Ni-Phosphine Complexes - Term Paper Example

Abstract Nickel (II) complexes are the most studied complexes in chemistry. This is because of their numerous uses that have been discovered and are still being discovered until today. The most common use of nickel(II) complexes is as a catalyst in cross-coupling reaction of Grignard reagents with alkyl and aryl halides and hydroboration, isomerization and hydrogenation reactions (Garner & Macdonald, 1962, p. 98). The preparation of Dichlorobis(triphenylphosphine)Nickel (II), and Dibromobis(triphenylphosphine)Nickel (II) which are some examples of nickel(II) complexes is carried out through a simple reaction process where the triphenylphosphine ligands substitute the water molecules that are bound to a hydrated solution of Nickel chloride (or bromide) (Garner & Macdonald, 1962, p. 187). Several other complexes are formed from these two through the same process of ligand exchange where one phenyl is substituted with another ligand. The structures and geometries of the new formed complexes are determined by way of magnetic moment calculation, observation of IR spectra, NMR spectra and UV-Visible spectra. The NMR and IR spectra aid in the determination of the functional groups present in every complex and their positions in the complex as well. The UV-Visible spectra and the magnetic moments aid in the determination of the geometry of the complex. Most of the nickel(II) complexes are tetrahedral. The successful testing of all the complexes will portray the success in the preparation of the complexes as well as the success in the methods of structure and geometry determination used (Garner & Macdonald, 1962, p. 208). Table of Contents Abstract 2 Table of Contents 2 Introduction 4 Methods of Determining the Structure and Geometry of Nickel Triphenylphosphine Complexes. 5 UV-Visible Spectroscopy 6 Magnetic Moment (μ) 7 Nuclear Magnetic Resonance (NMR) 8 Infra-Red Spectroscopy (IR) 9 Experimental 9 Preparation of Dichlorobis(triphenylphosphine)Nickel (II) 9 Preparation of dibromobis(triphenylphosphine) nickel(II) 10 Formation of new complexes from Dichlorobis (triphenylphosphine)Nickel (11) 11 Formation of new complexes from Dibromobis(triphenylphosphine)Nickel (11) 13 Investigation of the structures and geometries of the complex compounds 14 Results and discussion 15 Magnetic moments of each of the complexes 15 Discussion 16 Infra-red spectra of each of the complexes 17 Discussion 17 UV-Visible spectra of each of the complexes 18 Discussion 18 NMR spectra of each of the complexes 20 Discussion 21 Conclusion 22 References 23 Introduction The organometallic chemistry of nickel has been studied for many years in the past. In spite of the many years chemists have spent studying nickel and its compounds, new reactions are still being discovered frequently. The improvement of the understanding of the already discovered reactions is also raising and new ways of utilizing these reactions to aid in the alleviation of the challenges of organic synthesis are being discovered day in day out. Nickel catalysts provide preparative convenient reactions. The reactivity that can be achieved by nickel compounds cannot be achieved by any other transition metal. Nickel and its compounds is the most covered and the most promising area of chemistry (Garner & Macdonald, 1962, p. 98). Nickel (I) and nickel (II) species are of great interest because of their likely association of their oxidation states in nickel containing metalloenzymes. Triphenylphosphine nickel complexes are one of the most productive and selective catalysts employed in hydroboration, isomerization and hydrogenation reactions. Nickel phosphine complexes in particular are useful in the cross-coupling reactions of Grignard reagents with alkyl and aryl halides. The reaction below shows Dichlorobis(triphenylphosphine)Nickel (II) acting as a catalyst in a cross-coupling reaction of Grignard reagents with alkyl and aryl halides (Garner & Macdonald, 1962, p. 99). The coordination geometry of nickel(II) complexes is sensitive to alterations in the ligand set. The nickel complexes with a chloride or a bromide will have a tetrahedral structure due to the steric bulkiness of the chloride and bromide ions. The occupation of the antibonding orbitals in the complex is also unavoidable in this case. The tetrahedral complexes are paramagnetic with two unpaired electrons. Formation of square planar nickel (II) complexes is also possible. These form in the presence of small ligands or large ligand splitting. Large ligand splitting is caused by ligands with a strong negative charge and those that are able to move closer to the metal ion. Small highly charged ions make the d-orbital to be energetically unfavourable for an electron when the ion moves closer to the orbital. The number of complexes of the type [Ni(PR3)2X2] in solution that show a square planar to tetrahedral equilibrium