Complexes, ligands and changing colours
Introduced| English | Meaning |
|---|---|
| ligand/ˈlɪɡænd/ | An ion or molecule that binds to another species at a specific site, such as a metal centre or a receptor. |
| coordination number/kəʊˈɔːdɪneɪʃn ˈnʌmbə/ | The number of nearest neighbours around a particle, or of donor atoms directly bonded to the central atom in a complex. |
| octahedral/ˌɒktəˈhiːdrəl/ | Describing an arrangement with six surrounding positions directed toward the vertices of an octahedron. |
| Absorption/əbˈsɔːpʃn/ | Taking a substance or energy into another material or system. |
Count what actually bonds
- A ligand 配体 donates an electron pair to a metal ion. Coordination number 配位数 counts donor atoms directly attached to that metal.
- Three bidentate en ligands have six donor atoms. A six-coordinate complex often has octahedral 八面体的 geometry.
Three bidentate ligands necessarily give coordination number three.
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Each ligand contributes two donor atoms, giving six.
Keep atom and ion configurations separate
- Chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹. Remove 4s electrons before 3d to form positive ions.
- Cu²⁺ is d⁹ and meets the transition-element criterion. Zn²⁺ is d¹⁰; zinc does not meet that definition.
Which ion has an incomplete d subshell?
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Copper(II) is d⁹, satisfying the incomplete-d condition.
Solve charge before interpreting colour
- In [Cr(H₂O)₄Cl₂]⁺, neutral waters contribute zero and two chlorides contribute −2. Chromium is +3.
- Four water donors plus two chloride donors give coordination number six. Neither colour nor ligand count alone determines every structural detail.
For [Co(en)₃]³⁺, three neutral en ligands leave cobalt at +3 and give six donor atoms.
Find chromium oxidation state in [Cr(H₂O)₄Cl₂]⁺.
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x − 2 = +1, so x = +3.
Explain colour using energy levels
- Ligands split d-orbital energy levels. Absorption 吸收 of suitable visible light promotes an electron between levels.
- Changing ligand, oxidation state or coordination number can change the gap. A d⁰ or d¹⁰ ion has no suitable d–d transition.
Which two changes can alter d-orbital splitting?
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The ligand environment and oxidation state affect the d-orbital energy gap.
Separate ammonia’s two roles
- A small amount of ammonia precipitates copper hydroxide by accepting protons from coordinated water.
- Excess ammonia replaces water ligands and dissolves the precipitate to give a deep blue solution. The copper remains +2.
Four-coordinate complexes may be tetrahedral or square planar; the same count can have different geometry.
Count donor pairs and coordination sites, track the metal oxidation state, and distinguish precipitation from ligand replacement before interpreting a colour change.
Excess ammonia gives copper(II) a deep _____ solution.
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The dissolved ammine complex is deep blue, unlike the initial pale blue precipitate.