ContentIntroduction Scope of the Project Experimental SectionResult and Discussion Summary and ConclusionReferences

Introduction

Coordination complexes are extensively studied in the recent years due to their biological significance.

Cobalt exist in +2, +3 oxidation states. Cobalt(II) complexes are generally less stable than cobalt(III) Complexes.

Both the spin – free and spin-paired are formed by cobalt(II), but cobalt(III) form only spin free complexes.

Cobalt(III) has a d6 configuration and when the ligands are strong enough to cause spin pairing, gives electronic arrangement t2g

6 , eg0.

Larger CFSP, the color of the complex appears more red and absorbs in blue region.

Splitting of d - Orbitals in Octahedral Complexes

The d orbitals are often used in coordination complexes. Therefore it is important to study their shape and distribution in space.

The five d orbitals are not identical and the orbitals are divided into two sets.

The three t2g orbitals have identical shape and point between the axes x, y and z other two eg orbitals have different shapes and point along the axes.

In an octahedral complex, the metal is at the centre of the octahedran, and the ligands are at the six corners.

The approach of six ligands along x, y, z,-x, -y and -z directions will increase the energy of the dx

2-y

2 and dz

2 orbitals much more than the dxy, dyz and dzx

orbitals.

Examination of the spectra of a series of complex of the same metal with different ligands shows that the position of the absorption band maxima vary depending on the ligands.

Ligands which cause only a small degree of crystal field splitting are termed weak field ligands.

Ligands which cause a large splitting are called strong field ligands.

The common ligands can be arranged in ascending order of crystal field splitting ∆0.

Ligand strength has the following order for these electron donors: weak: iodine < bromine < fluorine < acetate < oxalate < water < pyridine < cyanide: strong

Electronic Absorption Spectra for Cobalt(III) Complexes

The ground state is always taken as the horizontal axes (X-axes) and provides a constant reference point. The other energy states are plotted relative to this

Low spin terms, i.e. states where the spin multiplicity is lower than the ground state are included.

In order to make the diagrams general for different metal ions with the same electronic configuration, And to allow for difference ligands, both of which affect Dq and B(or B’),the axes are plotted in units of energy /B and Dq/B.

Interpretation of Electronic Spectra of cobalt(III)

Nature of Ligands

A molecule or ion, which has optimum electro negativity can act as a ‘good’ ligand in other words form stable complexes.

Thus usually the donor atoms in a ligand are Halogens, Oxygen or Nitrogen. Further based on the structure, chelate effect may increase the stability of a complex.

This effect is well pronounced in the case of EDTA complexes and many macro cyclic ligands.

Due to the nature of the ligand formation of several types of isomers are possible in the case of Coordination compounds.

Ligand Substitution Reactions

The most fundamental reaction, a complex can undergo ligand substitution, a reaction in which one lewis base displaces another from lewis acid.

SN1 or dissociation mechanism

SN2 Association or Displacement mechanismSNCB mechanism.

Y + M – X M – Y + X

Ligand Substitution in Octahedral Complexes

Octahedral complexes occur for a wide variety of metals in a wide range of oxidation states and with a great diversity of bonding modes.

The analysis of rate laws for reactions that take place by such mechanism helps to formulate the precise condition for distinguishing these two possibilities and identifying the substitution as Ia or Id.

The difference between the two classes of reaction bring on whether rate determining step is the formation of the new Y…..M bond or the breaking of the old M……X bond.

Linkage Isomers for cobalt(III) Complexes

Co

NH3

NH3

NH3

NH3

NH3 ONO

2+

Co

NH3

NH3

NH3

NH3

NH3

2+

NO2

Hydrolysis Reactions

The substitution reactions in which a ligand is replaced by a water molecule or by OH- groups.

The reactions in which an aquo complex is formed as a result to the replacement of a ligand by H2O molecules are called acid hydrolysis while those in which a hydroxo complex is formed by the replacement of a ligand by OH- group called base hydrolysis reactions.

Acid HydrolysisBase hydrolysis

Acid Hydrolysis

[Co(NH3)5 X]2+ + H2O [Co(NH3)5 (H2O)]3+ + X-

Base Hydrolysis Reaction

[Co(NH3)5 Cl]2+ + OH- [Co(NH3)5OH]2+ + Cl-

Labile and Inert Complexes

The complexes in which the ligands are rapidly replaced by other are called labile complexes.

