The f- and d- block elements- Importance of atomic size in coordination compounds

  • Introduction:
    • Transition metals from the d-block of the periodic table.
    • Importance of atomic size in coordination compounds.
  • Definition of coordination compound:
    • Complexes containing a metal ion surrounded by ligands.
    • Ligands donate a pair of electrons to the metal ion.
  • Coordination number:
    • Number of ligand atoms bonded to the central metal ion.
    • Examples of coordination numbers include 2, 4, 6, etc.
  • Importance of atomic size in coordination compounds:
    • Larger atomic size results in higher coordination number.
    • Smaller atomic size limits coordination number.
  • Examples of coordination compounds with different coordination numbers:
    • CuSO4 (coordination number 4)
    • [Fe(CN)6]3- (coordination number 6)
    • [Co(NH3)6]2+ (coordination number 6)
  • Effect of changing ligands on coordination number:
    • Different ligands have different abilities to donate electrons.
    • Can result in change in coordination number.
  • Crystal field splitting:
    • Interaction between the ligands and the d-orbitals of the metal ion.
    • Results in energy splitting of the d-orbitals.
  • Crystal field splitting energy (Δ):
    • Energy difference between the e_g and t_2g levels.
    • Determines the color and magnetic properties of coordination compounds.
  • Charge transfer spectra:
    • Transition of electrons from ligand to metal or vice versa.
    • Results in absorption of specific wavelengths of light.
  • Summary:
    • Atomic size plays a crucial role in determining the coordination number.
    • Changing ligands can alter the coordination number.
    • Crystal field splitting energy affects the properties of coordination compounds.
  1. Crystal field splitting energy (Δ):
  • Energy difference between the e_g and t_2g levels.
  • Determines the color and magnetic properties of coordination compounds.
  • Example: [Fe(H2O)6]3+ has Δ value of 10Dq = 8000 cm-1.
  1. Factors affecting crystal field splitting energy:
  • Nature of ligands: Different ligands result in different Δ values.
  • Oxidation state of the central metal ion: Higher oxidation state leads to higher Δ.
  • Example: [Co(H2O)6]3+ has a larger Δ value than [Co(NH3)6]3+.
  1. Spectrochemical series:
  • Ranking of ligands based on their ability to split d-orbitals.
  • Ligands at the top have stronger σ-donor and π-acceptor properties.
  • Example: I- < Br- < SCN- < Cl- < F- < OH- < H2O < NH3 < en < NO2- < CN-.
  1. Colors of coordination compounds:
  • Transition metal complexes exhibit various colors.
  • Color arises due to excitation of electrons between d-orbitals.
  • Example: Cu2+ complexes are blue, while Fe3+ complexes are red.
  1. Magnetic properties of coordination compounds:
  • Paramagnetic: Unpaired electrons in d-orbitals, attracted to a magnetic field.
  • Diamagnetic: All electrons are paired, repelled by a magnetic field.
  • Example: [Fe(H2O)6]3+ is paramagnetic, [Zn(H2O)6]2+ is diamagnetic.
  1. Factors affecting magnetic properties:
  • Number of unpaired electrons: More unpaired electrons, higher magnetic moment.
  • Example: [Fe(H2O)6]3+ (5 unpaired electrons) has a higher magnetic moment than [Fe(H2O)6]2+ (4 unpaired electrons).
  1. Isomerism in coordination compounds:
  • Structural isomerism: Different arrangements of ligands around the central metal ion.
  • Example: cis and trans isomers in [Co(NH3)4Cl2]Br.
  1. Geometric isomerism: Different spatial arrangements of ligands.
  • Cis-trans isomerism: In complexes with coordination number 4.
  • Example: [Pt(NH3)2Cl2] can exist as cis or trans isomers.
  1. Optical isomerism: Non-superimposable mirror images of each other.
  • Requires the presence of chiral ligands or a chiral metal center.
  • Example: [Co(en)3]3+ can exist as two enantiomers.
  1. Summary:
  • Crystal field splitting energy (Δ) determines the properties of coordination compounds.
  • Factors such as ligands, oxidation state, and spectrochemical series affect Δ.
  • Coordination compounds exhibit various colors and magnetic properties.
  • Isomerism can occur in coordination compounds, leading to different structural arrangements. The f- and d- block elements- Importance of atomic size in coordination compounds
  1. Factors affecting the size of transition metal ions:
  • Effective nuclear charge: The attraction between the nucleus and the valence electrons.
  • Shielding effect: The repulsion between inner electrons and valence electrons.
  • Size increases with an increase in the number of inner shells.
  • Examples: Sc3+ has a larger size than Ti4+.
  1. Ionic radii of transition metal ions:
  • Ionic radii decrease from left to right across a period.
  • Example: Fe3+ has a smaller ionic radius than Mn2+.
  1. Coordination compounds with larger atomic size:
  • Coordination number is generally higher.
  • Examples: [Cu(NH3)4(H2O)2]2+, [Ag(NH3)2]+.
  1. Coordination compounds with smaller atomic size:
  • Coordination number is generally lower.
  • Examples: [Ti(H2O)6]3+, [Cr(NH3)4Cl2]+.
  1. Crystal field theory:
  • Describes the electronic structure of coordination compounds.
  • Interaction between ligands and d-orbitals causes energy splitting.
  • Resulting energy difference is denoted by Δ (delta).
  1. High spin and low spin complexes:
  • High spin: electrons occupy higher energy orbitals first.
  • Low spin: electrons occupy lower energy orbitals first.
  • Depends on the magnitude of Δ.
  • Example: [Fe(H2O)6]3+ is low spin, while [Fe(CN)6]3- is high spin.
  1. Calculation of Δ (delta):
  • Δ is determined by the nature of the ligands and oxidation state of the metal ion.
  • Actual value can be obtained from spectroscopic measurements.
  • Example: Δ is determined as 10Dq = 8000 cm-1 for [Fe(H2O)6]3+.
  1. Complexes with different spin states:
  • Transition metal complexes can show either high spin or low spin behavior.
  • Depends on the value of Δ.
  • Example: [Co(H2O)6]2+ is high spin, while [Co(NH3)6]2+ is low spin.
  1. Ligand field stabilization energy (LFSE):
  • Energy difference between the average energy of the complex and free ions.
  • LFSE = -0.4ΔO(n + 0.6l).
  • Determines the stability of a complex.
  • Example: LFSE is higher for low spin complexes.
  1. Summary:
  • Atomic size influences the coordination number in transition metal complexes.
  • Crystal field theory explains the electronic structure of coordination compounds.
  • Spin states (high spin or low spin) are determined by Δ.
  • LFSE is a measure of stability in coordination compounds.