Download 2B - ligands 2

2B-1
Chelating Ligands
H&S 7.11
Many ligands can bind to a metal through two or more atoms simultaneously.
Polydentate ligands can chelate to a metal. (Form a ring of atoms including the metal).
Ligand Denticity = # of donor atoms to a metal
Bidentate = 2 donor atoms
EXAMPLES:
1,2-dimethoxyethane (DME)
Ethanediamine (en)
(also known as ethylenediamine)
propanediamine
Bis(diphenylphosphino)ethane, dppm
2B-2
Chelating Ligands
H&S 7.11
Some ligands have a rigid structure and must have the donor atoms oriented correctly to form a
chelate.
2,2’-bipyridine (bipy)
Phenanthroline (phen)
4,4’-bipyridine
2B-3
Chelating Ligands
Delocalized bidentate ligands
(1)Carboxylates
acetic acid (a carboxylic acid)
H&S 7.11
acetate (a carboxylate) (LX)
Common carboxylates (RCO2-): formate (R = H), acetate (R = CH3), benzoate (R = Ph)
2. -diketonates
Acetylacetone (Hacac)
acetylacetonate (acac) (LX)
2B-4
Chelating Ligands
Tridentate = 3 donor atoms
Diethylene triamine (dien)
terpyridine (terpy)
H&S 7.11
2B-5
Chelating Ligands
Higher denticity ligands are also known:
e.g. triethylene tetramine (trien) is a tetradentate ligand
EDTA (ethylenediamine tetraacetic acid) is a hexadentate ligand:
H&S 7.11
2B-6
Macrocyclic ligands
Macrocyclic ligands are already cyclic as free ligands. Usually
a minimum of 3 donor atoms (usually O, N, S) - 4-6 donor
atoms are the most common ligand structure type
The “curvy bits” in the schematic diagram on the right are
usually carbon-based chains.
Examples:
2B-7
Ligands based on pi bonds
H&S 24.2
So far, all ligands we have examined mainly bond to metals by using a lone pair orbital to form
a sigma bond:
Some organic (carbon-based) ligands don’t have long pairs situated on individual donor atoms
yet can function as ligands for transition metals. These ligands make use of pi electrons and
are therefore known as “pi complexes”.
2B-8
Ligands based on pi bonds
H&S 24.2
Examples:
Ethylene, C2H4
Benzene, C6H6
cyclopentadienide,
C5H5cyclopentadiene,
C5H6
2 ethylene
complex
6 benzene
complex
5 Cp complex