Inorganic Lab Report

Aim To synthesise 4 different Ni(II) complexes: Ni(Et2en)2Cl2.2H2O, Ni(Et2en)2(NCS)2, Ni(Et2en)2(NO3)2 and Ni(Et2en)2I2 and ascertain the effect of the ligands on the electronic structure of the complexes by UV spectrometry and also through the magnetic moments of the complexes. Results and Calculation 1. Mass of Compounds obtained, and their respective percentage yields Compound A, Ni(Et2en)2Cl2.2H2O Mass of NiCl2.6H2O used = 0.79g Mass of Ni(Et2en)2Cl2.2H2O obtained = 1.05g NiCl2.6H2O + 2Et2en ↔ Ni(Et2en)2Cl2.2H2O + 4H2O

Amount of NiCl2.6H2O used = 0.79g / (58.7 + 2(35.5) + 6(18.0)) = 0.0032394 mol Amount of Et2en = 0.77g / (6(12.0) + 16(1.0) + 2(14.0)) = 0.0066379mol (limiting reagent) Theoretical Amount of Ni(Et2en)2Cl2.2H2O obtained = 0.0066379/2 = 0.0033190mol Theoretical mass of Ni(Et2en)2Cl2.2H2O obtained = 0.0032394 x (58.7 + 2(35.5) + 2(116) + 2(18.0)) = 1.2883g Percentage Yield of Ni(Et2en)2Cl2.2H2O = (1.05/1.2883) x 100% = 81.5% (3 s.f.)

Compound B, Ni(Et2en)2(NCS)2

Mass of Ni(Et2en)2(NCS)2 obtained = 0.76g Ni(NO3)2.6H2O + 2NaNCS  Ni(NCS)2 + 6H2O + 2NaNO3 Ni(NCS)2 + 2Et2en ↔ Ni(Et2en)2(NCS)2

Amount of Ni(NO3)2.6H2O used = 0.97g / (58.7 + 2(14.0 + 48.0) + 6(18.0)) = 0.0033368mol Amount of NaNCS = 0.54g / (23.0 + 14.0 + 12.0 + 32.1) = 0.0066584mol (limiting reagent) Theoretical Amount of Ni(NCS)2 obtained = 0.0066584/2 = 0.0033292mol

Amount of Et2en = 0.77g / (6(12.0) + 16(1.0) + 2(14.0)) = 0.0066379mol (limiting reagent) Theoretical mass of Ni(Et2en)2(NCS)2 obtained = 0.0033190 x (58.7 + 2(14.0 + 12.0 + 32.1) + 2(116))

= 1.3505 Percentage Yield of Ni(Et2en)2Cl2.2H2O = (0.78/1.3505) x 100% = 57.8 % (3 s.f.)

Compound C, Ni(Et2en)2(NO3)2

Mass of Ni(Et2en)2(NO3)2 obtained = 0.84g Ni(NO3)2.6H2O + 2Et2en ↔ Ni(Et2en)2(NO3)2 + 6H2O Amount of Ni(NO3)2.6H2O used = 0.97g / (58.7 + 2(14.0 + 48.0) + 6(18.0)) = 0.0033368mol Amount of Et2en = 0.77g / (6(12.0) + 16(1.0) + 2(14.0)) = 0.0066379mol (limiting reagent) Theoretical mass of Ni(Et2en)2(NO3)2 obtained = 0.0033190 x (58.7 + 2(116) + 2(14.0 +48.0)) = 1.3764 Percentage Yield of Ni(Et2en)2(NO3)2 = (0.84/1.3764) x 100% = 61.0% (3 s.f.)

Compound D, Ni(Et2en)2I2

Mass of Ni(Et2en)2I2 obtained = 0.76g Ni(NO3)2.6H2O + 2NaI  Ni(I)2 + 6H2O + 2NaNO3 Ni(I)2 + 2Et2en ↔ Ni(Et2en)2I2

Amount of Ni(NO3)2.6H2O used = 0.97g / (58.7 + 2(14.0 + 48.0) + 6(18.0)) = 0.0033368mol Amount of NaI = 1g / (23.0 +126.9) =0.0066711mol (limiting reagent) Theoretical Amount of Ni(I)2 obtained = 0.0066711/2 = 0.0033356mol

