Coordination Chemistry: Complexes and Ligands

Core Idea

Coordination complexes form when a central metal ion bonds with electron-donating ligands. Ligands donate electron pairs to the metal, forming coordinate covalent bonds. Complex ions have characteristic charges and geometries.

Explainer

From your study of ionic and covalent bonding, you know that atoms can transfer electrons (ionic) or share them (covalent). Coordination chemistry introduces a third variation: the coordinate covalent bond (also called a dative bond), where both electrons in the shared pair come from the same atom. This happens when a metal ion with empty orbitals meets a molecule or ion that has a lone pair to donate. The metal is a Lewis acid (electron pair acceptor), and the donor species is a Lewis base — called a ligand in coordination chemistry.

A coordination complex consists of a central metal ion surrounded by ligands. Consider the deep blue complex formed when ammonia is added to a solution of copper(II) sulfate: four NH₃ molecules each donate their lone pair to Cu²⁺, forming [Cu(NH₃)₄]²⁺. The metal ion is the center, the ligands are the attachments, and the whole assembly carries a charge equal to the metal's charge plus the charges of all ligands. The number of bonds from ligands to the metal is called the coordination number — copper in this example has a coordination number of 4. Common coordination numbers are 2, 4, and 6, with 6 being the most frequent for transition metals.

Ligands come in different varieties based on how many donor atoms they have. Monodentate ligands like NH₃, Cl⁻, and H₂O donate through a single atom. Bidentate ligands like ethylenediamine (en) have two donor atoms and grip the metal at two points, like a crab's claw — this is why multi-donor ligands are called chelating agents (from the Greek word for claw). Chelating ligands form more stable complexes than comparable monodentate ligands because detaching requires breaking multiple bonds simultaneously, an effect known as the chelate effect. EDTA, with six donor atoms, is a powerful chelating agent used in everything from water softening to medical treatment of heavy metal poisoning.

The geometry of a coordination complex depends on its coordination number: two ligands typically give a linear arrangement, four can give either tetrahedral or square planar geometry, and six ligands arrange octahedrally. These geometries determine the complex's physical properties — its color, magnetic behavior, and reactivity. The vivid colors of transition metal complexes (the green of chromium(III), the purple of permanganate, the blue of copper-ammonia) arise because d-electrons absorb specific wavelengths of visible light, and the energy gap between d-orbitals depends on the geometry and the identity of the ligands. This is why adding different ligands to the same metal ion can produce dramatically different colors.