coordination compound
Think of a small magnet surrounded by iron filings that all point inward and cling to it. A coordination compound is that idea built from chemistry: a central metal atom or ion with a set of molecules or ions gathered tightly around it, donating electron pairs to bind. The metal is the hub; the surrounding groups are passengers it has gathered. These compounds give us the deep blue of copper in ammonia, the red of blood's iron, the platinum drug that fights cancer, and the catalysts that make plastics.
More precisely, a coordination compound contains at least one coordination entity: a central metal bonded to a number of surrounding ligands, where each ligand donates a lone pair into an empty metal orbital, forming a coordinate (dative) bond. The metal plus its directly bound ligands is the coordination sphere, written inside square brackets, for example [Cu(NH3)4]SO4. If the coordination entity carries a charge it is a complex ion, and ions outside the brackets (here sulfate) are counter-ions that balance the charge but are not bonded to the metal. A neutral overall compound can be entirely one neutral complex, such as [Ni(CO)4] or [Co(NH3)3Cl3].
Coordination compounds are the central subject matter of inorganic chemistry, and a reminder that 'inorganic' does not mean lifeless: hemoglobin, chlorophyll, and the enzymes that fix nitrogen are all coordination compounds. Their structure (how many ligands and in what geometry), their naming, and their isomerism are the foundation this field builds; the colors and magnetism come later from ligand-field theory, and their reactions from mechanism studies.
Copper(II) sulfate dissolved in water is pale blue because of the aqua complex [Cu(H2O)4]2+ (with two more loosely held). Add ammonia and the deep royal blue [Cu(NH3)4]2+ forms as ammonia ligands replace water. Both are coordination compounds; only the ligands changed, and with them the color.
Swapping water ligands for ammonia turns pale-blue copper solution deep royal blue.
The bonds between ligand and metal are usually called coordinate or dative bonds because both shared electrons come from the ligand, but once formed they are ordinary covalent bonds — there is nothing weaker or 'special' about them by virtue of where the electrons started.