29. Transition Metals: Crystal Field Theory Part II by MIT OpenCourseWare

Summary by www.lecturesummary.com: 29. Transition Metals: Crystal Field Theory Part II by MIT OpenCourseWare

• • • Preface: Colour, Magnetism, Geometry, and Crystal Field Theory

      Discussions of colour, magnetism, and various geometries in transition metal complexes are continued in the lecture.

      The splitting energy of the d-orbitals in octahedral geometry depends on the kind of ligand (weak, intermediate, or strong field), but the geometry itself also influences this. Different splitting patterns result from different geometries.

      Nickel Chloride Demo: Ligand Strength & Initial Colour

      • The first compound of nickel chloride is greenish.
      • A greenish hue indicates that it absorbs the red complementary colour.
      • The wavelength of red light is long; a short frequency and little energy are associated with a long wavelength.
      • The nickel chloride complex absorbs a low energy, suggesting that the d-orbitals have a small splitting energy.
      • A weak field ligand is chloride (Cl-), which does not strongly split the d-orbitals, consistent with a small splitting.

      - Adding Water

      • The greenish nickel chloride compound becomes slightly more blue-green when water is added.
      • Orange-red light absorption is indicated by a blue-green hue.
      • The wavelength of orange-red light is shorter than that of red light.
      • Higher frequency and greater energy are associated with shorter wavelengths.
      • The blue-green solution absorbs more energy than the nickel chloride complex, suggesting a higher splitting energy.
      • Water is stronger than chloride and is referred to as an intermediate field ligand. The observed colour shift is consistent with a larger splitting caused by the stronger ligand.

      - Including EDTA

      • The solution gets even more blue when EDTA, a chelating agent, is added.
      • When a colour is more blue, it absorbs an even more orange colour.
      • This corresponds to an even shorter wavelength.
      • An even greater energy and, consequently, an even greater splitting energy are implied by an even shorter wavelength.
      • EDTA is predicted to be an even stronger ligand than water based on this observation of a larger splitting.
      • EDTA is a chelating agent that displaces other ligands, such as six water molecules, by binding to the metal at multiple points (six points of attachment).

      - Including Ammonia + Another Chelating Agent (DMG)

      • The nickel water complex (blue-green) turns red when another chelating agent (DMG, later assisted by ammonia) is added.
      • Red indicates that it absorbs green, which is its complementary colour.
      • The wavelength of green light is even shorter than that of orange light.
      • A very large energy is associated with this very short wavelength, suggesting a very large splitting energy.
      • This particular system (with DMG) is a square planar system rather than an octahedral one, implying a fairly large splitting of d-orbital energies.

      Demo Visual Confirmation

      • The first green nickel chloride complex is displayed.
      • The anticipated blue-green colour is produced by adding water.
      • One solution is mixed with EDTA, a powder that needs a base to dissolve.
      • This solution becomes somewhat more blue than the water complex.
      • The second solution, which has an unpleasant odour, is mixed with dimethylglyoxime (DMG). Since DMG finds it difficult to directly replace water, there isn't much of a colour change at first.
      • After that, ammonia is added, which aids in the displacement of water so that DMG can take its place. A change in colour is seen locally.
      • The solution eventually turns completely red when there is enough ammonia present. An equilibrium is involved in this reaction.

      Geometry's Effect on Splitting

      • The red colour created in the demonstration shows that the geometry of the complex is just as important as a ligand's strength or weakness.
      • We'll talk more about the red color's suggestion of a significant splitting in the square planar system.

      Tetrahedral Geometry

      • A model and drawing are used to describe a tetrahedral system. In tetrahedral geometry, ligands are oriented off-axis with respect to the coordinate frame.
      • The off-axis d-orbitals (dyz, dxy, and dxz) in this geometry are more affected and repelled by the ligands than the on-axis orbitals (dx^2-y^2, dz^2).

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        The tetrahedral geometry splitting diagram displays the off-axis orbitals (dyz, dxy, dxz, now called T2 set) as having higher energy (destabilized) and the on-axis orbitals (dx²-y², dz², now called E set) as having lower energy (stabilized). Both the T2 and E orbitals are degenerate with one another.

        Characteristics of Tetrahedral Splitting

        The fact that tetrahedral splitting is generally much smaller than octahedral splitting (Δ_t < Δ_o) is a very important characteristic. The reason for this is that none of the d-orbitals are directly pointed at by the ligands.

        Electron Pairing in Tetrahedral Complexes

        Because electrons have a small splitting energy, their pairing energy is always higher than the energy needed to move them up to a higher orbital level. Since electrons will fill singly before pairing, all tetrahedral complexes are high spin.

