Complex Ions: Transition metal ion form complex ions. Complex ions formed when ligands bond
to central metal ion by coordinate bond.
Ligands: Ligands are molecules or ions that donate lone pair of electrons to a central metal ion
to form coordinate bond or dative covalent bond.
Coordinate Bond: A shared pair of electrons in which one the bonded atoms provides both
electrons.
Nitrogen and oxygen often have lone pair of electrons, so attach them central metal ion.
Coordination Number: Number of coordinate bonds attached to central ion- coordinate
number.
Hydrated solid CrCl3. 6H2O dissolved in water to form blue solution containing [Cr(H2O)6]
3+
.
Chemical Formula: Square brackets enclose complex ion. Overall charge of complex ion is 3+,
the sum of the charges of central metal ion and ligands. Ligand shown in round brackets.
– When writing chemical formula, remember to account ligands and metal ion charge to find
overall charge.
This is how ethanedioate ion looks attached to metal ion. The atoms with the
charge will be attached. Ethanedioate has 2- charge and has two carbons in structure.
Monodentate Ligands: Examples are water, NH3 ammonia, Cl-, CN- cyanide and OH-. Water
cannot act as bidentate, although two lone pairs on O, because they repel. Can get hexadentate
and so on e.g. EDTA4-
.
If H2O, on the RHS written as OH2.
Bidentate Ligands: Some ligands are able to donate two lone pairs of electrons to central metal
ion to form two coordinate bonds.
Examples are 1,2- diaminoethane (shortened to –en) where each nitrogen atom has a lone pair
so bidentate donates two lone pairs to metal ion to form two coordinate bonds. Also
ethanedioate ion (aka oxalate ion).
Complex ion with bidentate ligand [Co(NH2CH2CH2NH2)3]
3+ shown above. Oxidation number of Co
must be 3+ as ligand is neutral. Coordinate number is 6 as 3 bidentate ligands so has octahedral
shape.
Need to balance the charges as well.