Transition Metals and Coordination Chemistry

• Crystal field theory

  Based on the premise that the metal ion and ligands can be treated as point charges and the spatial arrangement of these point charges will affect the energies of the orbitals for the central metal ions
• What does crystal field theory explain?

  • Explains colors, magnetic behaviour and structures that can be observed in coordination compounds
• Ligands


  Regarded as dipoles/anions with excess electron density that they donate to the metal ion, which repels existing electron density on the metal ion
• What are ligands considered as?

  Considered as point negative charges (i.e. no orbital overlap/electron-sharing)
• Repulsion between ligand electron density and metal ion electron density


  Causes the energy of certain orbitals to increase and not in an equal manner
• Ionic lattice


  Built from point positive and negative charges, a very basic ionic model (dramatic simplification as few complexes are purely ionic)
• Bonding energy


  Ionic electrostatic forces
• Octahedral crystal field theory


  • d-orbitals split into two sets (eg and t2g) with different energies due to the repulsion from the ligands
• Eg orbitals


  Point directly towards the ligands, have higher energy
• T2g orbitals


  Point between the ligands, have lower energy
• Crystal field splitting energy

  The difference in energy between the eg and t2g orbitals
• Spectrochemical series


  • I- < Br- < Cl- < F- < H2O < NH3 < en < NO2- < CN-
• Weak-field ligands have a small crystal field splitting energy, not enough to overcome pairing energy, so they form high-spin complexes
• Strong-field ligands have a large crystal field splitting energy, enough to overcome pairing energy, so they form low-spin complexes
• Tetrahedral crystal field theory


  • Ligands approach the orbitals between the axes rather than on the axes, so the higher energy orbitals are the opposite of octahedral
• Coordination number


  Determines the magnitude of the crystal field splitting energy, which in turn determines the electron configuration and other properties of the complex
• Crystal field theory helps determine the color of transition metal compounds by explaining the energy differences between orbitals that lead to the absorption and emission of visible light
• Even small changes in orbital energy can have a significant impact on a compound's color, and different oxidation states of the same element can have different colors
• Coordination complex


  A species formed by the interaction of a Lewis acid (central metal ion) and Lewis bases (ligands)
• Ligands


  • Must be capable of donating a pair of electrons to form a dative bond, and the central atom must have empty orbitals to accommodate the incoming electrons from the ligands
• Determining d-electron configuration

  1. Group number

  2. Oxidation state

  3. Pauli exclusion principle
• High-spin complexes


  Have unpaired electrons in the higher energy eg orbitals
• Low-spin complexes


  Have paired electrons in the lower energy t2g orbitals
• Tetrahedral complexes are always high-spin
• Diamagnetic

  No unpaired electrons
• Paramagnetic


  Has unpaired electrons
• The ligand field stabilisation energy (LFSE) and pairing energy determine the spin state of a complex
• For an octahedral Fe2+ complex, the LFSE is -0.4Δo for the high-spin configuration and -2.4Δo for the low-spin configuration
• The total energy of the complex is the sum of the LFSE and pairing energy