.H.S of the series are strong field ligands while that on the L.H.S are weak field ligands. Also, strong field ligands cause higher splitting in the d orbitals than weak field ligands. Strong ligands are able to produce strong field whereas weak ligands produce weak field.

Weak ligands can not cause pairing of electrons and form high spin complexes whereas strong field ligands can cause pairing of electrons and thus form low spin complexes.

29.  The difference between energies of two sets of d-orbitals is called crystal field splitting energy i.e. delta note. The degenerate d-orbitals (in a spherical field environment) split into two levels i.e., eg and t2g in the presence of ligands. The splitting of the degenerate levels due to the presence of ligands is called the crystal-field splitting while the energy difference between the two levels (eg and t2g) is called the crystal-field splitting energy. It is denoted by Δ0.

After the orbitals have split, the filling of the electrons takes place. After 1 electron (each) has been filled in the three t2g orbitals, the filling of the fourth electron takes place in two ways. It can enter the eorbital (giving rise to  like electronic configuration) or the pairing of the electrons can take place in the t2g orbitals (giving rise to  like electronic configuration). If the Δ0 value of a ligand is less than the pairing energy (P), then the electrons enter the eg orbital. On the other hand, if the Δvalue of a ligand is more than the pairing energy (P), then the electrons enter the t2g orbital.

Although orbital splitting energies are not sufficiently large to force pairing so low spin configuration are rarely observed.

30.  Cr is in the +3 oxidation state i.e., d3 configuration. Also, NH3 is a weak field ligand that does not cause the pairing of the electrons in the 3d orbital.

Therefore, it undergoes d2 sp3 hybridization and the electrons in the 3d orbitals remain unpaired. Hence, it is paramagnetic in nature.

In [Ni(CN)4]2-, Ni exists in the +2 oxidation state i.e., d8 configuration.

CN is a strong field ligand. It causes the pairing of the 3d orbital electrons. Then, Ni2+ undergoes dsp2 hybridization.

As there are no unpaired electrons, it is diamagnetic with square planar shape.

31.  In  is a weak field ligand. Therefore, there are unpaired electrons in Ni2+. In this complex, the d electrons from the lower energy level can be excited to the higher energy level i.e., the possibility of d – d transition is present. Hence, [Ni(H2O)6]2+ is coloured.

In [Ni(CN)4]2-, the electrons are all paired as CN is a strong field ligand. Therefore, d-d transition is not possible in [Ni(CN)4]2-. Hence, it is colourless.

32.  The colour of a particular coordination compound depends on the magnitude of the crystal-field splitting energy, Δ. This CFSE in turn depends on the nature of the ligand. In case of [Fe(CN)6]4- and [Fe(H2O)6]2+, the colour differs because there is a difference in the CFSE. Now, CN is a strong field ligand having a higher CFSE value as compared to the CFSE value of water. This means that the absorption of energy for the intra d-d transition also differs. Hence, the transmitted colour also differs.

33.  The metal-carbon bonds in metal carbonyls have both σ and ∏ characters. bond is formed when the carbonyl carbon donates a lone pair of electrons to the vacant orbital of the metal. A ∏ bond is formed by the donation of a pair of electrons from the filled metal d orbital into the vacant anti-bonding ∏* orbital (also known as back bonding of the carbonyl group). The σ bond strengthens the ∏ bond and vice-versa. Thus, a synergic effect is created due to this metal-ligand bonding. This synergic effect strengthens the bond between CO and the metal.

34.  (i) K2[Co(C2O4)3]

The central metal ion is Co.

Its coordination number is 6.

The oxidation state can be given as:

x – 6 = –3

x = +3

The d orbital occupation for Co3+ is 

(ii) cis-[Cr(en)2Cl2]Cl

The central metal ion is Cr.

The coordination number is 6.

The oxidation state can be given as:

x + 2(0) + 2(–1) = +1

x – 2 = +1

x = +3

The d orbital occupation for Cr3+ is .

(iii) (NH4)2 [CoF4]

The central metal ion is Co.

The coordination number is 4.

The oxidation state can be given as:

x – 4 = –2

x = + 2

The d orbital occupation for Co2+ is .

(iv) [Mn(H2O)6]SO4

The central metal ion is Mn.

The coordination number is 6.

The oxidation state can be given as:

x + 0 = +2

x = +2

The d orbital occupation for Mn is 

35.  (i) Potassium diaquadioxalatochromate (III) trihydrate.