Amount of Et2en = 0.77g / (6(12.0) + 16(1.0) + 2(14.0)) = 0.0066379mol (limiting reagent) Theoretical mass of Ni(Et2en)2I2 obtained = 0.0033190 x (58.7 + 2(116) + 2(126.9)) = 1.8072g Percentage Yield of Ni(Et2en)2I2 = (0.76/1.8072) x 100% = 42.1%

2. The preparation of Compounds A and B for Electronic Spectrometer Compound A, Ni(Et2en)2Cl2.2H2O

Amount of A, Ni(Et2en)2Cl2.2H2O needed = 3 x 10-2M x (10mL/1000LmL-1) = 3 x 10-4 mol Mass of A, Ni(Et2en)2Cl2.2H2O needed = 3 x 10-4 mol x (58.7 + 2(35.5) + 2(116) + 2(18.0)) = 0.11931g Compound B, Ni(Et2en)2(NCS)2 Amount of B, Ni(Et2en)2(NCS)2 needed = 3 x 10-2M x (10mL/1000LmL-1) = 3 x 10-4 mol Mass of B, Ni(Et2en)2(NCS)2 needed = 3 x 10-4 mol x (58.7 + 2(116) + 2(14.0 + 12.0 + 32.1)) = 0.12207g

3. Magnetic Moments of all Compounds Compound

A, B, Ni(Et2en)2Cl2.2H2O Ni(Et2en)2(NCS)2 0.9028 0.6680

Mass of filled tube/g 0.8001 0.6195 Mass of empty tube/g 0.1027 0.0485 Mass of sample used/g 23.5 21.0 Length of sample in tube/mm 0.0083 x 10-4 0.082 x 10-4 Magnetic Susceptibility Table 1: Magnetic Moment of Compounds A to D

C, Ni(Et2en)2(NO3)2 0.6966

D, Ni(Et2en)2I2

0.5984

0.7420

0.0982

0.1423

19.0

23.5

0.000 x 10-4

0.000 x 10-4

0.8843

From the lab manual, it is given that cM = cg x MW + cD + cTIP where cM is the molar susceptibility, cg is the magnetic susceptibility per unit mass, MW is the molecular weight of the substance, cD is the correction for intrinsic diamagnetism, and cTIP is the correction of temperature-independent magnetism (cTIP is assumed to be negligible). It can also be represented by cM = m(Nb2/3kT) ; m = 2.83√(cMT) where N is Avogadro’s Number, b is Bohr’s magneton (the magnetic moment), k is the Boltzmann Constant, and T is the absolute Temperature (in Kelvin). m = √[n(n+2)] where n is the number of unpaired electrons in the given species. Compound A, Ni(Et2en)2Cl2.2H2O

cD = χ(Ni2+) + 32χ(H) + 12χ(C) + 4χ(N) + 2χ(Cl-) + 2χ(H2O) = [(-12.8) + 32(-2.9) + 12(-6.0) + 4(-5.6) + 2(-23.4) + 2(13)](10-6) = -2.208 x 10-4 cM = cg x MW + cD + cTIP ≈ (0.0083 x 10-4 x (58.7 + 2(35.5) + 2(116) + 2(18.0))) + (-2.208 x 10-4) = 0.000109

m = 2.83√(cMT) = 2.83√(0.000109 x 298) = 0.51072 0.51072 = √n(n+2) By solving quadratically, we get n = 0.12287 ≈ 0 (value -2.1229 is rejected) Compound B, Ni(Et2en)2(NCS)2 cD = χ(Ni2+) + 32χ(H) + 14χ(C) + 6χ(N) + 2χ(S2-) = [(-12.8) + 32(-2.9) + 14(-6.0) + 6(-5.6) + 2(-38)](10-6) = -2.992 x 10-4 cM = cg x MW + cD + cTIP ≈ (0.082 x 10-4 x (58.7 + 2(14.0 + 12.0 + 32.1) + 2(116)) + (-2.992 x 10-4) = 0.0030374 (5sf)

