Coordinate Compounds - High and Low Spin Complex (Intro)

• Coordination compounds are formed by the reaction of a metal ion and one or more ligands.
• Ligands are molecules or ions that have lone pairs of electrons and can donate these electron pairs to form coordinate bonds with the metal ion.
• In coordination compounds, the metal ion is usually a transition metal, which has partially filled d orbitals.
• The arrangement of electrons in the d orbitals determines whether a coordination compound is a high spin or low spin complex.
• Let’s explore the concepts of high spin and low spin complexes in more detail.

High Spin Complexes

• High spin complexes are coordination compounds where the electrons occupy the available d orbitals in such a way that they have maximum spin.
• In high spin complexes, the electrons fill up the d orbitals in ascending order according to Hund’s rule, which states that electrons prefer to occupy separate orbitals before pairing up.
• Example: [Fe(H2O)6]2+
  • In this complex, iron(II) ion (Fe2+) is surrounded by six water molecules as ligands.
  • The electrons fill up the five d orbitals with parallel spins before pairing up, resulting in high spin.

Low Spin Complexes

• Low spin complexes are coordination compounds where the electrons occupy the available d orbitals in such a way that they have minimum spin.
• In low spin complexes, the electrons pair up in the d orbitals before filling the next available orbital, according to Hund’s rule.
• Example: [Fe(CN)6]4-
  • In this complex, iron(II) ion (Fe2+) is surrounded by six cyanide ions (CN-) as ligands.
  • The electrons fill up the d orbitals with parallel spins until all the orbitals are half-filled, resulting in low spin.

Factors Affecting High or Low Spin Complex Formation

• The factors that influence whether a coordination compound will be high spin or low spin are:
  1. Nature of the metal ion: The size and charge of the metal ion affect the energy gap between different d orbitals.
  2. Nature of the ligands: Ligands with strong field strength lead to a larger energy gap between different d orbitals, favoring low spin.
  3. Magnitude of crystal field splitting: The crystal field splitting energy, Δ, determines the energy difference between the d orbitals.

Crystal Field Splitting

• Crystal field splitting refers to the energy difference between the d orbitals in a coordination compound.
• Ligands create an electrostatic field around the metal ion, which splits the d orbitals into two sets of different energies.
• The energy difference between these sets of orbitals is denoted by Δ (delta).
• The magnitude of Δ determines whether the complex will be high spin or low spin.

Strong Field Ligands

• Strong field ligands are ligands that create a large crystal field splitting energy (Δ).
• These ligands usually have lone pairs of electrons available for donation to the metal ion.
• Example: CN-, CO, NO2-
• Strong field ligands favor low spin complexes since the energy required to pair up the electrons in the d orbitals is lower than the energy required to occupy the higher energy orbitals.

Weak Field Ligands

• Weak field ligands are ligands that create a small crystal field splitting energy (Δ).
• These ligands do not have lone pairs of electrons available for donation to the metal ion.
• Example: H2O, NH3, Cl-
• Weak field ligands favor high spin complexes since the energy required to pair up the electrons in the d orbitals is higher than the energy required to occupy the higher energy orbitals.

Factors Affecting Crystal Field Splitting of Ligands

• The factors that affect the crystal field splitting of ligands are:
  1. Charge: Ligands with a higher charge create a larger crystal field splitting energy.
  2. Electronegativity: Ligands with higher electronegativity create a larger crystal field splitting energy.
  3. Size: Ligands with a larger size create a smaller crystal field splitting energy.
  4. Shape: Ligands that are more compact or symmetrical create a smaller crystal field splitting energy.
  5. Multiple ligands: Coordination compounds with more than one ligand can have different crystal field splitting energies depending on their arrangement.

11. Factors Affecting Crystal Field Splitting of Ligands (Contd.):

• Coordination number: Ligands in a coordination compound can have different coordination numbers, which refers to the number of ligands surrounding the central metal ion.
• Example: [Co(NH3)6]3+ vs. [CoCl6]3+
  • In [Co(NH3)6]3+, the ammonia ligands have a smaller coordination number than the chloride ligands in [CoCl6]3+.
  • The smaller coordination number in [Co(NH3)6]3+ leads to a larger crystal field splitting energy.

