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Química Inorgânica · Coordination chemistry

Compostos de coordenação e teoria do campo cristalino

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O que você vai aprender

  • Name a coordination compound, find the metal's oxidation state and d-electron count, and identify the denticity of its ligands.
  • Predict the geometry of a complex and count its geometric, optical, linkage and ionization isomers.
  • Use crystal field splitting, the spectrochemical series and the pairing energy to decide high or low spin and to calculate CFSE.
  • Calculate spin-only magnetic moments and explain colour, absorption intensity and Jahn–Teller distortion from d-orbital occupancy.

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Ligands are Lewis bases

A coordination complex is a metal ion bonded to a set of ligands. Each ligand donates an electron pair into an empty metal orbital, so every metal–ligand bond is a Lewis acid–base (coordinate) bond. The number of donor atoms bonded to the metal is the coordination number: 2 (linear, as in [Ag(NH₃)₂]⁺), 4 (tetrahedral or square planar) and 6 (octahedral) are the most common. A ligand that binds through one atom is monodentate (NH₃, H₂O, Cl⁻, CN⁻). Ethylenediamine (en, left) binds through both nitrogens and closes a five-membered chelate ring; EDTA⁴⁻ (right) wraps one metal with two nitrogens and four carboxylate oxygens and is hexadentate. Count donor atoms, not ligands: [Co(en)₃]³⁺ has three ligands but coordination number 6. Because the metal pulls electron density from its ligands, coordinated water becomes acidic: [Fe(H₂O)₆]³⁺ has a pKa of about 2.

Names, oxidation states and d counts

Write the cation first and the anion second, as for any salt. Inside the complex, list ligands alphabetically, ignoring multiplying prefixes (di, tri, tetra, penta, hexa), then the metal, then its oxidation state in Roman numerals. Anionic ligands end in -o (chloro, cyano, oxalato; current IUPAC names are chlorido, cyanido), and neutral ligands keep special names: aqua (H₂O), ammine (NH₃), carbonyl (CO). An anionic complex takes the ending -ate, with the Latin stem for some metals: ferrate, cuprate, argentate, aurate. To find the oxidation state, take the charge of the complex ion (fixed by the counterions) and subtract the ligand charges: in K₃[Fe(CN)₆], potassium hexacyanoferrate(III), the complex is 3− and six cyanides give 6−, so iron is +3. The d-electron count is the group number minus the oxidation state: Fe(III) is d⁵, Co(III) d⁶, Cr(III) d³, Ni(II) d⁸, Cu(II) d⁹.

Geometry and isomerism

Isomers of complexes differ either in what is bonded (structural isomers) or in how the same bonds are arranged in space (stereoisomers). Ionization isomers swap a ligand with a counterion, [Co(NH₃)₅Br]SO₄ versus [Co(NH₃)₅(SO₄)]Br; hydrate isomers swap water and chloride between the inner sphere and the lattice; linkage isomers use different donor atoms of an ambidentate ligand such as nitrite (N- or O-bound) or thiocyanate (S- or N-bound). Square-planar MA₂B₂ and octahedral MA₄B₂ complexes have cis and trans isomers; octahedral MA₃B₃ has fac (three like ligands on one face) and mer (three like ligands in a plane) isomers. With more kinds of ligand, classify the isomers by which like pairs sit trans to each other. Octahedral complexes whose chelate rings twist like a propeller, such as cis-[Co(en)₂Cl₂]⁺ and [Co(en)₃]³⁺, have no mirror plane and exist as Δ and Λ enantiomers; trans-[Co(en)₂Cl₂]⁺, with both rings in one plane, has a mirror plane and is achiral. Chelating ligands need donor atoms placed to reach one metal: 2,2′-bipyridine (left) closes a five-membered ring, acetylacetonate (right) a six-membered ring.

Crystal field splitting

Crystal field theory treats ligands as negative charges that repel the d electrons. In an octahedron the ligands lie on the x, y and z axes, so the dz² and dx²−y² orbitals (eg) point at them and rise, while dxy, dxz and dyz (t2g) point between them and fall. The gap is Δo. Relative to the average d energy, each t2g electron is stabilized by 0.4Δo and each eg electron destabilized by 0.6Δo; the sum is the crystal field stabilization energy (CFSE), reported here as a negative number for stabilization and without pairing energy unless a question includes it. In a tetrahedron the pattern is inverted and smaller: e (−0.6Δt) below t2 (+0.4Δt), with Δt ≈ 4/9 Δo. In a square plane dx²−y² lies far above the other four orbitals, so d⁸ ions with large splittings (Ni(II) with CN⁻, Pd(II), Pt(II), Au(III)) prefer square-planar geometry. Δ grows with the ligand along the spectrochemical series, shown here from weak to strong field: I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO. Δ also grows with metal oxidation state and down a group (3d < 4d < 5d).