Oxidation state of chromium = 3

Electronic configuration: 

Coordination number = 6

Shape: octahedral

Stereochemistry:

IUPAC name: Pentaamminechloridocobalt(III) chloride

Oxidation state of Co = +3

Coordination number = 6

Shape: octahedral.

Electronic configuration: 

Stereochemistry:

Magnetic Moment = 0

(iii) CrCl3(py)3

IUPAC name: Trichloridotripyridinechromium (III)

Oxidation state of chromium = +3

Electronic configuration for 

Coordination number = 6

Shape: octahedral.

Stereochemistry:

Both isomers are optically active. Therefore, a total of 4 isomers exist.

IUPAC name: Caesium tetrachloroferrate (III)

Oxidation state of Fe = +3

Electronic configuration of 

Coordination number = 4

Shape: tetrahedral

Stereochemistry: optically inactive

Magnetic moment:

Potassium hexacyanomanganate(II)

Oxidation state of manganese = +2

Electronic configuration: 

Coordination number = 6

Shape: octahedral.

Streochemistry: optically inactive

Magnetic moment,

36.  The stability of a complex in a solution refers to the degree of association between the two species involved in a state of equilibrium. Stability can be expressed quantitatively in terms of stability constant or formation constant.

For this reaction, the greater the value of the stability constant, the greater is the proportion of ML3 in the solution.

Stability can be of two types:

(a) Thermodynamic stability:

The extent to which the complex will be formed or will be transformed into another species at the point of equilibrium is determined by thermodynamic stability.

(b) Kinetic stability:

This helps in determining the speed with which the transformation will occur to attain the state of equilibrium.

Factors that affect the stability of a complex are:

(a) Charge on the central metal ion: The greater the charge on the central metal ion, the greater is the stability of the complex.

(b) Basic nature of the ligand: A more basic ligand will form a more stable complex.

(c) Presence of chelate rings: Chelation increases the stability of complexes.

37.  When a ligand attaches to the metal ion in a manner that forms a ring, then the metal- ligand association is found to be more stable. In other words, we can say that complexes containing chelate rings are more stable than complexes without rings. This is known as the chelate effect.

For example:

38.  (i) Role of coordination compounds in biological systems:

We know that photosynthesis is made possible by the presence of the chlorophyll pigment. This pigment is a coordination compound of magnesium. In the human biological system, several coordination compounds play important roles. For example, the oxygen-carrier of blood, i.e., haemoglobin, is a coordination compound of iron.

(ii) Role of coordination compounds in medicinal chemistry:

Certain coordination compounds of platinum (for example, cis-platin) are used for inhibiting the growth of tumours.

(iii) Role of coordination compounds in analytical chemistry:

During salt analysis, a number of basic radicals are detected with the help of the colour changes they exhibit with different reagents. These colour changes are a result of the coordination compounds or complexes that the basic radicals form with different ligands.

(iii) Role of coordination compounds in extraction or metallurgy of metals:

The process of extraction of some of the metals from their ores involves the formation of complexes. For example, in aqueous solution, gold combines with cyanide ions to form [Au(CN)2]. From this solution, gold is later extracted by the addition of zinc metal.

39.  (iii) The given complex can be written as Co(NH3)6Cl2.

Thus, [Co(NH3)6]+ along with two Cl- ions are produced.

40.  (i) No. of unpaired electrons in [Cr(H2O)6]3+ = 3

(ii) No. of unpaired electrons in [Fe(H2O)6]2+ = 4

(iii) No. of unpaired electrons in [Zn(H2O)6]2+ = 0

Hence, [Zn(H2O)6]2+ has the highest magnetic moment value.

41.  We know that CO is a neutral ligand and K carries a charge of +1.

Therefore, the complex can be written as K+[Co(CO)4]. Therefore, the oxidation number of Co in the given complex is -1. Hence, option (iii) is correct.

42.  We know that the stability of a complex increases by chelation. Therefore, the most stable complex is [Fe(C2O4)3]3-

43.  The central metal ion in all the three complexes is the same. Therefore, absorption in the visible region depends on the ligands. The order in which the CFSE values of the ligands increases in the spectrochemical series is as follows:

Thus, the amount of crystal-field splitting observed will be in the following order:

Hence, the wavelengths of absorption in the visible region will be in the order:

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