m = 2.83√(cMT) = 2.83√(0030374 x 298) = 2.6924 (5sf) 2.6924 = √n(n+2) By solving quadratically, we get n = 1.8721 ≈ 2 (value -3.8721 is rejected) Compound C, Ni(Et2en)2(NO3)2 cD = χ(Ni2+) + 32χ(H) + 12χ(C) + 4χ(N) + 2χ(NO3-) = [(-12.8) + 32(-2.9) + 12(-6.0) + 4(-5.6) + 2(-18.9)](10-6) = -2.378 x 10-4 cM = cg x MW + cD + cTIP ≈ (0.000 x 10-4 x (58.7 + 2(116) + 2(14.0 +48.0)) + (-2.378 x 10-4) = -0.0002378 < 0 The substance is diamagnetic => n = 0 (no unpaired electrons) Compound D, Ni(Et2en)2I2 cD = χ(Ni2+) + 32χ(H) + 12χ(C) + 4χ(N) + 2χ(I-) = [(-12.8) + 32(-2.9) + 12(-6.0) + 4(-5.6) + 2(-50.6)](10-6) = -3.012 x 10-4 cM = cg x MW + cD + cTIP ≈ (0.000 x 10-4 x (58.7 + 2(116) + 2(126.9)) + (-3.012 x 10-4) = -0.0003012 < 0 The substance is diamagnetic => n = 0 (no unpaired electrons)

Discussion (a) Magnetic Susceptibility

Nickel is a group 8 transition element and so forms d8 complex. Transition elements have a partially filled d or f subshell in any common oxidation state, and thus, they form complex ions with various types of ligands. Ligands are groups of atoms or molecules that donate electrons to the central metal ion centre. The strength and type of these ligands can potential alter the structure and properties of the ions. This experiment shows the significance in the interactions between the type of ligands and the effect on colour and magnetic susceptibility of the compound. There are two types of magnetism under consideration: paramagnetism, the type of magnetism if there is an external magnetic field to align the randomly aligned electron spins, or diamagnetism, the type that creates a magnetic field that opposes any applied magnetic field due to the presence of any electron. All materials are thus diamagnetic, but they may paramagnetism as the effect of diamagnetism is overshadowed. NiCl2.6H2O + 2Et2en  Ni(Et2en)2Cl2.2H2O + 4H2O

Ni(NO3)2.6H2O + 2NaNCS



Ni(NCS)2 + 2Et2En

---(1)

Ni(NCS)2 + 2NaNO3 + 6H2O



Ni(Et2en)2(NCS)2 ----(2)

Ni(NO3)2.6H2O + 2Et2en  Ni(Et2en)2.(NO3)2 + 6H2O ---(3)

Ni(NO3)2.6H2O + 2NaI NiI2 + 2Et2en

 

NiI2 + 2NaNO3 + 6H2O Ni(Et2en)2I2 -----(4)

Crystal Field Splitting Theory—Octahedral Field In an octahedral field, where the ligands’ line of approach is along the x, y and z axes, five degenerate orbitals will split to two energy levels, comprising two eg and three t2g orbitals, with the eg orbitals’ energy raised above the energy of the degenerate d-orbitals, and the t2g orbital’s energy lowered below that. The two eg orbitals comprise of the dx2-y2 and the dz2 orbitals, while the three t2g orbitals comprise of the dxy dxz and dyz orbitals. This is because, upon the approach of ligands in an octahedral field, the dx2-y2 and the dz2 orbitals are being approached by the ligands head-on, as these two orbitals lie on the x, y and z axes, which are the axes along which the ligands are approaching. As for the dxy dxz and dyz orbitals, they lie in the space between the axes. Therefore, the electrons in the dx2-y2 and the dz2 orbitals experience much more repulsion from the ligands as compared to those in the dxy dxz and dyz orbitals. Hence, the energy of the dx2-y2 and the dz2 orbitals is raised, and relatively, the energy of the dxy dxz and dyz orbitals is lowered. The energy difference between the eg and t2g orbitals is the energy needed to excite the electron from the lower energy state, t2g to a higher energy state eg, in the process absorbing energy of a certain wavelength which corresponds to that in the visible spectrum. Therefore, most transition metals can

form colored complexes, and the color of the complex observed is the complementary color of that absorbed due to the excitation of the electron.

Figure 1: Splitting of d-orbitals in an Octahedral Field

Ni(II) is able to accommodate six ligands, and can form up to six coordinate bonds. The geometry around such a complex is octahedral if all the ligands are identical. However, if not all the ligands are identical, the strength and length of the coordinate bond differs. The outcome is a distortion of the octahedral geometry. In this experiment, the axial ligands used were varied for the complexes A to D, resulting in the different extent of distortion and the energy levels of the d-orbitals. Complexes Ni(Et2en)2Cl2.2H2O and Ni(Et2en)2(NCS)2 have 3 peaks in their UV spectra, suggesting a tetragonal structure, with electronic transitions between from a1g to b1g ; b2g to b1g and b2g to a1g. Complexes Ni(Et2en)2(NO3)2 and Ni(Et2en)2I2 however have 2 peaks, suggesting electronic transitions from b2g to b1g / and a1g to b1g. This is due to the d8 configuration of the Ni(II) ion, which gives rise to the electronic structure accordingly:

Figure 2: Electronic structure of the different compounds

Tetragonal Distortion in an Octahedral Field Tetragonal distortions, according to Jahn-Teller Theorem, are distortions that occur because a molecule in a degenerate electronic state undergoes geometrical distortion to remove that degeneracy. This

results in the splitting of the d-orbitals, i.e. the eg and t2g orbitals will split to different energy levels too, in an octahedral field. In this experiment, the axial ligands were varied, thus directly affecting the colour and magnetic properties of the compounds. When the two axial ligands (which approach the central metal ion along the z-axes) have larger strength, there would be more interaction and larger repulsion between the dorbitals along the z-axes and these stronger ligands, as compared to the relatively weaker ligands that are approaching along the x and y plane. The orbitals along the x and y axes would be drawn slightly inwards since they experience relatively less repulsion with the relatively weaker ligands and thus have shorter bonds as compared to orbitals with a ‘z’ component. Therefore, these ligands would tend to keep further from the d-orbitals along the z-axes and form longer bond lengths. The orbitals with a ‘z’ component would experience less actual repulsion than what is predicted in an octahedral field, and these orbitals include the dxz, dyz and dz2 orbitals. By keeping slightly further away, the energy can be slightly lowered. The dxz and dyz orbitals have eg symmetry and the dz2 orbital has a1g symmetry, and their energy levels after tetragonal distortion is lower than in a perfect octahedral field. On the other hand, orbitals dx2-y2 and dxy would experience more actual repulsion than what is predicted in an octahedral field. The dxy orbital has b2g symmetry and the dx2-y2 orbital has b1g symmetry, and their energy levels after tetragonal distortion are higher than that in a perfect octahedral field.

Figure 3: Tetragonal Distortion in an Octahedral Field

Therefore, the energy of the orbitals with ‘z’ component are lowered, while those without are raised, thus forming new energy levels. The strength of the axial ligand plays a role determining the magnetic property of a complex. The strength of the axial ligand determines the energy difference in the orbitals upon tetragonal distortion. This is because the position of the dz2 orbital is very sensitive to the strength of the axial ligands, as

they will approach each other head on during coordination. Relatively strongly coordinating axial ligands will result in stronger interaction and repulsion, creating a stronger tetragonal distortion. Conversely, relatively weaker coordinating axial ligands will result in less interaction and repulsion as compared to stronger axial ligands, and thus creating a weaker tetragonal distortion. This is very important as it determines whether the complex is a high-spin or low-spin complex, which orbitals the electrons will occupy, and finally, its magnetic property.

Figure 4: Weak and Strong Tetragonal Distortion in an Octahedral Field In an octahedral field with weak tetragonal distortion, the energy gap between the b1g and a1g orbitals is relatively small, and the magnitude of energy is likely less than the pairing energy. Since Ni2+ is a d8 compound, the 8th electron has a choice to fill either the b1g orbital, or pair up in the a1g orbital. The system would be relatively lower in energy overall if the electrons were excited to the b1g orbital (since the energy gap is relatively smaller than pairing energy), minimizing the repulsion due to pairing as well. This would result in two unpaired electrons (high spin compound), and thus the compound would be paramagnetic. However, if an octahedral field has a strong tetragonal distortion, the energy gap between the b1g and all the other orbitals is significantly large; the magnitude of energy is likely larger than the pairing energy. The system will then achieve a more stable system if the electrons pair up in the b2g orbital instead of exciting it and having two unpaired electrons, one in the b1g and the other in b2g orbital. The compound in this case would have no unpaired electrons (low spin compound), and is diamagnetic. Tetragonal Distortion in the Ni(II) Complexes prepared in this experiment

Ligands influenced the magnitude of the energy difference between the d-orbitals according to their ligand strength, and results in the various colors of metal-ligand complexes. According to the spectrochemical series, the ligand strength of the molecules used in the four Ni(II) complexes is in the following order: I- < Cl- < NO3- < H2O < NCS-. The experimental results obtained shows that compounds A (axial ligands are Cl-) and B ( axial ligands are NCS-) are paramagnetic, while compounds C (axial ligands are NO3-) and D (axial ligands I-) are diamagnetic. However, there is a discrepancy from the specchemical series. It is expected that B and C to be paramagnetic as their ligands are stronger than the ligands for A and D. Stronger ligands imply more interaction with the central metal ion and greater repulsion, leading to larger tetragonal distortion. In the calculations, the number of unpaired electrons in A is zero, B is two, C and D are zero. The erroneous value of zero for A, when in fact theoretically it is expected to be two, suggests experimental error while measuring the magnetic susceptibility. The 2 unpaired electrons in A and B was the cause for them to be paramagnetic, and as such should reflect a nonzero and positive magnetic susceptibility. C and D are diamagnetic because all the electrons are paired up. The spins in the complexes are paired, so they are repelled by the magnetic field.