are limited as the energy difference between the two geometries is very little. The more bulky the ligand bound to the nickel (II), the more favourable the tetrahedral geometry becomes. This is because the geometry of nickel (II) complexes relies on the steric and electronic effects that are produced by the ligands. The type of ligand bound to the nickel also plays a part in the determination of the geometry of the complex. Strong field ligands form low spin complexes while medium-weak field ligands form high spin complexes. High spin complexes tend to form square planar complexes (Ballhausen, 1962, P. 210). One of the nickel triphenylphosphine complexes, (NiCl2(PPh3)2) is a four coordinate Ni(II) complex. It exists as an equilibrium mixture of tetrahedral and square planar geometries. There are other five crystals that have this same characteristic. Some other NiCl2(PPh3)2 complexes display square planar geometry. They embody solvent molecules in their structures. The solvent molecules do not form a coordinate with the nickel complex but rather they solvate next to the molecules of the complex. This has a stabilizing effect on the complex hence the square planar geometry is achieved due to weak dipole interactions (Ballhausen, 1962, P. 217). Methods of Determining the Structure and Geometry of Nickel Triphenylphosphine Complexes. Every chemist at one point or another encounters the need to determine the structure of a compound. There are various methods that can be used to determine the structures and geometries of complexes such as the Nickel Triphenylphosphine complexes we are focusing on. UV-Visible spectroscopy and the calculation of magnetic moments are methods used to determine the geometry of complexes while NMR spectroscopy and Infra-Red spectroscopy are used to determine the structures of complexes. When determining the structure of a complex, it is advisable to carry out many different tests on the compound then consolidate the results because each test on its own may not be conclusive on the structure of the complex. UV-Visible Spectroscopy UV-Visible spectroscopy has been used for a long time in the determination of the coordinate geometries of transition metal complexes. In the UV-Visible spectrum, observation of colour bands is used to determine the geometry of the complex. The bands are caused by the movement of valance electrons between different energy levels. When a free metal ion interacts with ligands, the ligands locate themselves in the position of minimal electrostatic interaction thus the electrons of the transition metal ion occupy the orbitals with the least ligand charge. Such interactions occurring at the d-orbitals resulting in their splitting due to a ligand field can be observed in the visible region of the electromagnetic spectra. This is observed as different colours of the complexes. The presence of a ligand field tend to split the degenerate d-orbitals in to the eg and the t2g sets. The transition of an electron from a lower level (t2g) to a higher level (eg) is attained by the absorption of visible light. When a complex absorbs a colour from the visible region of the electromagnetic spectrum, we observe the compliment of the absorbed colour. It is clear that the geometrical arrangement of the complex determines the amount of energy required to elevate an electron to a higher level thus displaying the contrasting colour. The type of ligands bound to the transition metal also play a part in this because ligands of a stronger field have larger energy values (Δ) as compared to ligands of a weaker field. Magnetic Moment (μ) Most of the existing transition metal coordination compounds are paramagnetic. This means that the molecule has a permanent magnetic dipole and bulk samples of the material are attracted into the magnetic field. The magnetic field μ of transition metal coordination compounds is the outcome of the presence of unpaired electrons in the d-orbitals of the compound. The magnitude of μ is correlated to the number and nature of the unpaired electrons present in the coordination compound. The magnetic moment of the compound provides information on the structure and valency of the compound. Nickel (II) complexes can display high-spin paramagnetic character thus tetrahedral geometry or a low-spin, diamagnetic character, thus the square planar geometry. The geometry of the complexes is determined by the changes in ligand field strength and steric requirements. The magnetic moment of a compound is a function of the number of unpaired electrons. It can be calculated using the formula referred to as the spin-only formula: μ = [n (n +  = [s (s +  Where n= number of unpaired electrons μ= magnetic moment in Bohr magnetons s= sum of spin quantum numbers for individual electrons. The total magnetic moment µt is expressed in term of the spin (S) and orbital (L) angular momentum: µt = [4S (S+1) + L (L+1)]1/2 