Metal Complexes Oxidation State Analysis

Modified: 23rd May 2018
Wordcount: 3187 words

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In the Periodic Table, the elements can be grouped in blocks followed by the outermost electrons or valence shell. Transition elements are the main group in Periodic Table and those are the elements with a partly filled d or f sub shell. They all are metals with the similar characteristic properties. All can form the alloys with another and also with metallic main group element. They all able to conduct electricity well and appear high melting and boiling point (except mercury). Many of them will form the salts when react with mineral acids. They usually show more than one oxidation state. They also can form the colored compounds and have the magnetic properties. The coordination complexes have a central metal atom surrounded by different others atom or ligands. In the complex, the oxidation state of metal can be obtained by oxidation or reduction of the starting material. In transition metals complexes, the number of electrons various with the oxidation state of the metal. The magnetic properties of transition metals are interest in determining the oxidation state due to the electrons spin can generate a magnetic field. The magnetic moment of the metal is determined indirectly from the magnetic susceptibility. In this experiment, we had prepared manganese, cobalt and vanadium metals complexes. These three metals complexes are belonging to first row transition element. The first transition element series are those 3d electron shell consists between one and nine electrons.

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In experiment Part A, the complex of manganese are prepared, which is tris(acetyacetonato) manganese (III). Manganese is the third most abundant transition metal in the earth. The maximum oxidation state +7 is in the permanganates and the oxidation states from +7 to -3 are known. Complexes of manganese (III) were prepared in aqueous solution by chemical or electrolytic oxidation of Mn2+ . The black tris(acetyacetonato) manganese (III) is obtained by the oxidation of manganese (II) solutions containing an excess of acetylacetone.

In experiment Part B, the complexes of cobalt are prepared, chloropentaamminecobalt(III) chloride, which is cationic complex. Cobalt oxidation states in aqueous solution are +3 and +2. In the absence of complexing ligands, cobalt (II) is very stable in aqueous solution,

Eo [Co(H2O)6]3+ / [Co(H2O) 6]2+ = 1.84 V

However, if presence of some ligands, such as ammonia. The cobalt (III) becomes more stable and in octahedrally coordinated structure.

Eo [Co(NH3) 6] 3+ / [Co(NH3) 6] 2+ = 0.10 V

Cobalt (III) has a great affinity for nitrogen donors especially ammonia or amines. The hexammines are formed preferentially in the presence of charcoal catalyst and by using air as oxidant. When charcoal is absent and oxidation is carried out by hydrogen peroxide, the aquopentammines species predominates. The red chloropentaamminecobalt(III) chloride is obtained by treatment with concentrated hydrochloric acid. It is particular useful to start with cobalt (II) salts because cobalt (II) complexes being labile undergo rapid substitution reactions whereas cobalt (III) complexes are inert. The cobalt (III) complexes are thus produced by substitution of ligand molecules for coordinated water molecules around cobalt (II) followed by oxidation of the cobalt (II) ammine to cobalt (III) ammine.

In experiment Part C, the complexes of vanadium are prepared, which was quabis (acetylacetonato) oxovanadium (IV). The +4 oxidation state is the one most important for vanadium in aqueous solutions. It is neither strongly oxidizing nor reducing and acidified solutions are stable to atmospheric oxidation. +5 oxidation state of vanadium is then reduced by sodium sulphite to V+4. Excess sulphur dioxide is removed by boiling. In general the compounds have either five or six- coordination around the vanadium. The structures are usually based on that of the tetragonal pyramid with the oxygen atom of V=O bond. The five-coordination compounds can formed six-coordinate by accepting a ligand molecule in the “vacant” site trans to the oxygen atom of V=O bond. Then VO(acac)2 dissolves in nonpolar solvents. Oxovanadium (IV) complexes are usually most conveniently prepared from V2O5 due to it is reduced by many anions in acid solution.


  • Conical flask
  • Measuring cylinder
  • Glass rod
  • Hot-plate stirrer, magnetic bar
  • Thermometer
  • Beaker
  • Sintered funnel
  • FTIR spectrometer
  • Guoy balance
  • Round bottom flask with vertical condenser
  • pH paper


  • Manganese (II) chloride tetrahydrate
  • Sodium acetate trihydrate
  • Acetylacetone
  • Potassium permanganate solution
  • Acetone
  • Ammonium chloride
  • Concentrated ammonia solution
  • CoCl2. 6H2O
  • 30 % hydrogen peroxide
  • Concentrated HCL
  • 6M HCL
  • Alcohol
  • Vanadium (V) oxide
  • Absolute ethanol
  • Industrial ethanol
  • 16% w/v of sodium carbonate
  • Methylated spirit (for washing )
  • Dichloromethane
  • Diethyl ether
  • Concentrated sulphuric acid


Part A: Preparation of metal acetylacetonates

5g (0.025 mol) manganese (II) chloride tetrahydrate (M.W. 197.90) and 1.3g (0.0095 mol) sodium acetate trihydrate (M.W. 136.08) are dissolved in distilled water (200 cm3) and 21cm3 (~0.20 mol) acetylacetone is added slowly to the stirred solution (checked whether its purify is 99%, density 0.975gcm1; the actual purify and density of the acetylacetone is recorded ; M.W. 100.11).

