Transition Metal Complexes: Ligands and Coordination Number - SS3 Chemistry Lesson Note
Transition metal complexes are coordination compounds formed by the coordination of ligands to a central metal ion. These complexes are essential in various chemical, biological, and industrial processes, and they exhibit diverse properties and behaviours based on the ligands and coordination number. Let's delve into the concepts of ligands and coordination numbers in transition metal complexes:
Ligands: Ligands are molecules or ions with one or more lone pairs of electrons that can donate those electrons to the central metal ion to form coordinated covalent bonds. Ligands can be classified based on the number of donor atoms they possess:
● Monodentate ligands donate one lone pair (e.g., H2O, Cl-, NH3).
● Bidentate ligands donate two lone pairs through two donor atoms (e.g., ethylenediamine, en).
● Polydentate ligands (chelating ligands) donate multiple lone pairs through two or more donor atoms, forming stable chelate rings (e.g., ethylenediaminetetraacetate, EDTA).
● Ligands can also be classified as anionic, neutral, or cationic, depending on their charge.
Coordination Number: The coordination number refers to the number of coordinate covalent bonds formed between the central metal ion and the ligands. The coordination number determines the geometry of the complex and influences its stability and reactivity. Common coordination numbers for transition metal complexes are 4, 6, and 2, but other coordination numbers are also possible.
Geometry of Complexes: Complexes with a coordination number of 2 typically have linear geometry. Complexes with a coordination number of 4 often have tetrahedral or square planar geometry. For d8 and high-spin d6 metal ions, square planar complexes are more stable. Complexes with a coordination number of 6 can have octahedral, trigonal prismatic, or distorted octahedral geometries, depending on the ligands and metal ions.
Isomerism: Transition metal complexes can exhibit various types of isomerism, which arise due to differences in the spatial arrangement of ligands around the central metal ion.
● Geometric Isomerism: In octahedral complexes with bidentate ligands or complexes with chelating ligands, geometric isomers (cis-trans isomers or facial-meridional isomers) can be observed.
● Optical Isomerism: In certain octahedral or tetrahedral complexes with chiral ligands, optical isomers (enantiomers) can form.
Stability and Reactivity: The stability and reactivity of transition metal complexes are influenced by several factors, including the nature of ligands, the charge of the metal ion, and the coordination number. Chelating ligands increase the stability of complexes due to the formation of stable chelate rings, protecting the metal ion from hydrolysis or redox reactions.
Color and Spectroscopy: Transition metal complexes often exhibit intense colours due to electronic transitions within the d-orbitals of the metal ion. The study of electronic spectra and colour in transition metal complexes provides valuable information about their structures and properties.
Transition metal complexes play crucial roles in catalysis, bioinorganic chemistry, materials science, and medicinal chemistry. Their ability to form stable coordination compounds with a wide range of ligands and coordination numbers makes them versatile tools in various chemical and industrial processes. Understanding the interactions between metal ions and ligands in these complexes is fundamental to advancing our knowledge of coordination chemistry and its applications.