The orbital contribution to the magnetic moment can be ignored if the ground state is A1, A2 and E states because in these states, the spin-only formula is as good as the observed magnetic moment and T1 and T2 ground states have orbital angular moment contribution. The magnetic manners of dissimilar materials are differentiated by the temperature reliance of their magnetic receptiveness. The magnetic susceptibility of diamagnetic materials is not dependent on the temperature. For paramagnetic materials, magnetic susceptibility decreases with increasing temperature due to thermal effects which distort the placement of magnetic moments in an applied magnetic field. Nuclear Magnetic Resonance (NMR) Nuclear Magnetic Resonance (NMR) spectroscopy gives a variety of potent techniques for the determination of the structures of complex compounds (Akitt, 1973, P. 167). This technique exploits the magnetic properties of the atomic nuclei in the complex to enable the determination of the physical and chemical properties of the molecules in the complex. NMR spectroscopy provides data about the structure, dynamics and the chemical environment created by the molecules in the complex. The chemical environment of the complex determines the frequency at which different nuclei in the molecule will absorb electromagnetic radiation. The interaction of nuclear spins with the surrounding electronic field creates a chemical shift. Different chemical groups have different chemical shifts thus the resonance assignment to these chemical groups can be used to determine the structure ((Roberts, 1959, p. 99). The result of NMR is observed when nuclei aligned with the external field absorb energy and modify the orientation of their spin with respect to the magnetic field. Planck postulated that the energy variation between the two spin states is correlated to the electromagnetic radiation frequency. The difference between the square-planar and tetrahedral arrangements may therefore be indicated by the small disparities in energy that will be apparent in the NMR spectra. For the confirmation of the structure of the organic ligands, three different hydrogen environments of the P(ph)3 compound can be seen in the structure which gives arise to three signals in the NMR spectra (Akitt, 1973, P. 358). Infra-Red Spectroscopy (IR) Infra-Red spectroscopy is another commonly used method of determination of the structure of compounds. It deals with the Infra-red region of the electromagnetic spectrum. This method entails the measurement of different Infra-red frequencies by a sample positioned in the path of and IR beam (Hesse, Meier, & Zeeh 2008, p. 60). Different groups in the complex will absorb the IR radiation at different frequencies. This variation is used to determine the structure of the complex compound. IR absorptions are presented as wave numbers or wavelengths. At absolute zero temperatures, the atoms in a molecule are constantly vibrating. When exposed to IR radiation, the atom with a vibration frequency equal to that of the radiation absorbs the radiation. Three degrees of freedom, corresponding to the motions along the three Cartesian planes, are required to describe the motion of the molecule. The change in the absorption intensity is what entails the IR spectra . (Hesse, Meier, & Zeeh 2008, p. 77) Experimental Preparation of Dichlorobis(triphenylphosphine)Nickel (II) (Brammer & Stevens, 1989, p. 63) This complex is normally used as a catalyst in the aryl mesylates and allylis carbonates coupling reactions with lithium organoborates. It is also used as a coordination compound. It has three freely rotating bonds. Procedure 1.2g of nickel chloride was dissolved in 15cm3 of dry ethanol. This solution was then warmed gently. 2.8g of tryphenylphosphine and 30cm3 of isopropanol were placed in a round bottomed flask which was fit with a reflux condenser. The phosphine was dissolved by refluxing gently. When the phosphine had completely dissolved, the flask was removed from the heat and the nickel chloride solution which had been made earlier was carefully added. The mixture was further refluxed for another 10 minutes and then was allowed to cool at room temperature. The product was filtered in a Buchner flask. It was then washed with cold ethanol, 15cm3 then it was followed by 15cm3 of diethylether. A streat of air was drawn through the product to dry. Ph3P, AcOH NiCl2.6H2O NiCl2(PPh3)2 Preparation of dibromobis(triphenylphosphine) nickel(II) (Brammer & Stevens, 1989, p. 65) This complex is monoclinic with four molecules in the unit cell. The coordination of the complex round nickel is tetrahedral but it is distorted by the repulsion between the bromine atoms thus an enlarged Br-Ni-Br angle of 126o is formed (Angelici, 1977, p. 598). The procedure for the preparation of dibromobis(triphenylphosphine) nickel(II) is similar to that of dichlorobis(triphenylphosphine) nickel(II) only that the nickel chloride used in dichlorobis(triphenylphosphine) nickel(II) is replaced with nickel bromide trihydrate. Procedure 1.4g of nickel bromide trihydrate was dissolved in 15cm3 of dry ethanol. This solution was then warmed gently. 