nickel complexes

Assistant: José Gonzalez Student: Janosch Ehrenmann

Date: 29.01.2009

V.2 – Synthesis of nickel complexes

Introduction:

The nickel ion in nickel(II)-complexes exists in the coordination number of 4, 5 and 6. Its

octahedral, trigonal-bipyramidal, quadratic-pyramidal and tetrahedral complexes are

paramagnetic and have in the majority of cases a green or blue colour. The quadratic-planar

nickel complexes are diamagnetic and mostly have a yellow, red or brown colour.

The synthesis of nickel(II)-complexes passes over ligand substitution reactions where one or

several ligands are replaced by other ones. These reactions are equilibrium reactions.

Essential for the building of a new complex is the complex building constant (stability

constant). In all the following reactions a more basic ligand is removed by a lower basic one

or a less dentate one by a more dentate one. The whole experiment is done in a single botton.

At the end of the experiment a very stable Ni-EDTA complex will be built. It will also be

discussed how to get the used NiCl2 back (recycling).

Execution:

1.1 2,00g (8,4 mmol) NiCl2*6H2O are solved into 20ml water in a round bottom flask.

Concentrated NH3 is dropped in until a temporary appearing precipitate is completely solved.

The solution is no clear and has a deep blue colour caused by the built hexaamminnickel

complex. The passing reactions are the followings:

NiCl2 + 6 H2O → [Ni(H2O)6]Cl2

[Ni(H2O)6]Cl2 + 2 NH3 → Ni(OH)2↓ + 2NH4+ + 4 H2O

Ni(OH)2 + 4 NH3 + 2 NH4+ → [Ni(NH3)6]

2+ + 6 H2O

1.2 The solution is heated until boiling and a solution of 1,66 g (10 mmol) K2C2O4*H2O is

added. Afterwards the solution is cooled down for two hours in the fridge. Turquoise

crystals of a oxalatenickel complex are precipitating.

[Ni(NH3)6]

2+ + 2 C2O4

2- → [Ni(C2O4)2]

2- + 6 NH3

1.3 The solution is again warmed up on room temperature and a solution of 1,85 g (24,6

mmol) glycin H2NCH2COOH (Hgly) and a few drops of concentrated ammoniac are added.

The colour of the solution is turning into a clear light blue of nickel glycinate.

[Ni(C2O4)2]

2- + 2 Hgly → [Ni(gly)2] + 2 C2O4

2- + 2 H

+

1.4 2,36 ml (25,2 mmol) of acetylacetone are dropped into the solution. During 10 minutes

the solutions is stirred while a complex of light blue nickel acetylacetonate is precipitating.

[Ni(gly)2] + 2 Hacac → [Ni(acac)2] + 2 gly

- + 2 H

+


Date: 29.01.2009

1.5 A solution of 2,92 g (25,2 mmol) dimethylglyoxime (H2dmg) HON=C(CH3)-

C(CH3)=NOH in diluted NaOH is added to the suspension. Thereby the Ni(acac)2 precipitate

is converted into a raspberry red precipitate of bis(dimethylglyoximato)nickel(II).

[Ni(acac)2] + 2 H2dmg → [Ni(Hdmg)2] + 2 acac- + 2 H

+

1.6 By an addition of 30% NaOH the pH is raised onto 12. A solution of 6,26 g (16,8 mmol)

Na2H2EDTA in water is added and cooked on reflux until a clear blue solution of NiEDTA

appears.

[Ni(Hdmg)2] + Na2H2EDTA → [Ni(EDTA)]

2- + 2 Hdmg

– + 2 Na

+ + 2 H

+

Recycling:

The NiEDTA complex synthesized in the last step is dangerous for the environment even if it

is a very stable one. When the EDTA is broke down by micro organisms toxic Ni2+

gets free.

To avoid this case NiEDTA undergoes a recycling process.

First of all the NiEDTA solution is made highly acidic (pH<1) by adding concentrated H2SO4

until a green colour of the fluid phase and a precipitation of the organic ligand molecules is

observed.

[Ni(EDTA)]2-

+ 2 H2SO4 + 6 H2O → [Ni(H2O)6]2+

+ H4EDTA↓ + 2 SO42-

The solution is filtered and to the fluid phase are added 5 g of an activated carbon powder.

Subsequently the suspension is boiled and filtered hot. After the filtration another 5 g of

activated carbon are added. 35 ml of 30% H2O2 are added to the solution and the whole thing

is stirred for an hour. Then the solution is heated until boiling and filtered hotly. NaOH is

added to the filtrate until the pH has reached a value of 11-12. Light green Ni(OH)2 is

precipitating.