The resultant two-phase system is treated with potassium permanganate solution (1g in 50cm3 of water).

Small amounts is added with stirring and another sodium acetate solution (13g or 0.095 mol of sodium acetate trihydrate in 50cm3 of distilled water), after a few minutes.

The solution is heated to about 60oC with continuous stirring for about 30 minutes.

The resultant solution is cooled in ice-cold water and then the solid complex formed is filtered by suction filtration. The complex is washed with acetone and suck dry.

Part B: Preparation of chloropentaamminecobalt (III) chloride

6g ammonium chloride is dissolved in 40cm3 concentrated ammonium (d=0.880 gcm-3) in a 250 cm3 conical flask.

The solution was continually stirred while 12g of finely powdered CoCl2. 6H2O in small portion is added. (Each portion must dissolved before the next portion is added) A yellow precipitate of [CoCl(NH3)5]Cl2 will formed.

The slurry was warm in fume cupboard and 10cm3 of 30% hydrogen peroxide is slowly added from a burette, with vigorous swirling. An exothermic reaction occurred and a deep red solution is formed.

Effervescence has ceased, 40cm3 of concentrated hydrochloric acid is slowly added and then the product is heated 15 minutes on a steam bath.

25 cm3 of ice water, then 25cm3 6M HCL and then alcohol are to be used to cool, filter and wash it. The product was dry at 110oC for an hour.

Part C: Preparation of aquabis (acetylacetonato) oxovanadium (IV)

2g of vanadium (V) oxide, V2O5 is measured out into a 250cm3 conical flask.

A mixture of 5cm3 distilled water, 4cm3 concentrated sulphuric acid and 10 cm3 absolute ethanol are obtained and added these to the above oxide.

The mixture is boiled in a steam-bath with constant stirring (ensure all vanadium oxide is dissolved and the reaction of mixture did not allowed too much evaporation) until a dark blue solution is obtained. The deep blue solution contains the VO2+ ions. [Alternatively, a magnetic bar and a hot-plate stirrer are to be used in heating the oxide-sulphuric-ethanol mixture; heating under reflux.] The reduction of the vanadium (V) to vanadium (IV) is completed in one hour.

The solution is filtered; if necessary, a little ethanol is added to make the dark blue solution less viscous.

The filtrate is transferred to a 250cm3 beaker, 5cm3 acetylacetone is added and then a 16 % m/v of sodium carbonate (sodium carbonate solid used is anhydrous) is added to make the solution neutral.

Neutralize the solution slowly with constant stirring to avoid too much frothing. The addition of more carbonate solution did not produced more frothing at the completion of neutralization. The precipitation of complex is completed when the pH of the solution about pH5.5.

The precipitated bis(actylacetonato)oxovanadium (IV) is filtered.

Cold methylated is to be used to wash it and then cold ethanol (industrial grade or denatured) is to be used suction filtration.

Under suction to dry and the yield (g and %) is recorded.

Product was kept in a sample tube.

Half of the product was recrystallize. It is dissolved in minimum volume of dichloromethane, the impurities are filtered and diethyl ether is added until precipitation occurred. The product is filtered and washed it with the ether and air dry.


The results and the percentage yield of the complexes


Mn(acac)3 complex:

No. of mole of MnCl2.H2O = No. of mole of Mn(acac)3

No. of mole of MnCl2.H2O = 5 g _

197.90 g mol-1

= 0.025 mol

= No. of mole of Mn(acac)3

Mass of Mn(acac)3 = 0.025mol x 351.94 g mol-1

= 8.80 g

Percentage yield = ___Actual value x 100%

Theorectical value

= 3.53 g_ x 100%

8.80 g

= 40.11%

[CoCl(NH3)5]Cl2 complex:

No. of mole of CoCl2.6H2O = No. of mole of [CoCl(NH3)5]Cl2

No. of mole of CoCl2.6H2O = 12 g _

238.00 g mol-1

= 0.05 mol

= No. of mole of [CoCl(NH3)5]Cl2

Mass of [CoCl(NH3)5]Cl2 = 0.05 mol x 250.5 g mol-1

= 12.53 g

Percentage yield = ___Actual value x 100%

Theorectical value

= _7.74 g x 100%

12.53 g

= 61.77%

[Vo(acac)2(H2O)] complex:

