Lecture Seventeen - Ligands in coordination complexes and naming conventions Flashcards
What is a ligand?
A ligand is a molecule which acts as a lewis base. It has a lone pair of electrons to donate (on their donor atoms).
Can be neutral or anionic.
Can be:
Monodentate (1 donor atom).
Bi dentate (2).
Tridentate (3).
Tetradentate (4).
Penta-, hexa- …polydentate (n).
Describe bidentate ligands.
Bidentate ligans have two teeth.
Four atom chains which form 5-membered rings with metals.
A ligand like ethylenediamine (en) grabs metal ion like a crabs claw - chelate.
Describe polydentate ligands.
Can bond with many teeth.
Can form many bonds with a metal centre.
Forms extremly stable complexes.
Polydentate ligans which are cyclic are called macrocycles.
Important examples include porphyrin rings (tetradentate) and crowth ethers (multidentate, e.g. 18-crown-6).
What does chelate mean?
Chelate - form Greek meaning claw.
The attachement of two donor atoms from a bidentate ligand to the same metal forms a chelate ring.
E.g. En and acetylacetate (acac).
Chelate rings are thermodynamically very stable.
What are bridging ligands?
Bridging ligands bond to more than one metal.
Some monodentate ligands can coordinate more than once, i.e. they have multiple lone pairs.
The geometric arrangement of the lone pairs is such that they cannot all bond to the same metal more than once.
How are coordination complexes named?
As with ionic compounds, the cation is named first and the anion last.
Ligands are listed alphabtically before the metal.
Prefixes denoting the number of a particle ligand are ignored for the purpose.
Anionic ligans end in ‘o,’ neutral ligands are not changed.
If the complex is an anion, its ending is changed to -ate.
The oxidation number of the metal is given as a roman numeral in parentheses immediately after the name of the metal.
Coordination complex is written in square brackets.
For a complex ion, the charge is weitten outside the brackets, [CoCl(NH3)5]2+.