(b) Electronic Spectra of the Compounds An electronic spectrum is obtained upon the absorption and emission of electromagnetic radiation during changes in the electronic configuration of atoms or groups of atoms or ions. Although the Laporte Rule does not allow a transition between identical orbital type: s-s for example , d-d transitions have been observed experimentally. It has been suggested that the symmetry in the complex is not perfectly symmetrical therefore nullifying the Laporte’s rule. In this experiment, the electronic spectra of compounds A and B, and their peaks, were obtained. Wavelength/nm

Absorbance

670.50

0.2450

Table 2a: Peaks in the Electronic Spectra of Compound A, Ni(Et2en)2Cl2.2H2O Wavelength/nm

Absorbance

792.00

0.1653

579.50

0.2640

369.00

0.6500

Table 2b: Peaks in the Electronic Spectra of Compound B, Ni(Et2en)2(NCS)2 From Table 2a, it is seen that the wavelength of compound A falls within the visible spectrum, where the peak absorbance corresponds to the colour red (664.50 nm). This indicates that A absorbs red light,

reflecting the blue region. From Table 2b, the wavelength of compound B falls within the visible spectrum where the peak absorbance occur correspond to yellow (578.50 nm). This indicates that Compound B absorbed yellow light, reflecting the blue, purple region. It is theorized that the energy absorbed in the excitation of these compounds is relatively smaller than C or D. This implies that the compounds have smaller tetragonal distortion. Therefore, they are high spin compounds and have two unpaired electrons each, and are paramagnetic. This corresponds to the experimental results obtained. On the other hand, Compounds C (yellow) and D (brick-red) absorb in the violet and blue range respectively. The energy absorbed is larger than that of compounds A and B, implying a larger energy gap and so the compounds C and D must have a larger tetragonal distortion. Therefore, they are low spin compounds and have no unpaired electrons, making them diamagnetic. This corresponds to the experimental results obtained as well.

Precautions Taken, Sources of Error and Improvements The entire setup was performed under a hood with the glass cover pulled down to prevent any inhalation of any chemical (ethanol, particularly). This is done as any undesirable inhalation during the long hours may result in drowsiness. In preparing the chemicals for the UV spectrometer, toxic chemicals were used and so gloves were worn. This was to prevent the direct skin contact by the chemicals, which could result be dangerous. The number of unpaired electrons calculated for compound A, Ni(Et2en)2Cl2.2H2O was calculated experimentally to be zero. However, it does not correspond to its apparent paramagnetic properties. The error is likely to have arisen from the technique used to prepare the sample for magnetic susceptibility. Compound A should be powdered completely and packed compactly. Evidently, experimental technique should be improved in preparing this sample. More time and effort in ensuring complete packing of the powder is necessary to detect the correct magnetic susceptibility (improving accuracy). In addition, repeats ought to be done so the results would be more reliable. A highly accurate and very reliable set of data allows accurate analysis of the compound.

Conclusion The results of this experiment can be summed as follows:

Compound

Percentage yield (%)

Magnetism

Ni(Et2en)2Cl2.2H2O

81.5

Paramagnetic

Ni(Et2en)2(NCS)2

57.8

Paramagnetic

Ni(Et2en)2(NO3)2

61.0

Diamagnetic

Ni(Et2en)2I2

42.1

Diamagnetic

References [1] Harris, D.C.; Bertolucci, M.D. (1978). Symmetry and Spectroscopy. Oxford University Press [2] Housecroft, C.E. (2005). Inorganic Chemistry, 2nd Edition. Prentice Hall [3] Schläfer, H. L.; Gilemann, G. (1969) Basic Principles of Ligand Field Theory. John Wiley & Sons Parker, [4] Rayer-Canham, G.; Overton, T. (2003) Descriptive Inorganic Chemistry. 3rd Edition. W. H. Freeman & Company