No. of mole of V2O5 = No. of mole of [Vo(acac)2(H2O)]

No. of mole of V2O5 = 2 g _

182.00 g mol-1

= 0.01 mol

= No. of mole of [Vo(acac)2(H2O)]

Mass of [Vo(acac)2(H2O)] = 0.01 mol x 264.942 g mol-1

= 2.65 g

Percentage yield = Actual value x 100%

Theorectical value

= 0.29 g x 100%

2.65 g

= 10.94 %

The magnetic field of the complexes


Magnetic susceptibility = C (length of the tube with sample inside) (R-Ro)

109 M


C = calibration constant

R-Ro = reading of the magnetic susceptibility balance (product)

M = mass of product

Formula Xg = mass susceptibility or gram of sample (Telsa)

Mn(acac)3 complex:

The magnetic susceptibility of the Mn(acac)3 is

Magnetic susceptibility = 1(1.70cm)(+097)

109 (0.0555g)

= 2.97 x 10-6

[CoCl(NH3)5]Cl2 complex:

The magnetic susceptibility of the [CoCl(NH3)5]Cl2 is

Magnetic susceptibility = 1(2.20cm)(-001)

109 (0.0367g)

= -5.99 x 10-8

[VO(acac)2(H2O)] (pure) complex:

The magnetic susceptibility of the [VO(acac)2(H2O)] (pure) is

Magnetic susceptibility = 1(2.80cm)(+206)

109 (0.1022g)

= 5.64 x 10-6

[VO(acac)2(H2O)] (impure) complex:

The magnetic susceptibility of the [VO(acac)2(H2O)] (impure) is

Magnetic susceptibility = 1(1.90cm)(+112)

109 (0.0533g)

= 39.99 x 10-6

The magnetic moment of the complexes


Magnetic moment, µ = [n (n+2)]1/2 BM


n = number of unpaired electrons

BM = Bohr magneton

Mn(acac)3 complex:

Number of the unpaired electrons = 2

Magnetic moment =[n (n+2)]1/2 BM

= [2 (2+2)] ½

= 2.83 BM

[VO(acac)2(H2O)] (impure) complex:

Number of the unpaired electrons = 1

Magnetic moment = [n (n+2)]1/2 BM

= [1 (1+2)] ½

= 1.73BM

Results of FTIR Spectroscopy for each complex:

Part A : Tris (acetyacetonato) manganese(III), Mn (acac)3

Part B : Chloropentaamminecobalt(III) chloride, [CoCl(NH3)5] Cl2

Part C : Aquabis (acetylacetonato) oxovanadium(IV) , [VO(acac)2(H2O)], (for pure and impure)

  1. Pure
  2. Impure


According to the results that we obtained (Table 1), the percentage yield of the three complexes is calculated. The percentage yield of Mn(acac)3, [CoCl(NH3)5]Cl2 and [VO(acac)2(H2O)] (pure) are 40.11 %, 61.77 % and 9.10 % respectively. Based on the percentage yield that we calculated, the actual yield of these three complexes were less than the values of theoretical yield. This may due to some reason such as the procedure may not be conduct perfectly or completed when during the heating process. Another reason may be the reactant is not being pure, they may contain some contaminated. Products in the reaction may be lost during the procedure in the experiment. Hence, we should take note of these reasons in order to avoid it and to increase the precision of the results attained. We can minimize our technical error when doing the experiment such as heating or weighing the substance. This not only can help the results more accurate, this also can help to improve our experimental skills. For example, ensure the substances are heat completely. We also have to make sure the reactant is pure and no contain contaminant before starting the experiment. This also is one of the factors affect the actual yield of the experiment. Besides that, we also need to be careful handle the things during the experiment. For example, the product we must handle correctly, in order to avoid the products being lost. Therefore, the more accurate results can be obtained.