12. Factors Affecting Crystal Field Splitting of Ligands (Contd.):

• Organic ligands: Organic ligands, such as acetylacetonate (acac), EDTA, or porphyrins, can have different electronic structures compared to inorganic ligands.
• Chelating effect: Ligands that contain multiple donor atoms and can form multiple coordinate bonds to a metal ion are known as chelating ligands.
• Example: Ethylenediaminetetraacetate (EDTA) can form a complex with a metal ion by donating two pairs of electrons through its four carboxylate groups.
• Chelating ligands increase the crystal field splitting energy due to the increased number of coordinate bonds.

13. Crystal Field Splitting Energy Diagram: t₂g eg ↑ ↑ ↑ ↑ ▲ < Δ (energy gap) ↑ ↑ ↑ ↑ e_g t_2g

• In a crystal field splitting energy diagram, the energy levels of the d orbitals are shown.
• The lower energy set of d orbitals is labeled as t₂g, and the higher energy set of d orbitals is labeled as eg.
• The size of the energy gap (Δ) determines whether the complex will be high spin or low spin.

14. Spectrochemical Series:

• The spectrochemical series is a list of ligands arranged in order of increasing crystal field splitting energy (Δ).
• The series is used to predict the crystal field splitting energies of different ligands and determine whether a complex will be high spin or low spin.
• Example of spectrochemical series (in increasing order of Δ): I- < Br- < S2- < SCN- < Cl- < NO3- < N3- < F- < OH- < C2O42- < H2O < NCS- < CH3CN < py < NH3 < en < bipy < phen < NO2- < PPh3

15. Jahn-Teller Effect:

• The Jahn-Teller effect is the distortion of a coordination complex from a symmetric shape due to an uneven distribution of electrons.
• The effect occurs when a coordination complex has degenerate orbitals (orbitals with the same energy).
• The degeneracy is lifted by the distortion, resulting in a lower energy state.
• The Jahn-Teller effect is more pronounced in complexes with an odd number of electrons.

16. Ligand Field Theory:

• Ligand field theory (LFT) is a model that combines molecular orbital theory with crystal field theory to describe the electronic structure and properties of coordination compounds.
• LFT takes into account the interactions between the metal ion and the ligands, including the overlap of atomic orbitals and the formation of molecular orbitals.
• The theory provides a framework for understanding the color, magnetism, and reactivity of coordination compounds.

17. Color of Coordination Compounds:

• The color of a coordination compound is due to the absorption of certain wavelengths of light by the complex.
• The absorption of light corresponds to the energy difference between the ground state and an excited state of the complex.
• The energy difference is related to the crystal field splitting energy (Δ) and depends on the nature of the ligands and the metal ion.
• Example: The blue color of [Cu(H2O)6]2+ is due to the absorption of orange/yellow light.

18. Magnetic Properties of Coordination Compounds:

• Coordination compounds can exhibit different magnetic properties, depending on the number of unpaired electrons in the complex.
• Paramagnetic complexes have unpaired electrons and are attracted to a magnetic field.
• Diamagnetic complexes have all paired electrons and are not attracted to a magnetic field.
• The number of unpaired electrons can be determined by considering the crystal field splitting of the complex.

19. Reactivity of Coordination Compounds:

• The reactivity of coordination compounds is influenced by the nature of the ligands and the metal ion.
• Ligands can donate or accept electrons to form coordinate bonds, leading to the formation of new complexes.
• The coordination number, oxidation state, and electronic configuration of the metal ion also play a role in determining the reactivity.
• Coordination compounds can participate in various reactions, including redox, substitution, and isomerization reactions.

20. Applications of Coordination Compounds:

• Coordination compounds have various applications in different fields, including:
  • Catalysis: Some coordination compounds act as catalysts in chemical reactions, speeding up the reaction rate without being consumed.
  • Medicine: Certain coordination compounds are used as anticancer drugs, imaging agents, or contrast agents in medical imaging techniques.
  • Industrial processes: Coordination compounds are used in industrial processes, such as water treatment, dye synthesis, and metal extraction.
  • Material science: Coordination compounds are used to create materials with specific properties, such as magnets, conductors, or superconductors.

21. Coordination Isomerism:

• Coordination isomerism is a type of isomerism exhibited by certain coordination compounds.
• In coordination isomerism, both the ligands and the metal ion are different in the isomers.
• Example: [Co(NH3)5(NO2)]Br2 vs. [Co(NO2)5(NH3)]Br2
  • In the first isomer, NH3 is the ligand while NO2 is the anion, whereas in the second isomer, NO2 is the ligand while NH3 is the anion.