High spin, low spin and magnetism

For octahedral d⁴ to d⁷, the fourth electron can either pair in t2g (cost: the pairing energy P) or enter eg (cost: Δo). If Δo > P the complex is low spin; if Δo < P it is high spin. Strong-field ligands and heavier metals give low spin: [Co(NH₃)₆]³⁺ is low-spin d⁶ (t2g⁶, diamagnetic), while [CoF₆]³⁻ is high spin with four unpaired electrons. Tetrahedral complexes are almost always high spin because Δt is small. The spin-only magnetic moment is μ = √(n(n + 2)) BM, where n is the number of unpaired electrons: n = 1 gives 1.73 BM, 2 gives 2.83, 3 gives 3.87, 4 gives 4.90 and 5 gives 5.92 BM. Read backwards, a measured moment gives n and so the spin state.

Colour and the Jahn–Teller effect

A d–d transition promotes an electron from t2g to eg, so a complex absorbs light of energy close to Δo and shows the complementary colour: [Ti(H₂O)₆]³⁺ (d¹) absorbs near 493 nm and looks purple. A stronger-field ligand shifts the absorption to higher energy and shorter wavelength. d⁰ and d¹⁰ ions have no d–d transitions and are colourless unless another kind of transition occurs. Octahedral d–d bands are weak because they are Laporte forbidden (g → g in a centrosymmetric complex); tetrahedral complexes lack an inversion centre and absorb more strongly, and high-spin d⁵ ions such as [Mn(H₂O)₆]²⁺ are almost colourless because every d–d transition is also spin forbidden. Finally, an unevenly filled eg set (high-spin d⁴, low-spin d⁷, d⁹) makes a regular octahedron unstable: [Cu(H₂O)₆]²⁺ elongates along one axis, the Jahn–Teller distortion, because lowering the doubly occupied dz² outweighs raising the singly occupied dx²−y².

What to carry forward

Find the oxidation state and d count first, place the ligand on the spectrochemical series, compare Δ with the pairing energy, and fill the split d orbitals. That one diagram predicts the spin state, the magnetic moment, the CFSE, the colour and any Jahn–Teller distortion.

Problemas de prática (40)

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Preveja o produto 4

Crystal field splitting and spin state

  • Spin states of two cobalt(III) complexesPredict high- or low-spin configurations of two d⁶ complexes from the ligand field strength.

Geometry and isomerism

  • Isomers of [Co(NH₃)₃(NO₂)₃]Predict the fac and mer isomers of an octahedral MA₃B₃ complex.

Magnetism, colour and Jahn–Teller

  • Adding ammonia to aqueous nickel(II)Predict how replacing water by ammonia shifts the d–d absorption of an octahedral nickel(II) complex.

Ligands, names and d counts

  • Silver(I) in excess ammoniaPredict the formula, charge and geometry of the complex ion formed when ammonia acts as a Lewis base toward silver(I).

Classificar 9

Ligands, names and d counts

  • Which bipyridine chelates?Decide from donor-atom positions which bipyridine isomer can chelate a single metal ion.
  • Which ligand chelates?Recognize a chelating ligand from the placement of its donor atoms.
  • Find the hexadentate ligandCount the donor atoms of polydentate ligands to assign their denticity.
  • Name [Co(NH₃)₅Cl]Cl₂Order ligand names alphabetically and assign the oxidation state in a cationic complex salt.
  • Name K₃[Fe(CN)₆]Name a salt containing an anionic complex, using the -ate ending and the metal's oxidation state.

Geometry and isomerism

  • Which complex is chiral?Identify the chiral octahedral complex by looking for a mirror plane.
  • Which ligand gives linkage isomers?Recognize an ambidentate ligand that can produce linkage isomers.

Magnetism, colour and Jahn–Teller

  • Which ion is Jahn–Teller distorted?Identify the octahedral ion with an unevenly occupied eg set that undergoes a strong Jahn–Teller distortion.