2.8g of tryphenylphosphine and 30cm3 of isopropanol were placed in a round bottomed flask which was fit with a reflux condenser. The phosphine was dissolved by refluxing gently. When the phosphine had completely dissolved, the flask was removed from the heat and the nickel bromide solution which had been made earlier was carefully added. The mixture was further refluxed for another 10 minutes and then was allowed to cool at room temperature. The product was filtered in a Buchner flask. It was then washed with cold ethanol, 15cm3 then it was followed by 15cm3 of diethylether. A streat of air was drawn through the product to dry. Ph3P, AcOH NiB2.6H2O NiBr2(PPh3)2 Formation of new complexes from Dichlorobis (triphenylphosphine)Nickel (11) Formation of five new complexes by changing one of the phenyl group from nickel Triphenylphosphine complex [Dichlorobis (triphenylphosphine)Nickel (11)] with an Ar group (where Ar is 2- and 4- tolyl; and 2- , 3- and 4- chlorophenyl) is done by ligand exchange (Basolo & Pearson, 1986, p. 208). Ligand exchange reaction is the reaction whereby one ligand in a complex ion is replaced by a different ligand. A complex ion comprises of the central metal ion that is bound to several ligands (Basolo & Pearson, 1986, p. 217). If another type of ligand is added into this complex ion, and the second ligand is capble of forming stronger bonds (than the original ligands) with the metal ion, then the original ligands are replaced by the new ligand forming a new complex ion. When a ligand exchange reaction occurs, the geometry of the complex ion may be altered from its original geometry. This is dependent on the strength of the new ligands (Angelici, 1977, p. 618). Exchange of a phenyl group from NiCl2(PPh3)2 with a tolyl group Tolyl is a C7H7 group obtained from toluene. Exchange of one phenyl group with a tolyl group is possible via ligand exchange. One of the phenyl groups in NiCl2(PPh3)2 is substituted with a tolyl group. The tolyl group is supplied by toluene ((Basolo & Pearson, 1986, p. 273). 2-Tolyl group NiCl2(PPh3)2 NiCl2(PPh2-2-tolyl)2 Tolyl is more reactive than the phenyl group due to the lone pair of electrons, thus the substitution occurs due to the lone pair of electrons in the tolyl group. Substitution of a phenyl group in NiCl2(PPh3)2 with 3-tolyl and 4-tolyl also occur in the same way ((Basolo & Pearson, 1986, p. 310). 3-Tolyl group NiCl2(PPh3)2 NiCl2(PPh2-3-tolyl)2 4-Tolyl group NiCl2(PPh3)2 NiCl2(PPh2-4-tolyl)2 Exchange of a phenyl group from NiCl2(PPh3)2 with a chlorophenyl group Chlorophenyl is a group obtained from addition of chlorine to a phenyl group. Exchange of one phenyl group with a chlorophenyl group is possible via ligand exchange. One of the phenyl groups in NiCl2(PPh3)2 is substituted with a chlorophenyl group (Angelici, 1977, p. 625). 2-chlorophenyl NiCl2(PPh3)2 NiCl2(PPh2-2-chloride)2 Chlorophenyl is more reactive than the phenyl group due to the lone pair of electrons supplied by the chloride ion (Caldin, 1964, p. 450), thus the substitution occurs due to the lone pair of electrons in the chlorophenyl group. Substitution of a phenyl group in NiCl2(PPh3)2 with 3-chlorophenyl and 4-chlorophenyl also occur in the same way (Basolo & Pearson, 1986, p. 317). 3-2-chlorophenyl NiCl2(PPh3)2 NiCl2(PPh2-3-chloride)2 4-2-chlorophenyl NiCl2(PPh3)2 NiCl2(PPh2-4-chloride)2 Formation of new complexes from Dibromobis(triphenylphosphine)Nickel (11) Ligand exchange of one of the phenyl groups in Dibromobis(triphenylphosphine)Nickel (11) with either of the given substituent’s (2- and 4- tolyl; and 2- , 3- and 4- chlorophenyl) occurs just like the substitution of the phenyl group in Dichlorobis(triphenylphosphine)Nickel (11) highlighted above (Basolo & Pearson, 1986, p. 350). Exchange of a phenyl group from NiBr2(PPh3)2 with a tolyl group 2-Tolyl group NiBr2(PPh3)2 NiBr2(PPh2-2-tolyl)2 3-Tolyl group NiBr2(PPh3)2 NiBr2(PPh2-3-tolyl)2 4-Tolyl group NiBr2(PPh3)2 NiBr2(PPh2-4-tolyl)2 Exchange of a phenyl group from NiCl2(PPh3)2 with a chlorophenyl group 2-chlorophenyl NiBr2(PPh3)2 NiBr2(PPh2-2-chloride)2 3-2-chlorophenyl NiBr2(PPh3)2 NiBr2(PPh2-3-chloride)2 4-2-chlorophenyl NiBr2(PPh3)2 NiBr2(PPh2-4-chloride)2 Investigation of the structures and geometries of the complex compounds The structures and geometries of the complexes formed were investigated using various methods. The magnetic moments of each of the complexes was measured and calculated. The infra red spectra of the complexes were recorded as nujol mulls. The UV-Visible spectra of the complexes were observed and recorder and finally, the NMR spectra of the complexes were observed. Using the UV-Visible spectra, the