This particular set of experiment was aimed at preparing nickel NI complexes with different phosphine ligands and then studying the synthesized ligands using magnetic and spectral analysis techniques. Furthermore, the experiment is purposed at running spectroscopic scan on the metal complexes prepared to predict the stereochemistry or the arrangement around the nickel atom in the synthesized metal complexes. The analysis method used in this experiment include infra red, UV-Vis. From magnetic susceptibility value obtained, the magnetic moment of the transition metal complexes are to be obtained after synthesizing the complexes.
Nickel phosphine complexes under the influence of a catalyst find wide application in selective hydrogenation, hydroboration, and isomerization. The coordination of nickel (II) phosphine complexes is sensitive to alterations in the ligand set. The geometry of the complex formed is particularly for a four-coordinate nickel complex depends on the steric hindrance as well as electronic effect of the neighboring electrons of the combining molecules. As the phosphine ligands accumulate and become bulky, for instance tricyclohexylphosphine will shift the equilibrium in favor of the tetrahedral shape, whereas reducing the bulk makes the square planar shape preferred (Tamaru 15).
It is worthy noting that phosphine ligand, which is π-acceptor ligand when they bond with, nickel, they have the greatest capability of splitting d-orbitals. When nickel and phosphine form a bond, the distance of the bond formed is comparatively larger as compared to that of bromide and chloride and therefore these complexes will fall below that of bromide and chloride on spectrochemical series. Nickel donates to phosphine ligands from its d-orbitals through the vacant d-orbitals on the phosphorus atom.
Complexes of bromine/ chloride are more stable as compared nickel phosphine complexes. The stability of these nickel (II) bromide or chloride complexes can be credited to strong σ-bond electron donating nature of the 1-norbonyl group and chloride/bromide anion which offers the required electron density for making the nickel complex stable. Contrary to organic nickel (II) complexes such as NiBr2 (PPh3)2, which have a propensity of forming a square planar complexes only, inorganic nickel (II) complexes form different configurations. An example is [NiCl4]2- which forms tetrahedral configuration due to electronic repulsion. Therefore, nickel complexes that include chloride and bromide are more stable as compared to complexes that have phosphine, NiBr2 (4anysil) 2 among other similar complexes (Nicholls 51).
Methods of Determining the Structure of Nickel Complexes
Crucial factors to be considered when analyzing the complexes of nickel and particular in this experiment where were are dealing with phosphine ligands is the UV/visible spectra (absorbance at particular wavelength range) and magnetic properties. Whereas magnetic properties of the complex is the dependent on the differences in numbers of lone pairs of electron, UV/Vis and IR spectra of these complexes is determined by dipole-dipole interaction between nickel and the ligands.
UV/Visible spectroscopy technique has proved to be the best analytical tool in determining the geometric coordination of complexes around transition metal ions. The technique relies on the position, number, and intensity of the dipole-dipole interaction to elucidate the structure of the compound. The movement of either excited or those that have lost energy between energy levels of the nickel complexes in particular wavelength zone is the cause for specific UV/Visible spectra. Interaction between the d-orbitals of the ligand and transition metals are normally observed in the visible part of the transition metal. The interaction of d-orbitals and the resulting movement in the orbital is cause for the color of complexes formed.
When the complexes are exposed to electromagnetic radiation, the electrons will take in energy that will excite it to higher energy orbital, which is in the neighborhood or in the area of visible electromagnetic spectrum. The energy required in orbital splitting (Δo) when the atom is bonded to a ligand and when there is no ligand is same. From this, it is clear that geometric configuration of the nickel complexes is the determinant in the location of the peak in the spectrum. The degree to which the orbitals is split which in turn affects UV/Visible absorption is dependent on the geometrical arrangement of the ligands and nickel (Angelici 43).
Another method used in elucidating the structure of the ligand formed is nuclear magnetic resonance (NMR). The nuclei of hydrogen atom that is positively charged present in the nickel complexes spin about an axis. The spin of this proton is associated with electric charge circulation and it is this circulation that leads magnetic field spinning hydrogen atom leading to generation of magnetic moment. When this spinning hydrogen nuclei is put in an exterior magnetic field, they arrange themselves in the direction of the external field(Nicholls 56). The effect of nuclear magnetic resonance becomes evident when nuclei aligned in the direction of external magnetic field takes in energy and changes in direction of alignment in accordance to the applied external field. This is then related to Planck’s equation, which predicts that the difference in energy between two spin states is related to the frequency of the electromagnetic spectrum.
Angelici, Robert. Synthesis and technique in inorganic chemistry. Philadelphia: Saunders Company, 1977.Print.
Nicholls, David. Complexes and first row transition metals. London: Macmillan, 1974.
Tamaru, Yoshinao. Modern organonickel chemistry. Wenham: WILEY-VCH Verlag, 2005.