Electronic Effects of Axial Ligand Field Strength

Nickel is a group 8 transition metal which can form complex ions with various ligands as it has partially filled d orbitals. The regular octahedral complexes can either be high spin or low spin depending on their ligand field strength. A distortion of the regular ligand field can arise when a trans-axial ligand (X) is present in a 6-coordinate metal complex. The spin state can have important effects in terms of properties and reactivity.

The purpose of this experiment is to show the different electronic axial ligand effects and the effect on colour and magnetic susceptibility of the compound. There are two possible types of magnetism; paramagnetism or diamagnetism. Paramagnetism occurs when there is an external magnetic field to align the electron spins, which are randomly aligned. Diamagnetism creates a magnetic field that opposes any applied magnetic field due to the presence of any electron. There is a relationship between the ligand strength and observed colour of the complexes and this experiment will examine these.

Experimental Procedure

The standard PPE (personal protective equipment) are used. This means the wearing of a lab coat, safety specs and Nitrile gloves.

Precautions

• Nickel salts are potential carcinogens.
• Ethylenediamine is corrosive and has a strong, unpleasant odour.
• Hydrobromic acid (48%) is a concentrated acid and is corrosive.
• Tetrafluoroborate salts are potential sources of HF on hydrolysis and are corrosive.
• Methanol is toxic.
• Avoid skin contact with all the materials in this experiment.
• Wear gloves and perform all additions in a fume cupboard.

A) NiCl2 (Et2en)2•2H2O

NiCl2•6H2O (0.24g) lime green crystals was dissolved in ethanol (7cm3) which made the solution turn lime green. Et2en (0.23g) was added to the solution whilst stirring; this then caused a colour change to dark green. The solution was left at room temperature for about 20 minutes which formed a blue/green precipitate. The light blue crystals were isolated using a Buchner funnel, then washed with ethanol (1cm) and air dried by suction.

B) [Ni(NO3)2(et2en)2]

Ni (NO3)2•6H2O (0.29g) green crystals was dissolved in ethanol (5cm3) which made the solution turn light green. Et2en (0.23g) was added to the solution whilst stirring; this then caused a colour change to dark green. The solution was left at room temperature for about 20 minutes which formed an orange/green precipitate. The orange crystals were isolated using a Buchner funnel and then washed with ethanol (1cm) and air dried by suction.

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C) Ni(NCS)2(Et2en)2

Ethanolic solutions of Ni (NO3)2·6H2O (0.29g in 4 cm3) and NaNCS (0.16 g in 3cm3) were prepared. The solutions were mixed whilst stirring and left to cool for about 20minutes. The white precipitate was filtered off by passing it through a Pasteur pipette containing glass wool. Ethanol (ca. 2 mL) and then Et2en (0.23g) was added to the green filtrate whilst stirring. This was left for a few minutes and the violet crystals were filtered at the pump. It was then washed with chilled ethanol (2cm3) and dried under suction.

D) NiI2(Et2en)2

Ethanolic solutions of Ni(NO3)2·6H2O (0.29g in 4 cm3) and NaI (0.30 g in 3cm3) were prepared. The solutions were mixed whilst stirring and left to cool for about 20minutes. The white precipitate was filtered off by passing it through a Pasteur pipette containing glass wool. Ethanol (ca. 2 mL) and then Et2en (0.23g) was added to the lime green filtrate whilst stirring. This was left for a few minutes and the red crystals were filtered at the pump. It was then washed with chilled ethanol (2cm3) and dried under suction.

Results and calculations

NiCl2 (Et2en)2•2H2O

MK-24-I-1 formed light blue powder like crystals.

Yield: 0.18g

[Ni(NO3)2(et2en)2]

MK-24-I-2 formed orange powder like crystals

Yield: 0.22g

Ni(NCS)2(Et2en)2

MK-24-I-3 formed violet powder like crystals

Yield:

NiI2(Et2en)2

MK-24-I-4 formed red powder like crystals

Yield: 0.38g

Data Analysis

All Magnetic data collected at 292 K. N = number of unpaired electrons.

