Lesson 13 of 13 · 15 min
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Must-know facts
20 facts
- 01Werner: 1 mol each of CoCl₃·6NH₃, ·5NH₃ and ·4NH₃ give 3, 2 and 1 mol AgCl; they are 1:3, 1:2 and 1:1 electrolytes.
- 02Primary valence = ionisable = oxidation state; secondary valence = non-ionisable = coordination number.
- 03Double salts (Mohr's salt, carnallite, potash alum) give all their simple ions in water; complexes such as K₄[Fe(CN)₆] do not.
- 04EDTA⁴⁻ is hexadentate (2 N + 4 O); en and oxalate are didentate; NO₂⁻ and SCN⁻ are ambidentate.
- 05Coordination number counts σ-bonded donor atoms: [Co(en)₃]³⁺ and [Fe(C₂O₄)₃]³⁻ both have CN 6.
- 06Formula: metal first, ligands alphabetical. Name: ligands alphabetical, then metal; anionic complex ends in -ate (ferrate, argentate, cobaltate).
- 07aqua = H₂O, ammine = NH₃, carbonyl = CO, nitrosyl = NO; anionic ligands end in -ido in the 2004 IUPAC draft (chlorido, cyanido).
- 08Tetrahedral complexes show no geometrical isomerism; square planar [MABXL] has three geometrical isomers.
- 09[Co(NH₃)₃(NO₂)₃] has fac and mer isomers; [Co(en)₃]³⁺ is optically active; only cis-[PtCl₂(en)₂]²⁺ is chiral.
- 10Linkage: [Co(NH₃)₅(ONO)]²⁺ red, [Co(NH₃)₅(NO₂)]²⁺ yellow. Hydrate: [Cr(H₂O)₆]Cl₃ violet, [Cr(H₂O)₅Cl]Cl₂·H₂O grey-green.
- 11[Co(NH₃)₆]³⁺: d²sp³, inner orbital, diamagnetic. [CoF₆]³⁻: sp³d², outer orbital, 4 unpaired.
- 12[NiCl₄]²⁻: sp³, tetrahedral, 2 unpaired. [Ni(CN)₄]²⁻: dsp², square planar, diamagnetic. [Ni(CO)₄]: tetrahedral, diamagnetic, Ni(0).
- 13[Fe(CN)₆]³⁻ has 1 unpaired electron, [FeF₆]³⁻ has 5; [Mn(CN)₆]³⁻ has 2, [MnCl₆]³⁻ has 4.
- 14Octahedral splitting: eg +(3/5)Δo, t₂g −(2/5)Δo; tetrahedral Δt = (4/9)Δo, inverted, nearly always high spin.
- 15Spectrochemical series ends: I⁻ weakest ... en < CN⁻ < CO strongest; H₂O < NH₃ < en.
- 16Δo < P: high spin t₂g³eg¹ (weak field); Δo > P: low spin t₂g⁴eg⁰ (strong field).
- 17[Ti(H₂O)₆]³⁺ absorbs 498 nm (blue-green) and looks violet; [Cu(H₂O)₄]²⁺ absorbs 600 nm (red) and looks blue.
- 18Ruby: Al₂O₃ with 0.5-1% Cr³⁺; emerald: Cr³⁺ in beryl, Be₃Al₂Si₆O₁₈.
- 19Carbonyls: Ni(CO)₄ tetrahedral, Fe(CO)₅ trigonal bipyramidal, Cr(CO)₆ octahedral; bonding is synergic σ donation + π back donation.
- 20Wilkinson's catalyst [(Ph₃P)₃RhCl] hydrogenates alkenes; cis-platin fights tumours; EDTA treats lead poisoning; hypo gives [Ag(S₂O₃)₂]³⁻.
Common traps
Where marks are lost
Formulas
5 to know
Oxidation state of the central atom
charge on entity = x + Σ(ligand charges)
K₃[Fe(CN)₆]: −3 = x − 6, so x = +3.
Octahedral splitting
E(eg) = +(3/5)Δo; E(t₂g) = −(2/5)Δo
Measured from the average (spherical-field) energy of the d orbitals.
Tetrahedral splitting
Δt = (4/9)Δo
Same metal, ligands and metal-ligand distance; order of levels is inverted.
Spin state
Δo < P → high spin (t₂g³eg¹ for d⁴); Δo > P → low spin (t₂g⁴eg⁰)
P is the energy needed to pair two electrons in one orbital.
Spin-only magnetic moment
μ = √[n(n + 2)] BM
[MnBr₄]²⁻: 5.9 BM means n = 5, so the ion is tetrahedral.
Key terms
17 terms
- Coordination entity
- A central metal atom or ion together with the fixed set of ligands bonded to it.
- Ligand
- An ion or molecule that gives an electron pair to the central atom in a complex.
- Denticity
- The number of donor atoms through which one ligand binds a single metal ion.
- Chelate
- A complex in which one ligand grips the same metal through two or more donor atoms, forming a ring.
- Ambidentate ligand
- A ligand with two different donor atoms that can bind through either, such as NO₂⁻ or SCN⁻.
- Coordination number
- The number of ligand donor atoms σ-bonded directly to the central atom.
- Coordination sphere
- The central atom and its ligands, written inside square brackets.
- Counter ion
- An ionisable ion written outside the square bracket that balances the complex's charge.
- Coordination polyhedron
- The shape traced by the donor atoms around the central atom, such as an octahedron.
- Homoleptic / heteroleptic
- Having only one kind of ligand / having more than one kind.
- Primary valence
- Werner's ionisable valence, satisfied by anions; today the oxidation state.
- Secondary valence
- Werner's non-ionisable valence; today the coordination number.
- Inner / outer orbital complex
- A complex that hybridises (n−1)d orbitals (d²sp³, low spin) / nd orbitals (sp³d², high spin).
- Crystal field splitting (Δo)
- The energy gap between the eg and t₂g orbitals created by six ligands in an octahedron.
- Pairing energy (P)
- The energy cost of putting a second electron into an orbital that already holds one.
- Spectrochemical series
- Ligands ranked by the size of the d-orbital splitting they cause, found from absorption spectra.
- Synergic bonding
- σ donation from CO to the metal and π back donation from metal to CO, each strengthening the other.