complexes are observed and their colours noted. The colour visible for each complex is the contrasting colour of the one absorbed by the complex to raise an electron from a lower level to a higher level. The magnetic moments of the complexes are calculated using the formula μ = [n (n +  B.M. The first row of the transition metal complex has magnetic moments related to the number of unpaired electrons (spin-only formula). By carrying out a comparison of the measured magnetic moment with the spin only calculated value, the structure of the compound can be determined. It can also help in the determination of whether an octahedral complex is a high spin complex or a low spin complex (Smith, 1979, p. 98). IR energy in a spectrum is measured as wave number (cm-), the inverse of wavelength and propotional to frequency. Wavenumber (cm-) = 1/λ (cm) ((Bellamy, 1975, p. 27). The specific IR absorbed by a molecule is directly related to the molecule’s structure. Nujol mulls are used when measuring IR to make sure that they are neat samples. Nujol mulls/neat samples refer to liquid samples without any added solvent as presence of a solvent may dissolve the sample being tested. In the case of a solid sample, the sample is ground and mixed with nujol to form a suspension (Bellamy, 1975, p. 60). Nujol is a brand name of mineral oil that is obtained by distillation of petroleum. It is a mixture of alkanes which have molecular weights that are higher than those of kerosene but lower than paraffin. Using NMR spectroscopy, the chemical shifts observed will be different for each of the groups in each of the complexes. Thus the structure of the complex can be determined (Akitt, 1973, P. 255). Results and discussion Magnetic moments of each of the complexes The compounds gave varying magnetic moment values as follows: Complexes BM Geometry NiBr2(PPh2-2-tolyl)2 4.09 Tetrahedral NiBr2(PPh2-3-tolyl)2 5.15 Tetrahedral NiBr2(PPh2-4-tolyl)2 3.63 Tetrahedral NiBr2(PPh2-2-chloride)2 -1.1 Square planar NiBr2(PPh2-4-chloride)2 4.9 Tetrahedral NiCl2(PPh2-2-tolyl)2 5.4 Tetrahedral NiCl2(PPh2-3-tolyl)2 1.87 Square planar NiCl2(PPh2-4-tolyl)2 3.84 Tetrahedral Discussion The spin only magnetic moment can be calculated using the formula μ = [n (n +  B.M. Ni(II) has a configuration d8 which splits into (t2g6 eg2). The unpaired electrons are the eg2 thus n=2. Therefore; μ = [n (n +  B.M μ = [2 (2 +  B.M μ = [ B.M μ = 2.83 B.M For the Ni(II) d8 configurations the total magnetic moment µT is 4.47 and when the orbital contribution is ignored the spin-only moment µs is 2.83 as calculated above. The square planar Ni (II) complex will be diamagnetic as all spins are paired. The tetrahedral Ni(II) complexes have two unpaired electrons and therefore have the same spin contribution to the magnetic moment. The 3T1 ground state for the tetrahedral complex will give rise to an orbital contribution to the magnetic moment with µ greater than 2.83 BM. The magnetic behaviour of various materials can be differentiated by the temperature dependence of their magnetic susceptibility. The magnetic susceptibility of diamagnetic materials is not altered by temperature. For paramagnetic materials, magnetic susceptibility and temperature are indirectly proportional. This is because of the opposition on the alignment of the magnetic moments by temperature in an applied magnetic field. Infra-red spectra of each of the complexes Complexes IR Spectra (C=C)υ IR Spectra (=C-H)υ NiBr2(PPh3)2 1480 2964 NiBr2(PPh2-2-tolyl)2 1436 2957 NiBr2(PPh2-3-tolyl)2 1434 3054 NiBr2(PPh2-4-tolyl)2 1433 3052 NiBr2(PPh2-2-chloride)2 1433 3056 NiCl2(PPh2-2-tolyl)2 1433 3050 NiCl2(PPh2-3-tolyl)2 1435 2970 NiCl2(PPh2-4-tolyl)2 1433 2975 NiBr2(PPh2-4-chloride)2 1434 2971 Discussion Different functional groups absorb specific wavelengths of Infra-red radiation. The absorbance patterns of Infra-red radiation can reveal the identity of the functional groups in a complex. Different bonds in a complex vibrate at absolute temperature (Hesse, Meier, & Zeeh 2008, p. 71). These bonds absorb the radiation with the frequency as that of their vibration (Bellamy, 1975, p. 79). The chemical bonds present can then be determined from the observation of the vibration patterns. The C=C stretch did not have a significant shift (Smith, 1979, p. 78). This implies that the change from Cl to Br did not shift the IR resonance frequency. In order of confirming the structure of NiCl2(PPh3)2 by IR spectra, P-Ph can easily identified by the absorption around 1440 cm-1 as a sharp band. The =C-C stretch representing the aromatic rings is theoretically expected to have a peak at 3100-3000υ. The experimental figures obtained indicate the presence of aromatic rings (the phenyls and tolyls). The variation in these figures indicates the different aromatic groups present in the various complexes that is the phenyls and the tolyl’s (Bellamy, 1975, p. 77). UV-Visible spectra