Coordination Chemistry - CH5202 Lecture Notes

Coordination Chemistry - Module Summary

Reference Materials

• Supplement notes from various textbooks:
    - Advanced Inorganic Chemistry (Cotton and Wilkinson): Detailed general book.
    - Chemistry of the Elements (Greenwood and Earnshaw): Readable with interesting background material.
    - Inorganic Chemistry (Shriver and Atkins): Low on information, but good for concepts.
    - d-Block Chemistry (M. Winter): Affordable and well-written, recommended.
    - Complexes and 1st Row Transition Elements (Nichols): Out of print but readily available second-hand.

Class Test (January)

• Coordination chemistry will be assessed in a multiple-choice format covering:
    - Assignment of electron configurations.
    - Determination of oxidation states.
    - Identification of the number of unpaired electrons.
    - Calculation of magnetic moments.

Crystal Field Theory (CFT) vs. Molecular Orbital Theory (MO Theory)

Crystal Field Theory (CFT)

• Definition: Describes the energy variances of d-orbitals when ligands, treated as point charges, approach a charged transition metal ion.

• Example Representation:
    - Geometry: ML6
    - Energy interpretation: Energy increases due to electron (M) - electron (L) repulsion.

• Orbital Splitting:
    - eg (including $d_{x^2-y^2}, d_{z^2}$): Higher energy due to orientation along Cartesian axes.
    - t2g (including $d_{xy}, d_{xz}, d_{yz}$): Lower energy due to orientation between Cartesian axes.
    - Energy separation denoted as $riangle_0$ due to complex geometry.

Formation of Complexes

1. Complexes arise from electrostatic attraction between metal ions and charged or polar ligands.

2. Covalent Interactions:
      - Predominantly contribute to complex stability for neutral or low oxidation state metals and ligands, as well as very high oxidation states.

3. Ligand Characteristics:
      - All ligands (L) are Lewis bases (donating electrons).
      - Ligands can be π-Lean acids (e.g., $CN^-$), π-Lean bases (e.g., $Cl^-, SCN^-, ext{oxalate}$), or neutral (e.g., $NH_3, NR_3$).

Molecular Orbital Theory (MOT) for Transition Metal Complexes

• Metal-Ligand Bonds: Covalent bonds where ligands (Lewis bases) donate electrons to the metal (Lewis acid).

• Bonding Criteria:
    - Comparable energy and identical symmetry of orbitals on metal (M) and ligand(s) (L).
    - Availability of electrons for donation by ligands to metal.

Metal Valence Orbitals

• Used for 1st row metals: $3d, 4s, 4p$

• For 2nd row metals: $4d, 5s, 5p$

• For 3rd row metals: $5d, 6s, 6p$.

MO Diagram for σ-Bonding in Transition Metal Complexes

• Under octahedral symmetry (Oh):
    - Treat σ-Bonding and π-Bonding separately.
    - Metal Valence Orbitals:
      - $3d(5), 4s(1), 4p(3) = 9$ total metal valence orbitals.
    - Ligand Orbitals: Lone pairs are positioned along Cartesian axes for effective σ-bonding.
    - Ligand Group Orbitals: Constructed from ligand orbitals with the same symmetry as the metal's valence orbitals.

Bonding Combinations in Diagrams

• Combined Orbital Diagram:
    - Contains bonding, anti-bonding, and non-bonding orbitals: Total of 15 orbitals (6 ligand σ, 9 metal orbitals).

π-Bonding in Transition Metal Complexes

• π-Bonding modifies the π-bonding molecular orbital diagram.

• Interactions include:
     - t2g orbitals interacting with ligand π orbitals (bonding and anti-bonding combinations).
     - Ligand behavior (π-basic or π-acidic):
       - π-bases donate electrons.
       - π-acids accept electrons.

Effects of π-Bonding

• Determines energy levels in relationship to t2g orbitals:
    - Occupied t2g orbitals lead to lower Δo values (weak π-donors).
    - Vacant t2g orbitals yield higher Δo values (strong π-acceptors).

• The energy of Δo is largely affected by π-bonding.

Factors that Determine Magnitude of Δo

1. Electrostatic Field: Influenced by ligands and metal charge. Higher oxidation states raise Δo values due to increased Coulombic attraction.

2. Sterics: Small ligands create shorter M-L bonds, leading to larger Δo values due to reduced steric hindrance.

3. σ-Bonding Strength: Stronger σ-donors yield larger Δo values. Strong organic bases can act as effective ligands.

Spectrochemical Series

• Ranks ligands according to increasing Δo values:
    - List of ligands: I- < Br- < S2- < SCN- < Cl- < NO3 - < F- < C2O42- < H2O < NCS- < CH3CN < NH3 < en < bipy < phen < NO2 - < H- < PR3 < CN- < CO.

Kinetic Properties of Complexes

• Labile complexes: Rapid ligand exchange, primarily low LFSE values.

• Inert complexes: Slow ligand exchange, associated with high LFSE values.

• Henry Taube's Definition: Labile complexes have a half-life of 1 minute or less.

Optical Activity in Complexes

• Resolution of Optical Isomers: Notable for CoIII complexes due to their high LFSE.

Jahn-Teller Effect

• Distortion in non-linear molecules occurs to remove orbital degeneracy.
    - Examples include distortions in Cu2+ and Ti3+ complexes, resulting in unique geometries and bonding structures.

Summary of Geometries

• Common geometries such as Octahedral, Trigonal Prismatic, Square Planar, Tetrahedral, and their associated MO diagrams are summarized.

Conclusion

• Understanding the interactions and structural properties in Coordination Chemistry is crucial for predicting complex stability, reactivity, and magnetic properties. Use references to reinforce concepts and develop a deeper comprehension.