= Cl-

N=

a= 1 b=2 c= (3.27)2 = 10.6929

N=

N=

N = 2.419 (rounded to 2) or N= -4.419 (cannot be a negative number hence n=0)

This shows that Ni(Et2en)2Cl2 is a paramagnetic complex

= I-

N2 + 2N – 2 = 0

N2 + 2N = 0

N= -2

Because you can’t have a negative value of unpaired electrons n must = 0

As they are no unpaired electrons, Ni(et2en)2I2 is a diamagnetic complex

= SCN-

N=

a= 1 b=2 c= (3.20)2 = 10.24

N=

N=

N = 2.35 (rounded to 2) or N= -4.419 (cannot be a negative value hence N=0)

Because there are unpaired electrons present, the complex Ni(et2en)2(SCN)2 is paramagnetic

= NO3-

N2 + 2N – 2 = 0

N2 + 2N = 0

N2 = -2N

N= -2

Because you can’t have a negative value of unpaired electrons n must = 0

As they are no unpaired electrons, Ni(et2en)2(NO3)2 is a diamagnetic complex

2089 r-N-C isocyanide

1150-1032 CN stretches

3100-2965 N-H stretches

Discussion

The crystal field theory explains the octahedral complexes in terms of the positively charged metal centres and the negatively charged ligand which are surrounding it. There are 5 degenerate orbitals and these are split into two energy levels; the eg and the t2g which are high spin and low spin respectively. The eg orbitals are closer to the ligands and experience much more repulsion hence have higher energy than that of the degenerate d orbitals. The tg orbitals are further away meaning that they have less energy than that of the d orbitals. Moving of electrons between the energy levels is what gives the complexes there different colours.

Ni(II) can form up to six coordinate bonds. The complex is only octahedral if all the ligands are identical, however if all the ligands are not identical then the strength and length of the coordinate bonds differ and this results in distortion. In this experiment different axial ligands were used hence the extent of distortion and energy levels of the d-orbitals differed. This in turn affects the colour and magnetic properties of the compounds.

Strong axial ligands means there is strong interactions and hence there is a large repulsion which causes strong tetragonal distortion between the d-orbitals. The energy levels in distorted tetragonal are lower than those in a perfect octahedral. Similarly weak coordinating axial ligands result in less interaction and repulsion which causes a weaker tetragonal hence the energy levels are higher than that in a perfect octahedral field. The strength of the axial ligands helps to determine the energy difference in the orbitals and this in turn determines whether the complex is high spin or low spin.

Since Ni2+ is a d8 compound, the 8th electron can either fill the b1g orbital or it can pair up in the a1g orbital. If the electrons are excited to the b1g orbital, the system will become lower in energy and hence minimise the amount of distortion. This results in weak tetragonal distortion. The resulting two unpaired electrons results in a high spin compound which is paramagnetic. The colour of the compound appears to be a blue or purple colour.

However, if there is strong tetragonal distortion in an octahedral field, the energy gap between the b1g and all the other orbitals is significantly large. If the electrons in the b2g orbitals are paired up, more stability can be achieved, instead of having two unpaired electrons one in the b1g and one in the b2g orbital. This results in diamagnetism as there are no unpaired electrons and hence the compound is low spin. The colour of the compound appears to be between red-yellow.

According to the spectro-chemical series, the ligand strength of the molecules are ranked in the following order: I-< Cl-< NO3-< H2O < NCS- .The results obtained show that Compounds A (axial ligands are Cl-) and C (axial ligands are NCS) are paramagnetic, while compounds B (axial ligands are NO3-) and D (axial ligands I-) are diamagnetic.

Precautions Taken, Sources of Error and Improvements

Toxic chemicals were used and hence it was vital to wear safety goggles, gloves and a lab coat. This was so that there was no direct skin contact by the chemicals. Mixing of all the chemicals were done in a fume cupboard and it was important that the glass cover was pulled down to ensure that no chemicals were inhaled.

The magnetic properties of the complexes prepared match those of the literature reference hence the experiment went as desired. The range of the yields obtained differed enormously and hence this indicates that the experimental technique should be improved in preparing the samples. More accuracy and attention to detail should be ensured so that the maximum yield is obtained.