of each of the complexes Nickel (II) has 8 electrons in its d-orbital (d8) thus when ligands are introduced, the d-orbital splits to the (t2g6 eg2) arrangement. The presence of two unpaired electrons result in the paramagnetic property of the complexes formed. The colour bands displayed were as follows: Complexes Products colour Possible Geometry NiBr2(PPh3)2 Dark green Tetrahedral NiBr2(PPh2-2-tolyl)2 Green Tetrahedral NiBr2(PPh2-3-tolyl)2 Olive green Tetrahedral NiBr2(PPh2-4-tolyl)2 Green Tetrahedral NiBr2(PPh2-4-chloride)2 Dark green Tetrahedral NiCl2(PPh2-2-tolyl)2 Green Tetrahedral NiCl2(PPh2-4-tolyl)2 Olive green Tetrahedral NiBr2(PPh2-2-chloride)2 Brown Squire planar NiCl2(PPh2-3-tolyl)2 Red complex Square planar NiCl2(PPh2-2-chloride)2 No crystals Red solution- green NiCl2(PPh2-4-chloride)2 No crystals Red solution- green Discussion Colours in transition metal complexes are brought about by the excitement of an electron from one d-orbital to another d-orbital of higher energy. The energy required for this transition is absorbed as a colour in the visible spectrum. The colour observed is the complementary colour of the one absorbed. The amount of d-orbital splitting is dependent on the type of ligand bound to the transition metal and thus the colour observed represents the different splitting energies of different ligands. The value of splitting of the crystal field determines whether the electrons in the d-orbital of the metal ion pair up or obey the Hund’s rule. The degree of splitting is dependent on the type of ligand bound to the transition metal (Ballhausen, 1962, P. 261). For the tetrahedral geometry, the d-orbitals dx2-y and dz2 are further displaced from the ligands than the orbitals dxy, dxz and dyz. Therefore, the eg orbitals in tetrahedral complexes correspond to a lower energy value while the t2g orbitals are characterized by higher energy. For a square-planar arrangement, it is observed that d(x2–y2) will be positioned above the other orbitals. The effect of this is that the complex possesses no unpaired spins and there is pairing up of all the eight electrons in the low energy d-orbitals. It is complicated to describe spectra in relation to the electronic transitions. This is due to the fact that the electrons are delocalized and positioned on the ligands which have high affinities for electrons. The square-planar arrangement is only possible with strong-field ligation (Ballhausen, 1969, P. 210). The highest value of energy transferred occurs at wavelengths of 500-700 nm, which lies in the middle of the visible region of the spectrum. This represents the tetrahedral complexes. For green colour to be observed, red colour must have been absorbed. Red has energy of 650nm which is the highest of all the colours in the visible spectrum. This is an indication of a tetrahedral geometry. When red colour is observed, blue-green colour is absorbed which indicates an absorption of 490nm. This indicates a square planar complex. Tetrahedral complexes were determined as NiBr2(PPh3)2, NiBr2(PPh2-2-tolyl)2, NiBr2(PPh2-3-tolyl)2, NiBr2(PPh2-4-tolyl)2, NiBr2(PPh2-4-chloride)2, NiCl2(PPh2-2-tolyl)2 and NiCl2(PPh2-4-tolyl)2 since they were all green complexes. The spectra of the square-planar geometries show high values of energy transfer in the blue/near-UV region of the spectrum with wavelengths of 300 to 450 nm. This is exhibited in the UV-Visible spectra of NiBr2(PPh2-2-chloride)2 and NiCl2(PPh2-3-tolyl)2 ((Hesse, Meier, & Zeeh 2008, p. 39). The complexes that exhibit a square planar- tetrahedral equilibrium are affected by the change in temperature. The last two products in this experiment [NiCl2(PPh2-2-chloride)2 and NiCl2(PPh2-4-chloride)2] exhibit this characteristic. The solution at high temperatures is red (indicating the square planar geometry) but when it cools, it turns to green (indicating the tetrahedral geometry). These two complexes were unstable. NMR spectra of each of the complexes 1H NMR NiBr2(PPh2 -2-tolyl)2- 7.8-7.1 (1H, m), 6.9 (1H, t), 6.8 (1H, t) NiBr2(PPh2 -3-tolyl)2- 7.3 (1H, m), 7.1 (1H, t), 6.8 (1H, d) NiBr2(PPh2 -4-tolyl)2- 7.8 (1H, t), 6.9 (1H, t), 6.8 (1H, t) NiBr2(PPh2 -2-chloride )2- 7.3 (1H, m), 7.1 (1H, t), 6.8 (1H, d) NiCl2(PPh2 -2-tolyl)2- 7.4 (1H, t), 7.1 (1H, t), 6.8 (1H, t) NiCl2(PPh2 -3-tolyl)2- 7.4 (1H, t), 6.8 (1H, t) NiCl2(PPh2 -4-tolyl)2- 7.3 (1H, m), 6.9 (1H, t), 6.6(1H, d) Discussion In NMR spectroscopy, there is a spinning charge that is responsible for the generation of a magnetic field. In NMR spectroscopy, the complex is exposed to external magnetic field (Bo). In the presence of this field, there exists two spin states (; one is aligned to the external magnetic field while the other is opposed to it. The nuclei of the hydrogen atoms present in the structural formula of the complex spin about an axis (Akitt, 1973, P. 508). Since these nuclei are positively charged, this spin is associated