Lecture 3 ligand classification

Lecture Overview

• Lecture 3: Classification and Nomenclature in Coordination Chemistry

Ligand Classification

• Coordination Chemistry and Ligands

  • i. Monodentate Ligands: Bind to a metal ion through a single donor site.

  • ii. Ambidentate Ligands: Can bind to a metal ion through one of two different binding sites, but not both simultaneously.

  • iii. Bridging Ligands: Bind to two different metal ions simultaneously, important in biological applications.

  • iv. Multi- and Polydentate Chelating Ligands: Ligands that can form multiple bonds with a metal ion allowing for more complex structures.

  • v. Macrocyclic Ligands: Large ring-like structures that enhance stability in metal complexes.

Historical Background of Coordination Chemistry

• Alfred Werner (1866 – 1919):

  • Developed coordination theory in 1893 at age 26.

  • Awarded the first Nobel Prize in Inorganic Chemistry in 1913.

  • Example: CoCl3(aq) reacts with NH3.

Ligand Characteristics

• A ligand is an ion or molecule that binds to a metal ion to form a coordination complex.

• Term "ligand" originates from the Latin word "ligare" meaning "to bind" (first used in 1916).

• Ligands can be anionic or neutral, essentially acting as Lewis bases through electron pair donation to metal cations.

Types of Ligands

1. Monodentate Ligands

• Example: NH3, binds through single donor site.

• Common ligands: Chloro, Cyano, and H2O (changes to aqua or aquo).

• Complex Example: [Cu(H2O)6]2+

2. Bridging Ligands

• Bind to two different metal ions simultaneously through two donor atoms.

• Notation: Preceded by Greek letter mu (μ) indicating the number of metal ions.

  • Example: Two terminal chloro ligands and bridging chloro in complexes.

3. Ambidentate Ligands

• Feature two different types of donor atoms.

• Common examples include NO2−, CN−, NCS−, can switch binding sites.

4. Multi-/Polydentate Chelating Ligands

• Example: Ethylenediamine (en), which is bidentate, forms 5-membered rings.

• Process of chelation: Ligands' multiple binding sites trap the metal ion, increasing stability.

• EDTA as an example of a hexadentate ligand binding through six donor atoms.

5. Macrocyclic Ligands

• Larger cyclic ligands offer higher stability, pre-organized for binding with less entropy penalty.

• Example: Heme, a tetradentate macrocyclic ligand, has strong affinity for Fe2+ cations in biology.

Naming Metal Complexes

• Basic Rules:

  1. Cation first, anion last (e.g. K2[PtCl4]).

  2. Ligands named before metals, in alphabetical order.

  3. Use prefixes (di-, tri-, tetra-, penta-, hexa-) for counting ligands.

  4. Complex anions → add suffix -ate to metal name if from Latin/Greek.

  5. Indicate oxidation state with Roman numerals in parentheses.

Examples of Naming

• For [Cr(H2O)4Cl2]+: tetrraaquadichlorochromium(III) ion.

• For [CoCl4]2-: tetrachlorocobaltate(II) ion.

Greek/Latin Nomenclature

• Common Metal Roots:

  • Fe → ferrum → ferrate

  • Cu → cuprum → cuprate

  • Ag → argentum → argentate

  • Au → aurum → aurate

Distinguishing Ambidentate Ligands

• Key to identifying ambidentate ligands such as SCN- versus NCS- is determining which atom is attached to the metal.

• Examples include:

  • SCN-→ thiocyanato

  • CN- → cyano

  • NO2- → nitro

  • NO2- → nitrito

Isomerism in Metal Complexes

• Identifying isomers requires prefixes to denote spatial arrangement (cis/trans; fac/mer).

• Example: [Co(NH3)4Cl2]+ has two isomers:

  • cis-tetraamminedichlorocobalt(III)

  • trans-tetraaminedichlorocobalt(III)

Summary of Coordination Compounds

• Examples of complex ions:

  • Complex cation: [Co(NH3)6]3+

  • Complex anion: [CoCl4(NH3)2]-

  • Neutral complex: [CoCl3(NH3)]

Additional Nomenclature

• Denticity: Indicated by kappa (κ) notation, specifies which atoms are bonded in polynuclear complexes (e.g., EDTA as κ6).

• Hapticity: Describes the structures of organometallic complexes using the eta (η) notation, illustrating how many contiguous atoms are coordinating to a metal center (e.g., ferrocene as η5).