22. Linkage Isomerism:

• Linkage isomerism is a type of isomerism observed in coordination compounds where the ligands can coordinate to the metal ion through different atoms.
• In linkage isomerism, the ligands are the same, but their coordination mode differs.
• Example: [Co(NO2)5(NH3)]Cl vs. [Co(NO2)5(NH2)]Cl
  • In the first isomer, NH3 coordinates to the metal ion through its nitrogen atom, while in the second isomer, NH2 coordinates through its nitrogen atom.

23. Ionization Isomerism:

• Ionization isomerism is a type of isomerism found in coordination compounds in which anionic and neutral ligands can interchange their position by ionization.
• In ionization isomerism, the ligands may be the same, but their position in the complex changes due to ionization.
• Example: [Cr(NH3)5Cl]Cl2 vs. [Cr(NH2)5Cl]Cl2
  • In the first isomer, NH3 is neutral and Cl is anionic, while in the second isomer, NH2 is neutral and Cl is anionic.

24. Hydrate Isomerism:

• Hydrate isomerism is a type of isomerism observed in coordination compounds where water molecules can either be coordinated to the metal ion or present as non-coordinated solvent molecules.
• In hydrate isomerism, the number of water molecules coordinated to the metal ion differs.
• Example: [Cr(H2O)6]Cl3 vs. [Cr(H2O)5Cl]Cl2(H2O)
  • In the first isomer, all six water molecules are coordinated to the metal ion, while in the second isomer, only five water molecules are coordinated, and one remains as a non-coordinated solvent molecule.

25. Stereoisomerism:

• Stereoisomerism is a type of isomerism observed in coordination compounds, where isomers have the same connectivity of atoms but differ in the spatial arrangement.
• In stereoisomerism, the arrangement of ligands around the central metal ion differs.
• Two common types of stereoisomerism are geometric isomerism and optical isomerism.

26. Geometric Isomerism:

• Geometric isomerism is a type of stereoisomerism in coordination compounds in which isomers have different spatial arrangement due to restricted rotation around a coordination bond.
• In geometric isomerism, the ligands can be arranged in a cis or trans configuration.
• Example: [PtCl2(NH3)2] vs. [Pt(NH3)2Cl2]
  • In the first isomer, the chloride ligands are cis to each other, while in the second isomer, they are trans to each other.

27. Optical Isomerism:

• Optical isomerism is a type of stereoisomerism observed in coordination compounds in which isomers have different spatial arrangements that are non-superimposable mirror images of each other.
• In optical isomerism, the coordination compound must possess an asymmetric carbon atom or a chiral center.
• Example: [Co(en)3]Cl3
  • In this complex, each of the three en (ethylenediamine) ligands can have two possible arrangements, resulting in two optical isomers.

28. Chiral Complexes:

• Chiral complexes are coordination compounds that exhibit optical isomerism due to the presence of an asymmetric carbon atom or a chiral center.
• Chiral complexes are optically active and rotate the plane of polarized light.
• These complexes have non-superimposable mirror images (enantiomers) that cannot be interconverted without breaking any bonds.
• Example: [Pt(NH3)2Cl2L]+ (L = any chiral ligand)
  • This complex exhibits chiral properties due to the presence of a chiral ligand L.

29. Racemic Mixture:

• A racemic mixture is a mixture of equal amounts of both enantiomers of a chiral compound.
• Racemic mixtures are optically inactive since the rotation of plane-polarized light by one enantiomer is canceled out by the other enantiomer.
• Racemic mixtures are represented by a (±) prefix.
• Example: (±)-Tartaric acid
  • It is a racemic mixture consisting of equal amounts of both (+)-tartaric acid and (-)-tartaric acid.

30. Resolution of Enantiomers:

• Resolution is the process of separating an enantiomeric mixture into its individual enantiomers.
• Resolution can be achieved through techniques such as selective crystallization, chromatography, or reaction with an enantiomerically pure compound.
• Once separated, the individual enantiomers can exhibit different physical and chemical properties.
• Example: Separation of (+)- and (-)-limonene using chiral stationary phase chromatography
  • This technique separates the enantiomers of limonene based on their different interactions with a chiral stationary phase.