with circulation of electric charge, circulating charges result in magnetic fields so the spinning hydrogen nucleus has a magnetic moment (Roberts, 1959, p. 129). When placed in an external magnetic field, the nuclei tend to realign themselves into preferred positions whereby the nuclear magnet is aligned with the external field. Another less favored position occurs when the nuclear magnet is aligned against the external magnetic field. These are the only allowed orientations for nuclei to follow, according to the laws of quantum mechanics. The energy difference between the two spin states is highly dependent on the external magnetic field. The location of NMR signals is dependent on the external magnetic field strength and the frequency. Theoretical NMR figures indicate that the NMR spectra for aromatics should be between 6.0-8.0 ppm. The results obtained were all well within this range thus indicating the presence of aromatic ligands in the complexes (Hesse, Meier, & Zeeh 2008, p. 77). Conclusion The synthesis of new complexes by the exchange of ligands (one phenyl with a tolyl and a chloro-phenyl) was successful. This is evident because all the tests done to determine the structures of the new complexes were successful except for two complexes, NiCl2(PPh2-2-chloride)2 and NiCl2(PPh2-4-chloride)2 which were characterized by high solubility thereby rendering it impossible to perform the tests for structural confirmation. The UV-Visible spectra of these two complexes showed that they were in a geometrical equilibrium which was dependent on temperature. At high temperatures, the structure of the complex was square planar while at low temperatures, the complex attained the tetrahedral structure. The successful production of these complexes further shows the potency of ligand exchange in transition metal complexes. All the four methods applied to determine the structure of the complexes were successful as well. The UV-Visible spectra displayed an array of colours that were correspondent with the anticipated structure of the complex (Hesse, Meier, & Zeeh 2008, p. 59). Most of the complexes were green in colour meaning they absorbed red colour. This implies that they were of tetrahedral geometry. To confirm these findings, the magnetic moments results produced the same inference where most of the complexes formed were tetrahedral and only three had the square planar geometry. The similarity of results from these two methods confirms the accuracy of these methods for the determination of complex compounds geometries. The IR and NMR spectra gave conclusive information on the identities of the ligands attached to the central ion. In conclusion, new complexes can be successfully produced from dibromobis(triphenylphosphine) nickel(II) and dichlorobis(triphenylphosphine) nickel(II) by exchanging one of the phenyl groups with another group. The structures and geometries of complex structures can be determined by the use of UV-Visible spectra, IR, NMR spectra and the calculation of magnetic moments. Future works will entail the synthesis of nickelocene with NiBr2L2. References Akitt, J., (1973). NMR and Chemistry; An Introduction to Nuclear Magnetic Resonance Spectroscopy. Chapman and Hall, London. Angelici, R.J., (1977). Synthesis and Technique in Inorganic Chemistry. 1st ed., W.B. Saunders Company, Philadelphia. Ballhausen, C.J., (1962). Introduction to Ligand Field Theory. Mcgraw Hill Book Co., New York. Basolo, F. & Pearson, R.G., (1986). Mechanism of Inorganic Reaction. 2nd ed., John Wiley, New York. Bellamy, L.J., (1975). The Infrared Spectra of Complex Molecules. John Wiley, New York. Belluco, U., (1965). Mechanistic and Structural Aspects of Coordination Chemistry. Bressanone, Italy. Brammer, L. & Stevens, E. D. (1989). Structure of Dichlorobis(triphenylphosphine) nickel(II). Acta Cryst. C45. Caldin, E.F., (1964). Fast Reactions in Solution. Blackwell Scientific calculations, Oxford. Corain, B., Longato, B., Angeletti, R., & Valle, G. (1985). Trans [diclorobis(triphenylphosphine)nickel(II)]·(C2H4Cl2)2: a Clathrate of the Allogon of Venanzi’s Tetrahedral Complexes. Inorg. Chim. Acta, 104. Errington, R. J. (1997). Advanced Practical Inorganic and Metalorganic Chemistry. 1st ed, London. Garner, C.S. & Macdonald, D.J., (1962). Advances in Coordination Compounds. Macmillan, New York. Garton, G., Henn, D. E., Powell H. M., & Venanzi, L. M. (1963). Tetrahedral Nickel(II) Complexes and the Factors Determining Their Formation. Part V. The Tetrahedral Co- ordination of Nickel in Dichlorobistriphenylphosphinenickel. J. Chem. Soc. Chem. Comm.UN. Hesse, M., Meier, H. and Zeeh, B. (2008). Spectroscopic methods in organic chemistry. 