The complexes in which substitution takes place slowly are called inert complexes.

IR Spectrum Infrared spectroscopy gives information on

molecular vibrations or more precisely on transitions between vibrational and rotational energy levels in molecules.

The absorption region between 490 cm-1 and 395 cm-

1 shows that the complex is in the form of [XY5Z]2+.

This technique is highly useful for understanding the nature of ligand as far as coordination compounds are concerned.

Highly sophisticated instrument may be useful in

understanding M-L bond, but not widely in use

Conductometry

There shall be a change in the value of electrical conductance of the solutions in which complex formation occurs.

This is due to disappearance or production of ions

comparatively high mobility.

This method can also be used to give useful information’s about the total no of ions given by a complex in solution and change type on each ion.

Scope of the Project Cobalt(III) complexes are extensively studied in

the recent years owing to their application in biological fields, photochemical studies, electrochemical studies etc.

In the present investigation we have studied formations of simple new molecules of Cobalt(III) complexes by ligand substitution.

Ligands ammine, aqua, nitro, nitrato, 2-cyanopyridine, 4-aminopyridine, isonicotnamide, were substituted in place of carbonato.

Cobalt(III) complexes are extensively studied in

the recent years owing to their application in biological fields, photochemical studies, electrochemical studies etc.

Experimental SectionReagents

Cobalt(II) nitratehexahydrate, ammonium carbonate, 4-aminopyridine, isonicotinamide, 2-cyanopyridine, nitric acid, concentrate ammonia, hydrochloric acid and hydrogen peroxide were used without any further purification.

Solvents

Acetone (LR), rectified sprit, ether, N,N-dimetyl formamide and acetonitrile was used without further purification.

Synthesis of Complexes

carbonatopentaamminecobalt(III) nitrate 2Co(NO3)2 + 6NH3(aq) + 2(NH4)2CO3 + 1/2O2

2[Co(NH3)5CO3]NO3 + 2NH4NO3 + H2O

5g (0.0173M) of cobalt(II) nitrate hexahydrate, 7.54g (0.1049M) of ammonium carbonate was taken in the ratio of 1:4 and dissolved in 50ml of concentrated ammonia.

The resulting mixture is oxidized by air oxidation for 48 hours at room temperature.

Then the solution is kept for silent for 24 hours, carmine purple crystals are formed from violet red.

aquopentaamminecobalt(III) nitrate

[Co(NH3)5CO3]NO3 + 2HNO3 [Co(NH3)5H2O](NO3)3 + CO2

+ H2O 10g (0.036 mol) of carbonatopentaamminecobalt(III)

nitrate is suspended in 25 ml of water and 20 ml of colorless nitric acid (1:1 contraction acid and water) is added with stirring.

The evolution of carbon dioxide has stopped for (20

minutes), 100 ml of methanol was added, the aquopentaamminecobalt(III) nitrate settles down at the bottom.

The crude material was separated by filtration and washed with alcohol and ether.

4-aminopyridinepentaamminecobalt(III) nitrate Aquopentaamminecobalt(III) nitrate and a 40 fold

excess of 4-aminopyridine were dissolved in a minimum amount of DMF at 1000C.

The solution was cooled and combined with an

equal amount of water.

The iodine salts of 4-aminopyridinepentaammine was precipitated from the solution of KI to an excess addition.

The precipitate is then treated with AgNO3.

The AgI was removed by filtration and the purple crystals of 4-aminopyridinepentaamminecobalt(III) nitrate was crystallized by concentration slow evaporation.

isonicotineamidepentaamminecobalt(III) nitrate Aquopentaamminecobalt(III) nitrate and a 40 fold

excess of isonicotineamide were dissolved in a minimum amount of DMFat 1000C.

The solution was cooled and combined with an equal amount of water.

The iodine salts of isonicotineamidepentaammine were precipitated from the solution of KI to an excess addition.

The precipitate is then treated with AgNO3.The AgI was removed by filtration and the purple crystals of isonicotineamidepentaamminecobalt(III) nitrate was crystallized by concentration slow evaporation.

