Periodic Chemistry
Coordination Complex
Metal ions surrounded by ligands — colorful chemistry of transition metals
A coordination complex is a structure with a central metal ion bonded to surrounding ligands (Lewis bases). Bonds are coordinate covalent (both electrons from ligand). Common ligands: H₂O, NH₃, Cl⁻, CN⁻, EDTA. Coordination number: number of ligand bonds (typically 4 or 6). Geometries: tetrahedral, square planar, octahedral. Properties: distinctive colors (d-d transitions), magnetism (high vs low spin), bioactivity. Examples: hemoglobin (Fe-based), chlorophyll (Mg-based), vitamin B12 (Co-based), platinum drugs (Pt-based — cisplatin).
- ComponentsCentral metal + ligands
- BondCoordinate covalent (Lewis acid-base)
- Coordination numbers4 or 6 most common; 2-12 possible
- GeometriesTetrahedral, square planar, octahedral
- Examples[Cu(H₂O)₆]²⁺, hemoglobin, [PtCl₂(NH₃)₂] (cisplatin)
- PropertiesColor, magnetism, catalysis, biological function
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Why coordination complexes matter
- Biology. Hemoglobin, chlorophyll, B12.
- Drugs. Cisplatin, MRI contrast agents.
- Catalysis. Many industrial catalysts.
- Color. Pigments, dyes, gem stones.
- Analytical. Detection of metals.
- Materials. Coordination polymers, MOFs.
- Environmental. Heavy metal remediation via chelation.
Common misconceptions
- All metals form complexes. Specific transition metals; alkali less.
- Coordination = ionic. Coordinate covalent.
- Color from metal alone. Ligand affects via splitting.
- Geometry is fixed. Multiple possible per coordination number.
- Cis-trans not important. Critical (cisplatin example).
- Chelation just stronger. Specifically multidentate; entropy effect.
Frequently asked questions
What's a coordination complex?
Central metal ion + surrounding ligands. Ligands donate lone pair to metal — coordinate covalent bond. Metal acts as Lewis acid; ligands as Lewis bases. Typical: transition metals (have d-orbitals to accept electrons). Counter-ions balance charge if any. [Cu(H₂O)₆]²⁺ written: 6 water ligands around Cu²⁺.
What are common ligands?
Many. Monodentate (1 bond): H₂O, NH₃, Cl⁻, CN⁻, OH⁻, CO. Bidentate (2 bonds): ethylenediamine (en), glycinate. Polydentate: EDTA (4-6 bonds). Heme (4 N donors). Crown ethers. Specifically: nitrogen, oxygen, sulfur most common donor atoms. Strong ligands: CN⁻, CO. Weak: H₂O, halides.
Why are they colored?
d-d transitions. Ligands split d orbitals (Crystal Field Theory). Energy gap matches visible light. Absorption of certain wavelengths → complementary color seen. Examples: [Cu(H₂O)₆]²⁺ blue (absorbs orange). [Cu(NH₃)₄]²⁺ deep blue (absorbs orange-yellow more). Different ligands → different splittings → different colors.
What's Crystal Field Theory?
Approach to bonding in complexes. Ligands generate electric field; splits 5 d-orbitals into different energies. Octahedral: 3 lower (t₂g) + 2 higher (eg). Energy gap Δ depends on: metal, ligand, geometry. Stronger ligand field: larger Δ. Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < CN⁻ < CO. Predicts: color, magnetism.
What's hemoglobin?
Iron-based coordination complex. Heme group: porphyrin (4 N donors) + Fe²⁺ + 5th ligand (proximal histidine of protein) + 6th ligand (O₂ or CO). Reversible O₂ binding for transport. CO binds 200× stronger than O₂ — toxic (poisoning). Each hemoglobin: 4 heme units; cooperative binding makes O₂ uptake/release efficient.
What's cisplatin?
Platinum-based anticancer drug. [Pt(NH₃)₂Cl₂] — square planar geometry. Cl⁻ ligands replaced by water; binds DNA; prevents replication. Transplatin (trans isomer): doesn't work. Cis-trans isomerism critical. One of most successful chemotherapy drugs (testicular cancer cure rate >95%). Side effects significant; newer Pt drugs: carboplatin, oxaliplatin.
What's chelation?
Multidentate ligand wraps around metal — forms ring. Stronger bond than separate ligands. Examples: EDTA — 6 donors; binds metals tightly. Used: heavy metal poisoning (Pb, Hg, Cd) treatment, water softening (binds Ca, Mg), analytical chemistry (titrating metals). Chelate effect: entropy favors fewer particles (6 H₂O released for 1 EDTA bound).