A complex ion forms when several molecules or ions that carry a lone pair attach to a metal ion. The metal ion at the center is the central metal ion, and the particles around it are the ligands.
The charge on the whole ion is the charge on the central metal ion plus the charges on the ligands. The formula is written inside square brackets, with that total charge at the top right.
A ligand holds a lone pair and hands the whole pair to the central metal ion. A bond in which one atom supplies both of the shared electrons is a coordinate bond.
Once it exists, the bond cannot be told apart from an ordinary covalent bond. Only its origin differs; the strength and the length are the same.
The number of ligands bound to the central metal ion is the coordination number. It is fixed mainly by the metal ion: 2 for silver, 4 for copper, zinc and aluminium, 6 for iron.
Ligands push each other as far apart as they can, so the coordination number fixes the geometry. Two give a linear ion, four a tetrahedral one, six an octahedral one. Copper(II) is the exception: with four ligands it is square planar.
These are the complex ions worth knowing. The name gives the number of ligands as a Greek numeral, then the ligand, then the metal. When the whole ion carries a negative charge, the metal takes the -ate ending.
The color comes from the pairing of central metal ion and ligand. Zinc gives two different complex ions depending on the ligand, and both of them are colorless.
The deep blue of the copper complex and the yellow of the iron complexes help tell solutions apart. Adding a little ammonia to copper(II) sulfate solution gives a pale blue precipitate, and adding more dissolves it again into a deep blue solution.