        Energy Distribution in Tetrahedral Complexes

        Overall energy is maintained:

        • 3/5 of the splitting energy (-3/5) stabilizes the two E orbitals
        • 2/5 (+2/5) destabilizes the three T2 orbitals
        • The octahedral energy distribution is in opposition to this

        Example: Chromium 3+ in Tetrahedral Complex

        The d³ electron count of chromium 3+ (Cr³⁺) is used as an example. The three electrons will be positioned singly in the lowest energy levels for a d³ system in a tetrahedral (high spin) complex. According to the diagram, this indicates that one electron enters the T2 level and two enter the E level.

        Electron Configuration Notation

        E² T₂¹ is the notation for the d electron configuration. Three unpaired electrons are present in such a complex (E² T₂¹).

        Absorption Characteristics

        A tetrahedral Cr³⁺ complex with chloride is demonstrated, with a wavelength of light that is most intensely absorbed at 740 nm. The red region is at 740 nm. Green, the complementary color, is the complex's anticipated color. As would be expected for a tetrahedral complex with a weak field ligand like chloride, green is linked to a shorter wavelength than red, and the absorption of a long wavelength (red) is consistent with a very tiny splitting energy.

        Square Planar Geometry

        The square planar system is defined by having ligands on the x and y axes (on-axis) and nothing on the z-axis.

        Destabilization of Orbitals

        • The d-orbital most destabilized by repulsion from the ligands is dx²-y².
        • Dxy is the next most destabilized orbital; it is in the xy plane, but its lobes are off-axis (45 degrees away).
        • Since there are no ligands along the z-axis, the dz² orbital is less repelled than the dx²-y² orbital.
        • The orbitals with the least amount of repulsion, dyz and dxz, are typically the most stable.

        Energy Splitting in Square Planar Complexes

        The square planar splitting diagram indicates that dx²-y² has the highest energy, followed by dxy. Dz² and dyz/dxz are below dxy; they are frequently listed with dz² above dyz/dxz, though the precise arrangement of the lower orbitals can change.

        In general, the energy splitting in square planar complexes can be very large, especially the gap between the lowest or next highest orbitals (dxy) and the highest orbital (dx²-y²). This is consistent with the large splitting suggested by the red color (absorbing green/very short wavelength) in the demonstration and necessitates a large photon energy to cause a transition to the highest level.

        Square Pyramidal Geometry (Short)

        A square planar geometry with an extra ligand along the z-axis is described as a square pyramidal case.

        Destabilization in Square Pyramidal Geometry

        The presence of a ligand along the z-axis in square pyramidal geometry would destabilize the dz² orbital in comparison to the square planar case. Moreover, orbitals with z components (dyz, dxz) would not be degenerate and would be destabilized.

        Examples of Nickel Enzymes in Biology

        Importance of Geometry and Transition Metals: Examples from biology highlight the significance of geometry and transition metals.

        Role of Nickel Enzymes:

        • Buffering System: By forming a buffering system, nickel enzymes help H. pylori bacteria endure the stomach's low pH. This is crucial because stomach acid destroys antibiotics, making treatment of H. pylori infections challenging.
        • Gas Transformation: Nickel-dependent enzymes in microbes transform carbon monoxide (CO) and carbon dioxide (CO2) into acetate, significantly contributing to the removal of these gases from the environment. Millions of tonnes of CO are thought to be removed each year, while trillions of kilograms of acetate are produced.
        • Research Interests: Using microbes to turn CO2 into biofuels or creating small molecule catalysts based on these nickel centers are two areas of research interest.
        • Spectroscopy Applications: Spectroscopy can predict the geometry of the metal center in an enzyme in the absence of a crystal structure, particularly by determining whether a metal center is paramagnetic or diamagnetic.

        Determining Geometry with Magnetism (d8 Ni2+)

        • Diamagnetic Observation: Spectroscopy reveals that a Nickel +2 (Ni2+) d8 system is diamagnetic. This raises the question: Is it possible to rule out common geometries based on this observation?
        • Octahedral d8 System: Regardless of the size of the splitting, the electron configuration for an Octahedral d8 system is paramagnetic, indicating the presence of unpaired electrons. (Note: The configuration/magnetism in this instance is unaffected by the size of the splitting.)
        • Square Planar d8 System: All electrons in a Square Planar d8 system are paired when the eight electrons are inserted into the lower energy orbitals of the square planar splitting diagram. Thus, a square planar d8 complex is Diamagnetic, aligning with the nickel enzyme's spectroscopic observations.
        • Tetrahedral d8 System: Due to the small splitting, a Tetrahedral d8 system is always high spin. Unpaired electrons are found in the T2 level when the eight electrons are filled in high-spin fashion into the tetrahedral splitting diagram (E lower, T2 higher). Consequently, a tetrahedral d8 complex is Paramagnetic.

        Ni2+ Geometry Conclusion

        • Square Planar Consistency: The finding that the Ni2+ (d8) center is diamagnetic is consistent only with a Square Planar system when comparing the common geometries.
        • Accurate Prediction: This prediction regarding the square planar geometry was accurate, as depicted in the enzyme catalyst's square planar nickel complex.