Complex Ions
Definition and meaning of Complex Ions in chemistry.
Complex ions are charged species formed when a central metal ion (typically a transition metal) is surrounded by and bonded to a fixed number of ligands through coordinate (dative) covalent bonds.
In more detail
Each ligand donates a lone pair of electrons into an empty orbital on the metal, so all the bonding electrons originate from the ligand rather than being shared equally. The number of ligands attached (the coordination number, commonly 4 or 6) and their spatial arrangement gives the complex a defined geometry, such as octahedral, tetrahedral, or square planar. Because ligand-field splitting of the metal's d orbitals allows visible-light absorption, many complex ions are intensely colored, which makes them useful in qualitative analysis and industrial catalysis. Complex ion formation also explains why many "insoluble" metal hydroxides redissolve in excess ammonia or hydroxide solution.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| General formula | [M(L)x]n+ or [M(L)x]n- |
| Common coordination numbers | 4 (tetrahedral/square planar) and 6 (octahedral) |
| Bond type | Coordinate (dative) covalent bonds |
Adding excess aqueous ammonia to a solution containing pale blue [Cu(H2O)6]2+ ions displaces the water ligands and produces the deep blue tetraamminecopper(II) complex ion, [Cu(NH3)4(H2O)2]2+.
Frequently asked questions
Why are many complex ions colored?
Ligands split the metal's d orbitals into different energy levels; electrons absorb specific wavelengths of visible light to jump between these levels, and the transmitted light produces the observed color.
What counts as a ligand?
A ligand is any ion or molecule with at least one available lone pair, such as water, ammonia, chloride, or cyanide, that can donate electron density to a central metal ion.