Crystal field splitting and spin state

  • Which orbitals form the eg set?Identify the d orbitals raised in energy by an octahedral crystal field.

Comparar 5

Ligands, names and d counts

  • Chelate ring sizeCount chelate ring sizes to pick the ligand that forms a six-membered ring.

Geometry and isomerism

  • Cis and trans [PtCl₂(NH₃)₂]Compare cis and trans square-planar isomers by the symmetry-based property that differs between them.

Crystal field splitting and spin state

  • Δo across metal ionsRank Δo for one ligand as the metal oxidation state and period change.
  • Spectrochemical seriesRank common ligands by the size of the crystal field splitting they produce.

Magnetism, colour and Jahn–Teller

  • Rank by spin-only magnetic momentRank octahedral complexes by spin-only magnetic moment after assigning d count and spin state.

Calcular 11

Crystal field splitting and spin state

  • CFSE of [Fe(H₂O)₆]³⁺Show that a high-spin d⁵ ion has zero crystal field stabilization.
  • CFSE of [Co(NH₃)₆]³⁺Calculate the CFSE of a low-spin octahedral d⁶ ion in units of Δo.
  • CFSE of tetrahedral [CoCl₄]²⁻Calculate the CFSE of a tetrahedral d⁷ ion using the inverted tetrahedral splitting.
  • Splitting versus pairing for a d⁶ ionCalculate the energy difference between low- and high-spin d⁶ configurations from Δo and the pairing energy.

Ligands, names and d counts

  • Coordination number of [Co(en)₃]³⁺Find a coordination number by counting donor atoms rather than ligands.
  • d count in tris(oxalato)chromate(III)Count the d electrons of the metal in a complex with a dianionic chelating ligand.

Geometry and isomerism

  • Geometric isomers of [Co(NH₃)₂(H₂O)₂Cl₂]⁺Enumerate the geometric isomers of an octahedral MA₂B₂C₂ complex by trans-pair analysis.
  • Stereoisomers of [Co(en)₂Cl₂]⁺Count all stereoisomers of a bis-chelate octahedral complex, including enantiomers.

Magnetism, colour and Jahn–Teller

  • Spin-only moment of [Fe(CN)₆]³⁻Calculate the spin-only magnetic moment of a low-spin d⁵ complex.
  • Spin-only moment of [Fe(H₂O)₆]³⁺Calculate the spin-only magnetic moment of a high-spin d⁵ ion.
  • Δo of [Ti(H₂O)₆]³⁺ from its absorptionConvert the d–d absorption wavelength of a d¹ complex into Δo in kJ/mol.

Encontre o erro 1

  • Find the error in an oxidation-state countDiagnose an oxidation-state error caused by ignoring the counterion of a complex salt.

Interprete as evidências 2

Geometry and isomerism

  • Geometry from magnetic dataUse magnetic data to assign square-planar or tetrahedral geometry to four-coordinate d⁸ complexes.
  • Silver chloride from a hydrate isomerPredict the moles of AgCl precipitated from the number of chloride ions outside the coordination sphere.

Trabalhe de trás para frente 1

  • Formula from a nameWrite the formula of a complex salt from its name by balancing the metal oxidation state and ligand charges.

Planeje uma síntese 2

Crystal field splitting and spin state

  • Choose a ligand for a low-spin iron(II) complexChoose a ligand from the spectrochemical series that makes an octahedral iron(II) complex low spin.

Geometry and isomerism

  • Tell two ionization isomers apartPlan a precipitation test that distinguishes two ionization isomers.

Explique por quê 5

Crystal field splitting and spin state

  • Why 4d and 5d complexes are low spinExplain why second- and third-row transition-metal complexes are almost always low spin.
  • Why tetrahedral complexes are high spinExplain why tetrahedral complexes of first-row metals are almost always high spin.

Ligands, names and d counts

  • Why [Fe(H₂O)₆]³⁺ is an acidExplain the acidity of an aqua ion from metal charge density and keep the metal's oxidation state straight when a water ligand loses a proton.

Magnetism, colour and Jahn–Teller

  • Why [Cu(H₂O)₆]²⁺ elongatesExplain the energy gain that drives Jahn–Teller elongation of a d⁹ octahedral complex.
  • Why [Mn(H₂O)₆]²⁺ is almost colourlessExplain the very weak colour of high-spin d⁵ complexes using the spin and Laporte selection rules.

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