[Ni(H2O)6]2+

+ 2 NaOH → Ni(OH)2 + 2 Na+ + 6 H2O

The precipitate is separated, washed sulphate free and solved in HCl. The resulting solution is

evaporated so only the yellow NiCl2 is remaining. After a few day on air the colour of the salt

turns into green because the [Ni(H2O)6]Cl2 complex is built.

Ni(OH)2 + 2 HCl → Ni2+

+ 2 H2O + 2Cl-

Ni2+

+ 2 H2O + 2Cl- → NiCl2 + 2 H2O↑


Date: 29.01.2009

Structure and colour

The bonding between the centred atom and the ligand in metal complexes are the result of

electron donor and acceptor interactions. The centred metal atom represents a Lewis acid and

the ligand a Lewis base. From the negative charged ligand acts a repulsing force on the

electron orbitals of the centred atom. On the other hand the positive charge of the centred

atom affects an attracting force on the ligand. As well as the repulsing force of the negative

charged ligand has no energetic effect on the spherical-symmetrical s-orbital of the centred

metal ion, it has an energetic effect on the d-orbitals. Depending on the structure, diameter

and charge of the ligand the energy of some d-orbitals is raised and of some others it is

abased, means the orbitals are heavier or easier to fill up with electrons.

According to the Hund rule the orbitals with the lowest energy are filled firstly. If the spin

matching energy is higher than the single occupying one for the orbital with the highest

energy, all orbitals firstly are filled with a single electron. The resulting complex has high

spin. The octahedral and tetrahedral complexes in fig. 2.0 are examples for high spin

complexes. All orbitals are firstly filled single.

If the spin matching energy is lower than the double occupying one for the orbital with the

lowest energy, some orbitals will already been filled double while some orbitals are not yet

singly occupied and the complex has low spin. The quadratic-planar complex in fig. 2.0 is an

example for such a low spin complex.

The high and low spin model also explains magnetic behaviour of metal complexes. The

tetrahedral and octahedral complexes in fig. 2.0 both are paramagnetic, means have no

magnetic effect. This is caused by the homogenous distribution of the electrons over the

whole complex. The quadratic-planar complex is diamagnetic. Its electrons are not

homogenously distributed over the complex. The double occupancy of some orbitals while

others are still unoccupied results in majority of electrons on one side. The complex this way

gets a negative charge on one side while the other side appears positively charged. This

division of charge gives the complex two poles of different charge what makes him

diamagnetic.

fig. 2.0


Date: 29.01.2009

With the ligand field theory also the colour of complexes can be explained. The electrons in a

complex can be transferred form a low energy orbital into a higher energetic one. Therefore a

specific amount of energy is needed. This energy is calculated as d = h*λ where h is the Plank

constant and λ the wave length of an electromagnetic ray. The energy of d depends on the

mass of the ligand field splitting which is dependent on the structure, charge and diameter of

the ligand. This means that a strong ligand for example splits the ligand field more than a

weak one and the activating energy to move an electron from a lower energy state into a

higher is bigger.

The energy for transferring an electron into a higher energetic orbital is taken from

electromagnetic waves. If a complex absorbs a wavelength within the visible spectrum the

complex appears coloured. Important is to notice that the observed colour of a complex is the

complementary colour to the colour of wavelength by the complex. The NiEDTA complex for

example absorbs red light so it appears green.

Complex stability

The reactions in this experiment are all ordered the way every next complex is more stable

than the previous. The hexaaquanickel complex is the least stable one, NiEDTA is the most

stable one. This means that the complex stability constant for every next synthesized complex

is higher than for the previous. A higher complex stability is either reached by a stronger

ligand substituting a weaker one like it is the case when hexaamminnickel is synthesized from

hexaaquanickel or by a higher dentate ligand replacing a lower dentate one. The strength of a

ligand is given from its acidity as Lewis acid. A less basic ligand is a more acidic ligand and

as this a stronger one.

Multiple dentate ligands, so called chelat ligands, form more stable complexe because of the

entropy effect. This is explained as following: When a few single ligands are substituted by a

chelat ligand the entropy of the system arises because the number of products is higher than

the number of educts. A back reaction is more improbable because therefore a few ligands

have to replace one chelat ligand at the same time where the chelat ligand himself can replace

a few ligands at any time. The substituting reaction from single ligands by a chelat ligand

happens then more often than the back reaction what makes the complex stable.

Below, there are demonstrated the structures of the synthesized complexes in this experiment.

2+

[Ni(H2O)6]2+

Octahedral

[Ni(NH3)6]2+

Octahedral


Date: 29.01.2009

[Ni(C2O4)2]2-

Quadratic-planar

[Ni(acac)2]

Tetrahedral

[Ni(Hdmg)2]

Quadratic-planar

[Ni(EDTA)]2-

Octahedral

[Ni(gly)2]

Quadratic-planar


Date: 29.01.2009

nickel complexes

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