Structure of the complex

Mn(acac)3 [CoCl(NH3)5]Cl2 [VO(acac)2(H2O)]

According to the results that we obtained (Table 2), the magnetic susceptibility of the Mn(acac)3, [CoCl(NH3)5]Cl2 and [VO(acac)2(H2O)](pure and impure) are 2.97 x 10-6, -5.99 x 10-6, 5.64 x 10-6 and 39.99 x 10-6 respectively. The magnetic properties of substances arise principally from the charge and from the spin and orbital angular momenta of electrons. Diamagnetic materials (negative of magnetic susceptibility) are repelled by a magnetic field (no unpaired electrons) whereas paramagnetic materials (positive of magnetic susceptibility) are attracted by a magnetic field (have unpaired electrons). Therefore, from the reading of the magnetic susceptibility we would know that both Mn(acac)3 and [VO(acac)2(H2O)] were paramagnetic and only [CoCl(NH3)5]Cl2 was diamagnetic. The reason can be due to the electron configuration of these three metal ions with their ligands. The manganese +3 ion is [Ar] 3d4, it has two unpaired electrons in the d-orbital. Hence, Mn(acac)3 was show the paramagnetic properties. In addition, the Mn(acac)3 has the higher paramagnetic susceptibility than [VO(acac)2(H2O)] due to the extra two unpaired electrons. As the same reason with the Mn(acac)3, the [VO(acac)2(H2O)] also shown the paramagnetic properties because the electron configuration of vanadium +4 is [Ar] 3d1and it has a unpaired electron in the s-orbital. However, different with another two complexes, the [CoCl(NH3)5]Cl2 was show the diamagnetic properties because the cobalt +3 is [Ar] 3d6, which has full filled in the d-orbitals so no unpaired electrons. Co complex also is a perfectly octahedral structure.

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Valence electron configuration of the complex

In the FTIR spectrum diagram, we can see the manganese complex and vanadium complex were similar. This is because both of them have the same ligand (acac). However, the cobalt complex was much difference between manganese and vanadium complex. Based on the FTIR diagram that we obtained, the IR spectrum of the manganese complex, where the sp3 C-H stretching was 2908.8 cm-1, which was at the absorption band ~2900 cm-1 regions and 1381.0 cm-1 and 1394 cm-1 are occurred between the range 1400 cm-1 and 1380 cm-1, means the methyl group is presented. Thus, the C-O stretching also appeared in the region of absorption band about 1250 cm-1-1050 cm-1, where the IR spectrum was 1257 cm-1. For the second complex, [CoCl(NH3)5]Cl2 , the region of the absorption band between 3000 cm-1 – 3500 cm-1 was NH3 (ligand of the complex), the IR spectrum that we obtained was 3268.0 cm-1. The Co complex is occurred at the absorption band ~800 cm-1, which was 845.3 cm-1. Same as the complex of manganese, the methyl group of pure [VO(acac)2(H2O)] also occur at the region of absorption band 1400 cm-1 and 1380 cm-1, they were 1418.5 cm-1 and 1372.3 cm-1. Then, for the V-O stretching, IR spectrum 484.9 cm-1 is appeared at the region ~480 cm-1. On the other hand, the impure of the [VO(acac)2(H2O)] were almost the same as pure complex, only the O-H stretching is presented in the absorption band between 3200 cm-1 – 3600 cm-1 because contain the water molecules (before recrystallization). , The methyl group occurs at the region of absorption band 1400 cm-1 and 1380 cm-1, they were 1418.9 cm-1 and 1376.2 cm-1. Then, for the V-O stretching, IR spectrum 487.0 cm-1 is appeared at the region ~480 cm-1 .

In this experiment, we can observed that the Part C experiment (preparation of the aquabis(acetylacetonato) oxovanadium (IV) , [VO(acac)2(H2O)]) was more difficult than another two experiments (Part A : preparation of metal tris(acetyacetonato) manganese(III), Mn (acac)3 and Part B : preparation of chloropentaamminecobalt(III) chloride, [CoCl(NH3)5] Cl2 ). This can be explained through their reaction process. In experiment Part A and B, they were used oxidation reaction to produce the complex. However, synthesis [VO(acac)2(H2O)]) , the reduction reaction was used. Reduction process needs more energy to release the electron and the starting materials of vanadium oxide have a stable electron configuration. Therefore, there is harder to add in more electrons to distort the stable electron configuration. Hence, the [VO(acac)2(H2O)]) was more difficult to form than another two complexes.

In this experiment, the liagand of the manganese and vanadium is acetylacetone (acac). Acetylacetone has a CH2 group, which is adjacent to two carbonyl groups. In acetylacetone anion ([acac-]), one of the protons can be easy to remove due to the anion is stabilized by delocalization of the negative charge at the two oxygen (O) atoms. Therefore, the acetylacetone can be formed in three different forms called resonance form.


The percentage yield of the Mn(acac)3, [CoCl(NH3)5]Cl2 and [VO(acac)2(H2O)] (pure) are 40.11 %, 61.77 % and 9.10 % respectively and the magnetic susceptibility of the Mn(acac)3, [CoCl(NH3)5]Cl2 and [VO(acac)2(H2O)] (pure and impure) are 2.97 x 10-6, -5.99 x 10-6, 5.64 x 10-6 and 39.99 x 10-6 respectively.


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