What are Coordination Compounds?
Coordination compounds consist of a central metal atom or ion surrounded by ligands—polar molecules or ions that donate electron pairs. They form colorful, often soluble complexes used in medicine, industry, and research.
A coordination compound is formed when ligands (Lewis bases) bond to a central metal ion (Lewis acid) through coordinate covalent bonds. Ligands donate electron pairs to the metal, forming complex ions like [Fe(CN)₆]⁴⁻ or [Cu(NH₃)₄]²⁺.
- •[Fe(CN)₆]³⁻ — 6 cyanide ligands
- •[Al(H₂O)₆]³⁺ — 6 water ligands
- •Square planar arrangement in space
- •[ZnCl₄]²⁻ — 4 chloride ligands
- •[Ni(CO)₄] — 4 carbonyl ligands
- •Tetrahedral arrangement in space
Step-by-step worked examples
Determine the coordination number and identify the ligands in [Ni(NH₃)₄(H₂O)₂]²⁺.
Count the number of ligands bonded to the central metal (Ni). Ligands: 4 NH₃ molecules + 2 H₂O molecules = 6 total. Coordination number (CN) = 6. Ligands are: ammonia (NH₃) and water (H₂O).
Write the formula for a complex formed by Fe³⁺ ion with 6 cyanide ligands.
Central metal: Fe³⁺ Ligand: CN⁻ (cyanide, charge −1) Number of ligands: 6 Formula: [Fe(CN)₆]ⁿ⁺ Charge on complex: 3+ − 6(1−) = 3− charge. Complete formula: [Fe(CN)₆]³⁻
Is [CuCl₄]²⁻ octahedral or tetrahedral?
Count the coordination number. Cl⁻ ligands: 4 Coordination number = 4. With CN = 4, the geometry is tetrahedral. [CuCl₄]²⁻ is tetrahedral, not octahedral.
Flashcards
Quick quiz
Q1.In the complex [Cu(NH₃)₄]²⁺, what is the coordination number?
Q2.Which of the following is a bidentate ligand?
Q3.Calculate the charge on the complex [Fe(CN)₆] if the central Fe is Fe²⁺.
Q4.What geometry is typical for coordination number 4?
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Common mistakes
Confusing coordination number with oxidation state. — Correct: Coordination number is the number of ligands; oxidation state is the charge on the metal.
Assuming all monodentate ligands donate one electron pair. — Correct: All monodentate ligands donate exactly one electron pair; bidentate ligands donate two.
Forgetting to account for the charge of the ligands when calculating complex charge. — Correct: Complex charge = metal charge − sum of ligand charges.
Thinking only 6-coordination is stable. — Correct: Coordination numbers of 2, 4, 5, 6, 7, and 8 are all possible depending on metal and ligand.
FAQ
What is a chelate complex?
A complex where a polydentate ligand forms two or more coordinate bonds with a single metal ion, creating a ring structure.
Why do coordination compounds form?
Metal ions are Lewis acids (electron pair acceptors) and ligands are Lewis bases (electron pair donors). They combine for stability.
How does coordination number affect complex geometry?
CN = 4 gives tetrahedral or square planar; CN = 6 gives octahedral; CN = 2 is linear.
What is a complex ion?
A complex that has a net electric charge, such as [Cu(NH₃)₄]²⁺ or [Fe(CN)₆]⁴⁻.