Coordination Chemistry
Asst. Prof. Dr. Abbas A. S. Al-Hamdani
Lecture : Complexes & Introducing the Ligands
Lewis Model of coordinate Bonding
Neutral and Anionic Ligands
Mono- and bidentate ligands
Multidentate ligands
Inner and outer sphere coordination
Definition the Complex:
A complex or coordination compound is a compound in which an atom (called “central atom”) is bound to more groups (called “ligands”) than expected with respect to its charge and position in the periodic table.
The number of ligands around a central atom is called the “coordination number”.
Coordination Chemistry
• Transition metals act as Lewis acids
• Form complexes/complex ions
Complex contains central metal ion bonded to one or more molecules or anions
Lewis acid = metal = center of coordination
Lewis base = ligand = molecules/ions covalently bonded to metal in complex
Metal-Ligand Bond
• This bond is formed between a Lewis acid
and a Lewis base.
– The ligands (Lewis bases) have nonbonding
electrons.
– The metal (Lewis acid) has empty orbitals.
Coordinate bonding
Each ligand donates both electrons to the bond with the metal centre
Metal-Ligand Bond
The metal’s coordination ligands and geometry can greatly alter its properties, such as color, or ease of oxidation.
Lewis Model of Bonding
Neutral ligands containing Lone pairs of electrons
Ligands are bonded to the metal centre via a donor atoms
Anionic ligands containing Lone pairs of electrons
These ligands are s-bonded to the metal centre
Nomenclature of Coordination Compounds
• As always the name of the cation appears first; the anion is named last.
• Ligands are listed alphabetically before the metal. Prefixes denoting the number of a particular ligand are ignored when alphabetizing.
• The names of anionic ligands end in “o”; the endings of the names of neutral ligands are not changed.
• Prefixes tell the number of a type of ligand in the complex. If the name of the ligand itself has such a prefix, alternatives like bis-, tris-, etc., are used.
• If the complex is an anion, its ending is changed to -ate.
• The oxidation number of the metal is listed as a Roman numeral in parentheses immediately after the name of the metal.
• The basic protocol in coordination nomenclature is to name the ligands attached to the metal as prefixes before the metal name.
• Some common ligands and their names are listed above.
Examples
p - bonded ligands
The electrons in multiple bonds can act as a lone pair of electrons
eta-two ethene means the C2H4 is bonded via two atoms to the metal
Ligand Denticity
Denticity = number of donor atoms with which a ligand can bind to a metal centre
Monodentate one donor atom per ligand
Bidentate two donor atoms per ligand
Tridentate three donor atoms per ligand
Multidentate many donor atoms per ligand
Denticity refers to the number of "teeth" a ligand has Chelating ligand: a ligand which binds to the same metal centre with more than one donor atom
• Ligands
– classified according to the number of donor
atoms
– Examples
• monodentate = 1
• bidentate = 2
• tetradentate = 4
• hexadentate = 6
• polydentate = 2 or more donor atoms
Comments
Post a Comment