2-cyanopyridinepentaamminecobalt(III) nitrate

Aquopentaamminecobalt(III) nitrate and a 40 fold excess of 2-cyanopyridine were dissolved in a minimum amount of DMF at 1000C.

The solution was cooled and combined with an equal amount of water.

The iodine salts of 2-cyanopyridinepentaammine was precipitated from the solution of KI to an excess addition.

The precipitate is then treated with AgNO3.The AgI was removed by filtration and the purple crystals of 2-cyanopyridinepentaamminecobalt(III) nitrate was crystallized by concentration slow evaporation.

nitropentaamminecobalt(III) nitrate

[Co(NH3)5CO3]NO3 + 2HNO3 + NaNO2

[Co(NH3)5NO3](NO3)2 +NaNO3+ CO2 + H2O

10g (0.036mol) of carbonatopentaminecobalt(III) nitrate is suspended in 25ml of water, 10g of sodium nitrite and 10ml of colorless nitric acid (1:1 concentrated acid and water) are added.

The mixture was stirred for 15 minutes at room temperature and then 200 ml of methanol was added to the slurry.

The precipitate was collected by filtration, washed with methanol and ether and then kept for dry.

Result and Discussion

carbonoatopentaamminecobalt(III) nitrate The spectrum shows two ligand field bands

having maxima at 516 and 315 nm

Also a structureless charge transfer band was observed.

The low energy ligand field band is assigned to 1A1g 1T1g and the higher energy band is assigned to 1A1g 1T2g.

These observations suggest that the complex having octahedral geometry.

Reference sample is acetonitrile.

400.0 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700.0

0.00

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.00

nm

A

516.91

nitropentaamminecobalt(III) nitrate

390.0 450 500 550 600 650 700.0

0.00

0.2

0.4

0.6

0.8

1.0

1.2

1.41.50

nm

A

456.29

isonicotinamidepentaamminecobalt(III) nitrate

400.0 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700.0

0.00

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.00

nm

A

517.56

4-aminopyridinepentaamminecobalt(III) nitrate

400.0 450 500 550 600 650 700.0

0.00

0.1

0.2

0.3

0.4

0.50

nm

A

507.74

2-cyanopyridinepentaamminecobalt(III) nitrate

420.0 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 646.5

0.118

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0.44

0.46

0.48

0.500

nm

A

511.71

Infrared Spectrum

carbonatopentaamminecobalt(III) nitrate

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0

3.2

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100.0

cm-1

%T

3441.49

3278.08

2927.08

2375.112109.28

1626.14

1496.34

1381.40

1062.99

856.34

692.85 480.41

nitropentaamminecobalt(III) nitrate

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0

0.0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100.0

cm-1

%T

3450.48

3260.07

2925.85

2426.022362.082344.58

2093.42

1636.10

1384.08

1326.69

1048.41

830.73

551.80

isonicotinamidepentaamminecobalt(III) nitrate

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0

0.0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100.0

cm-1

%T

3746.43

3438.08

2950.57

2924.37

2882.622853.58

2498.402425.332393.27

2363.892344.93

2140.54

1628.77

1419.92

1384.09

1117.06

828.34

746.86712.83

691.72580.47

477.58463.68

453.76427.79420.93

409.84

4-aminopyridinepentaamminecobalt(III) nitrate

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0

3.2

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100.0

cm-1

%T 3436.59

3260.95

2928.26

2424.802364.58

2091.30

1628.05

1383.42

1023.73

838.21

581.74

2-cyanopyridinepentaamminecobalt(III) nitrate

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0

0.0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100.0

cm-1

%T

3445.47

3273.92

2924.47

2853.26

2425.85

2359.842342.93

2059.89

1633.48

1484.02

1384.02

1316.25

1271.23

1121.25

1018.06

840.49

799.91

499.94

Summary and Conclusion The substitution of carbonato groups in

[Co(NH3)5CO3]2+heterocyclic ligands 4-aminopyridine, 2-cyanopyridine and isonicotinamide has been studied.

The complexes were characterized by electronic absorption spectra and IR spectra.

This observation clearly infers that there complexes are having pseudooctahedral geometry and Jahn Teller distortions are to a negligible level.

The infrared red spectra supports information obtained from electronic spectra.

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