Transition Metals and Coordination Compounds: Structure, Bonding, and Properties

Transition Metals and Coordination Compounds

Introduction to Coordination Chemistry

Coordination chemistry is the study of compounds formed between metal ions and ligands. These compounds, known as coordination complexes, play a significant role in both biological and industrial processes.

• Coordination Compound: A compound consisting of a central metal atom or ion bonded to surrounding molecules or ions called ligands.

• Ligand: An ion or molecule capable of donating a pair of electrons to the metal center to form a coordinate bond.

• Example: $[Co(NH_3)_6]Cl_3$

Additional info: Coordination chemistry was pioneered by Alfred Werner, who explained the structures and bonding in these compounds.

Types of Ligands

• Monodentate Ligands: Ligands that donate only one pair of electrons to the metal center (e.g., $NH_3$, $Cl^-$).

• Bidentate Ligands: Ligands that donate two pairs of electrons from two donor atoms (e.g., $C_2O_4^{2-}$, ethylenediamine).

• Polydentate Ligands: Ligands with more than two donor atoms (e.g., EDTA).

Classification of Coordination Compounds

• Double Salts: Compounds that lose their identity in solution (e.g., $K_2SO_4 imes Al_2(SO_4)_3 imes 24H_2O$).

• Complex Salts: Compounds that retain their identity in solution (e.g., $K_4[Fe(CN)_6]$).

Types of Isomerism in Coordination Compounds

• Structural Isomerism: Includes ionization, linkage, coordination, and hydrate isomerism.

• Stereoisomerism: Includes geometrical and optical isomerism.

Coordination Number and Geometry

The coordination number (C.N.) is the number of ligand donor atoms bonded to the central metal ion.

• C.N. = 2: Linear geometry (e.g., $[Ag(NH_3)_2]^+$)

• C.N. = 4: Tetrahedral (e.g., $[Ni(CO)_4]$) or square planar (e.g., $[PtCl_4]^{2-}$)

• C.N. = 6: Octahedral geometry (e.g., $[Fe(CN)_6]^{4-}$)

Crystal Field Theory (CFT)

Crystal Field Theory explains the electronic structure of transition metal complexes by considering the effect of ligands as point charges that split the d-orbital energies of the metal ion.

• Key Points:

  • Ligands cause splitting of the metal d-orbitals into sets of different energies.

  • The magnitude of splitting depends on the nature of the ligand and the geometry of the complex.

  • Strong field ligands (e.g., CN-) cause large splitting; weak field ligands (e.g., H2O) cause small splitting.

• Advantages of CFT:

  • Explains color, magnetism, and stability of complexes.

  • Accounts for high-spin and low-spin configurations.

Crystal Field Splitting in Octahedral Complexes

In an octahedral field, the five d-orbitals split into two sets: $t_{2g}$ (lower energy) and $e_g$ (higher energy).

• Crystal Field Splitting Energy ($riangle_o$): The energy difference between $t_{2g}$ and $e_g$ orbitals.

Diagram:

• Filling of d-orbitals:

  • High-spin complexes: Weak field ligands, maximum unpaired electrons.

  • Low-spin complexes: Strong field ligands, minimum unpaired electrons.

Crystal Field Stabilization Energy (CFSE)

CFSE is the stabilization gained by the arrangement of electrons in the split d-orbitals.

• Formula for Octahedral Complexes: $CFSE = (-0.4 imes n_{t_{2g}} + 0.6 imes n_{e_g}) riangle_o + P$ where $n_{t_{2g}}$ and $n_{e_g}$ are the number of electrons in $t_{2g}$ and $e_g$ orbitals, and $P$ is the pairing energy.

Magnetic Properties of Transition Metal Complexes

• Paramagnetic Compounds: Contain unpaired electrons and are attracted to magnetic fields.

• Diamagnetic Compounds: All electrons are paired; repelled by magnetic fields.

• Magnetic Moment ($ext{BM}$): $\mu = \sqrt{n(n+2)}$ where $n$ is the number of unpaired electrons.

Color of Transition Metal Complexes

The color of a complex is due to d-d transitions, where electrons absorb visible light to move between split d-orbitals.

• Complementary Colors: The observed color is the complement of the absorbed wavelength.

Observed Color	Absorbed Color	Wavelength (nm)
Red	Green	620–750
Orange	Blue	590–620
Yellow	Violet	570–590
Green	Red	495–570
Blue	Orange	450–495
Violet	Yellow	400–450

Lab Techniques and Calculations

• Determination of Absorbance: $A = \varepsilon c l$ where $A$ is absorbance, $\varepsilon$ is molar absorptivity, $c$ is concentration, and $l$ is path length.

• Calculation of Energy and Wavelength:

  • Energy of a photon: $E = h\nu = \dfrac{hc}{\lambda}$

  • Wavelength: $\lambda = \dfrac{hc}{E}$

Summary Table: Common Geometries and Examples

Coordination Number	Geometry	Example
2	Linear	$[Ag(NH_3)_2]^+$
4	Tetrahedral	$[Ni(CO)_4]$
4	Square Planar	$[PtCl_4]^{2-}$
6	Octahedral	$[Fe(CN)_6]^{4-}$

Conclusion

Coordination compounds and transition metal chemistry are central to understanding the structure, bonding, color, and magnetism of many important chemical species. Crystal Field Theory provides a powerful model for predicting and explaining these properties.