What is a complex ion?

A complex ion is a central ion with attached ligands, the ligands attach themselves to the central ion by using a lone pair of electrons forming a Dative/Coordinate bond.

A ligand is a molecule or atom that attaches itself using using a lone pair of electrons (Dative bond) with a central ion. They are usually neutral or negative. Different ligands form different numbers of dative bonds with the central ion, a hydroxide ion forms one so is called monodentate. An ligand that forms two would be called bidentate and six would be hexadentate.
The Coordinate number
This is the number of ligands attached to the central ion, it is called this as the bonds that are used to connect the ligands to the central atom are called coordinate bonds. The number of ligands attached to the central ion (coordination number) can determine the shape of the complex.
Shapes
A complex ion of coordination number six will be octahedral and all ligands will be at 90° from each other.
A coordination number of four could be a Square planar molecule with all molecules at 90° or tetrahedral at 104.5° from each other.
A coordination number of two would be a linear molecule with the ligands at 180° from each other.
Chelate ring- When a Bidentate ligand forms two bonds with a metal ion from the lone pairs of two oxygen or nitrogen atoms, the metal is held in a five membered ring.
Ligand substitutionLigands can be substituted with other ligands is the product complex ion is more stable than before. Not all of the ligands need to be replaced so there can be a mix of different types of ligands on one complex ion.