2nd edition, Thieme. Porri, L., M. C. Gallazzi, and G. Vitulli. (1967). Complexes of Nickel(II) with Triphenylphosphine. Chemical Communications (London) 5. Roberts, J.D., (1959). Nuclear Magnetic Resonance: Applications to Organic Chemistry, McGraw-Hill Co., New York. Sletten, J. & Kovacs, J. A. (1993). Structure of trans-[dichlorobis(triphenylphosphine) Nickel(II)]·2CH2Cl2. J. Crystallogr. Spectrosc. Res., 23. Smith, A.L., (1979). Applied Infrared Spectroscopy, Fundamentals, Techniques, and Analytical Problem Solving. John Wiley, New York. Steed, J. W. & Atwood, J. L. (2000). Supramolecular Chemistry. John Wiley & Sons, England. Venanzi, M. L. (1958). Tetrahedral Nickel(II) Complexes and the Factors Determining Their Formation Part I. Bistriphenylphosphine Nickel(II) Compounds. J. Chem. Soc. Xiao, S-X., Trogler, W. C., Ellis, D. E. & Berkovich-Yellin, Z. (1983). Theoretical Study of the Frontier Orbitals of Metal-Phosphine Complexes. J. 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The number of complexes of the type [Ni(PR3)2X2] in solution that show a square planar to tetrahedral equilibrium are limited as the energy difference between the two geometries is very little. The more bulky the ligand bound to the nickel (II), the more favourable the tetrahedral geometry becomes. This is because the geometry of nickel (II) complexes relies on the steric and electronic effects that are produced by the ligands. The type of ligand bound to the nickel also plays a part in the determination of the geometry of the complex.

Strong field ligands form low spin complexes while medium-weak field ligands form high spin complexes. High spin complexes tend to form square planar complexes (Ballhausen, 1962, P. 210). One of the nickel triphenylphosphine complexes, (NiCl2(PPh3)2) is a four coordinate Ni(II) complex. It exists as an equilibrium mixture of tetrahedral and square planar geometries. There are other five crystals that have this same characteristic. Some other NiCl2(PPh3)2 complexes display square planar geometry.

They embody solvent molecules in their structures. The solvent molecules do not form a coordinate with the nickel complex but rather they solvate next to the molecules of the complex. This has a stabilizing effect on the complex hence the square planar geometry is achieved due to weak dipole interactions (Ballhausen, 1962, P. 217). Methods of Determining the Structure and Geometry of Nickel Triphenylphosphine Complexes. Every chemist at one point or another encounters the need to determine the structure of a compound.

There are various methods that can be used to determine the structures and geometries of complexes such as the Nickel Triphenylphosphine complexes we are focusing on. UV-Visible spectroscopy and the calculation of magnetic moments are methods used to determine the geometry of complexes while NMR spectroscopy and Infra-Red spectroscopy are used to determine the structures of complexes. When determining the structure of a complex, it is advisable to carry out many different tests on the compound then consolidate the results because each test on its own may not be conclusive on the structure of the complex.

UV-Visible Spectroscopy UV-Visible spectroscopy has been used for a long time in the determination of the coordinate geometries of transition metal complexes. In the UV-Visible spectrum, observation of colour bands is used to determine the geometry of the complex. The bands are caused by the movement of valance electrons between different energy levels. When a free metal ion interacts with ligands, the ligands locate themselves in the position of minimal electrostatic interaction thus the electrons of the transition metal ion occupy the orbitals with the least ligand charge.

Such interactions occurring at the d-orbitals resulting in their splitting due to a ligand field can be observed in the visible region of the electromagnetic spectra. This is observed as different colours of the complexes. The presence of a ligand field tend to split the degenerate d-orbitals in to the eg and the t2g sets. The transition of an electron from a lower level (t2g) to a higher level (eg) is attained by the absorption of visible light. When a complex absorbs a colour from the visible region of the electromagnetic spectrum, we observe the compliment of the absorbed colour.

It is clear that the geometrical arrangement of the complex determines the amount of energy required to elevate an electron to a higher level thus displaying the contrasting colour. The type of ligands bound to the transition metal also play a part in this because ligands of a stronger field have larger energy values (Δ) as compared to ligands of a weaker field. Magnetic Moment (μ) Most of the existing transition metal coordination compounds are paramagnetic. This means that the molecule has a permanent magnetic dipole and bulk samples of the material are attracted into the magnetic field.

The magnetic field μ of transition metal coordination compounds is the outcome of the presence of unpaired electrons in the d-orbitals of the compound.

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